Light and diurnal cycle affect human heart rate: possible role for the circadian pacemaker

Association between outdoor light at night exposure and executive function in Chinese children

BACKGROUND

Recent evidences highlight the potential impact of outdoor Light at Night (LAN) on executive function. However, few studies have investigated the association between outdoor LAN exposure and executive function.

METHODS

We employed data from 48,502 Chinese children aged 5-12 years in a cross-sectional study conducted in Guangdong province during 2020-2021, to examine the association between outdoor LAN and executive function assessed using the validated parent-completed Behavior Rating Inventory of Executive Function. We assessed children's outdoor LAN exposure using the night-time satellite images based on the residential addresses. We used generalized linear mixed models to estimate the association between outdoor LAN exposure and executive function scores and executive dysfunction.

RESULTS

After adjusting for potential covariates, higher quintiles of outdoor LAN exposure were associated with poorer executive function. Compared to the lowest quintile (Q1), all higher quintiles of exposure showed a significant increased global executive composite (GEC) score with β (95% confidence intervals, CI) of 0.58 (0.28, 0.88) in Q2, 0.59 (0.28, 0.9) in Q3, 0.85 (0.54, 1.16) in Q4, and 0.76 (0.43, 1.09) in Q5. Higher quintiles of exposure were also associated with higher risks for GEC dysfunction with odd ratios (ORs) (95% CI) of 1.34 (1.18, 1.52) in Q2, 1.40 (1.24, 1.59) in Q3, 1.40 (1.23, 1.59) in Q4, and 1.39 (1.22, 1.58) in Q5. And stronger associations were observed in children aged 10-12 years.

CONCLUSIONS

Our study suggested that high outdoor LAN exposure was associated with poor executive function in children. These findings suggested that future studies should determine whether interventions to reduce outdoor LAN exposure can have a positive effect on executive function.

Bright light increases blood pressure and rate-pressure product after a single session of aerobic exercise in men

This study aimed to test whether bright light (BL) exposure attenuates the reduction in blood pressure (BP) postexercise compared to dim light (DL). Twenty healthy men (27 ± 5 years) randomly underwent two experimental sessions: one under BL (5000 lux) and another under dim light (DL <8lux). In each session, subjects executed a bout of aerobic exercise (cycle ergometer, 30 min, moderate intensity). BP (oscillometric) and heart rate (HR monitor) were measured, and rate-pressure-product (RPP) was calculated. Additionally, a 24-h ambulatory blood pressure monitoring (ABPM) was conducted after the sessions. Systolic BP decreased while HR increased significantly and similarly after the exercise in both sessions. Additionally, systolic BP levels were higher in BL than DL throughout the experimental session (Psession = 0.04). Diastolic (Pinteraction = 0.02) and mean (Pinteraction = 0.03) BPs decreased after exercise in DL (at 30 min), and increased in BL (at 60 and 90 min). RPP increased in both sessions postexercise, but with a main effect revealing higher levels throughout the experimental session in BL than DL (Psession = 0.04) and during the first 3 h of ABPM (p = 0.05). In healthy men, BL exposure increased systolic BP and cardiac work, and abolished the postexercise decreases of diastolic and mean BPs.

Circadian Regulation of Cardiac Arrhythmias and Electrophysiology

Circadian rhythms in physiology and behavior are ≈24-hour biological cycles regulated by internal biological clocks (ie, circadian clocks) that optimize organismal homeostasis in response to predictable environmental changes. These clocks are present in virtually all cells in the body, including cardiomyocytes. Many decades ago, clinicians and researchers became interested in studying daily patterns of triggers for sudden cardiac death, the incidence of sudden cardiac death, and cardiac arrhythmias. This review highlights historical and contemporary studies examining the role of day/night rhythms in the timing of cardiovascular events, delves into changes in the timing of these events over the last few decades, and discusses cardiovascular disease-specific differences in the timing of cardiovascular events. The current understanding of the environmental, behavioral, and circadian mechanisms that regulate cardiac electrophysiology is examined with a focus on the circadian regulation of cardiac ion channels and ion channel regulatory genes. Understanding the contribution of environmental, behavioral, and circadian rhythms on arrhythmia susceptibility and the incidence of sudden cardiac death will be essential in developing future chronotherapies.

Heart rate patterns of captive Asian elephant (Elephas maximus) in their natural habitat at Wild Elephant Valley, Xishuangbanna of China

There are few studies on the changes of heart rate of the Asian elephant (Elephas maximus), one of the largest tropical terrestrial mammals, with its self-factors and external environment. By measuring the heart rate of 35 Asian elephants, ranging in age from 4 months to 52 years, using a non-invasive electrocardiogram sensor in their natural habitat at Wild Elephant Valley, Xishuangbanna of China, we found factors that significantly influenced the HR were season, phase of the day, age, body weight, and the interaction between some of the above factors. We also observed that Asian elephants had lower resting heart rate in the morning of hot season than the cold and mild season, and the differences were significant, but the heart rate increased to similar levels in the afternoon regardless of the season. HR also decreased with age in all seasons and phases of the day. However, there was no significant effect of sex. This study reveals the adaptability of Asian elephant to tropical environment, and provides a basic reference for heart rate of Asian elephant under various natural conditions.

Thermosensation and Temperature Preference: From Molecules to Neuronal Circuits in Drosophila

Temperature has a significant effect on all physiological processes of animals. Suitable temperatures promote responsiveness, movement, metabolism, growth, and reproduction in animals, whereas extreme temperatures can cause injury or even death. Thus, thermosensation is important for survival in all animals. However, mechanisms regulating thermosensation remain unexplored, mostly because of the complexity of mammalian neural circuits. The fruit fly Drosophila melanogaster achieves a desirable body temperature through ambient temperature fluctuations, sunlight exposure, and behavioral strategies. The availability of extensive genetic tools and resources for studying Drosophila have enabled scientists to unravel the mechanisms underlying their temperature preference. Over the past 20 years, Drosophila has become an ideal model for studying temperature-related genes and circuits. This review provides a comprehensive overview of our current understanding of thermosensation and temperature preference in Drosophila. It encompasses various aspects, such as the mechanisms by which flies sense temperature, the effects of internal and external factors on temperature preference, and the adaptive strategies employed by flies in extreme-temperature environments. Understanding the regulating mechanisms of thermosensation and temperature preference in Drosophila can provide fundamental insights into the underlying molecular and neural mechanisms that control body temperature and temperature-related behavioral changes in other animals.

Association of exposure to artificial light at night during adolescence with blood pressure in early adulthood

Artificial light at night (ALAN) is related to various diseases, such as cancer, obesity, and coronary heart disease. However, its impact on blood pressure in adolescents is not well understood. To investigate this, we conducted a cross-sectional study with a nationwide sample of college students in China, who were freshmen from four disperse universities during Sep. and Oct. 2018. Mean levels of ALAN at participants' residential addresses during 2013-2018 were estimated using time-varying satellite data. The association of the 6-y average of ALAN with blood pressure was estimated by using generalized linear mixed models. A total of 17 046 participants (18.2 ± 0.7 y of age, 46.79% female) from 2,412 counties and cities were included in the final analysis. After a full adjustment for potential confounders, ALAN was positively associated with systolic blood pressure (β = 0.20, p = 0.032) and pulse pressure (β = 0.28, p  = 0.001), but there was no association between ALAN and diastolic blood pressure (β = -0.08, p = 0.213). In the sensitivity analysis, the results consistent with the main analysis were observed. The blood pressure of males and those with a BMI ≤24 kg/m2 were more susceptible to ALAN exposure. Our findings highlight the importance of ALAN management for blood pressure control, particularly among male and normal-weight individuals.

The Role of the Circadian Rhythm in Dyslipidaemia and Vascular Inflammation Leading to Atherosclerosis

Cardiovascular diseases (CVD) are among the leading causes of death worldwide. Many lines of evidence suggest that the disturbances in circadian rhythm are responsible for the development of CVDs; however, circadian misalignment is not yet a treatable trait in clinical practice. The circadian rhythm is controlled by the central clock located in the suprachiasmatic nucleus and clock genes (molecular clock) located in all cells. Dyslipidaemia and vascular inflammation are two hallmarks of atherosclerosis and numerous experimental studies conclude that they are under direct influence by both central and molecular clocks. This review will summarise the results of experimental studies on lipid metabolism, vascular inflammation and circadian rhythm, and translate them into the pathophysiology of atherosclerosis and cardiovascular disease. We discuss the effect of time-respected administration of medications in cardiovascular medicine. We review the evidence on the effect of bright light and melatonin on cardiovascular health, lipid metabolism and vascular inflammation. Finally, we suggest an agenda for future research and recommend on clinical practice.

Effects of Post-awakening Light Exposure on Heart Rate Variability in Healthy Male Individuals

Light-induced effects on the autonomic nervous system (ANS) are assumed to be mediated by retinal projections to the hypothalamic suprachiasmatic nucleus (SCN) via different routes. Light information for the circadian system is detected by a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs), however, inconsistency exists in research concerning the effects of light exposure on heart rate variability (HRV). Two within-subject experiments were conducted in a standardized sleep laboratory to investigate effects of light intensity (study I, n = 29: 2 days dim vs. bright light) and spectral composition (study II, n = 24: 3 days using red vs. blue vs. green light) on HRV parameters (RMSSD, LF, HF-HRV, LF/HF ratio). Light exposure was conducted for one-hour in the post-awakening phase at 5:00 AM. Results revealed no significant light intensity effect comparing dim light versus bright white light on HRV parameters. Light color of different wavelengths significantly influenced all HRV parameters except the low frequency, with moderate to large effect sizes. RMSSD values were elevated for all three colors compared to norm values, indicating stronger parasympathetic activation. LED light of different spectral compositions demonstrated bidirectional effects on spectral components of the HRV. Red light decreased the LF/HF ratio within 30 min, whereas with blue light, LF/HF ratio consistently increased across 40 min of light exposure.

Measure by measure: Resting heart rate across the 24-hour cycle

BACKGROUND

Photoplethysmography (PPG) sensors, typically found in wrist-worn devices, can continuously monitor heart rate (HR) in large populations in real-world settings. Resting heart rate (RHR) is an important biomarker of morbidities and mortality, but no universally accepted definition nor measurement criteria exist. In this study, we provide a working definition of RHR and describe a method for accurate measurement of this biomarker, recorded using PPG derived from wristband measurement across the 24-hour cycle.

METHODS

433 healthy subjects wore a wrist device that measured activity and HR for up to 3 months. HR during inactivity was recorded and the duration of inactivity needed for HR to stabilise was ascertained. We identified the lowest HR during each 24-hour cycle (true RHR) and examined the time of day or night this occurred. The variation of HR during inactivity through the 24-hour cycle was also assessed. The sample was also subdivided according to daily activity levels for subset analysis.

FINDINGS

Adequate data was obtained for 19,242 days and 18,520 nights. HR stabilised in most subjects after 4 minutes of inactivity. Mean (SD) RHR for the sample was 54.5 (8.0) bpm (day) and 50.5 (7.6) bpm (night). RHR values were highest in the least active group (lowest MET quartile). A circadian variation of HR during inactivity was confirmed, with the lowest values being between 0300 and 0700 hours for most subjects.

INTERPRETATION

RHR measured using a PPG-based wrist-worn device is significantly lower at night than in the day, and a circadian rhythm of HR during inactivity was confirmed. Since RHR is such an important health metric, clarity on the definition and measurement methodology used is important. For most subjects, a minimum rest time of 4 minutes provides a reliable measurement of HR during inactivity and true RHR in a 24-hour cycle is best measured between 0300 and 0700 hours. Funding: This study was funded by Google.

Melatonin improves cardiac remodeling and brain-heart sympathetic hyperactivation aggravated by light disruption after myocardial infarction

Light in the external environment might affect cardiovascular function. The light disruption seems to be related to changes in cardiovascular physiological functions, and disturbing light may be a risk factor for cardiovascular diseases. Prior studies have found that light disruption after myocardial infarction (MI) exacerbates cardiac remodeling, and the brain-heart sympathetic nervous system may be one of the key mechanisms. However, how to improve light-disrupted cardiac remodeling remains unclear. Melatonin is an indoleamine secreted by the pineal gland and controlled by endogenous circadian oscillators within the suprachiasmatic nucleus, which is closely associated with light/dark cycle. This study aimed to explore whether melatonin could improve light-disrupted cardiac remodeling and modulate the brain-heart sympathetic nervous system. Our study revealed that light disruption reduced serum melatonin levels, aggravated cardiac sympathetic remodeling, caused overactivation of the brain-heart sympathetic nervous system, exacerbated cardiac dysfunction, and increased cardiac fibrosis after MI, while melatonin treatment improved light disruption-exacerbated cardiac remodeling and brain-heart sympathetic hyperactivation after MI. Furthermore, RNA-Seq results revealed the significant changes at the cardiac transcription level. In conclusion, melatonin may be a potential therapy for light-disrupted cardiac remodeling.

Objective Measures of Immediate “Energizing” Effect of Light: Studies Review and Data Analysis

While the energizing effect of light has been known since the early years of light therapy, its reliable detection using objective measures is still not well-established. This review aims to ascertain the immediate energizing effect of light and determine its best indicators. Sixty-four articles published before July 2022 were included in the review. The articles described 72 (sub-)studies performed in healthy individuals. Fourteen measures were analyzed. The analysis showed that light causes an energizing effect that can be best documented by measuring core (rectal) body temperature: the proportion of the studies revealing increasing, unchanging, and decreasing rectal temperature was 13/6/1. The second most suitable indicator was heart rate (10/22/1), which showed concordant changes with rectal temperature (a trend, seven mutual studies). There is no evidence from the reviewed articles that oxygen consumption, skin conductance, blood pressure, heart rate variability, non-rectal inner temperature (combined digestive, tympanic, and oral), skin temperature, or cortisol levels can provide light effect detection. Four other measures were found to be unsuitable as well but with less certainty due to the low number of studies (≤3): skin blood flow, noradrenaline, salivary alpha-amylase, and thyroid-stimulating hormone levels. On the other hand, light exposure had a noticeable effect on sympathetic nerve activity measured using microneurography; however, this measure can be accepted as a marker only tentatively as it was employed in a single study. The analysis took into account three factors—study limitation in design/analysis, use of light in day- or nighttime, and relative brightness of the light stimulus—that were found to significantly influence some of the analyzed variables. The review indicates that the energizing effect of light in humans can be reliably detected using rectal temperature and heart rate.

A Study on Nonvisual Effects of Natural Light Environment in a Maternity Ward of a Hospital in Cold Area

Thanks to the discovery of human eye photoreceptor cells, ipGRC and human nonvisual channels, the nonvisual effects of light have gradually come to our vision and been rationally utilized. Along with this trend, people have expanded their demand for the light environment to both visual and nonvisual needs from only visual needs. With a good natural daylighting, parturients will find their psychological pressure and physiological pain relieved, their rehabilitation rate increased, and they get rehabilitated more quickly. This study was carried out in a representative maternity ward in Harbin based on the latest research results on nonvisual effects at home and abroad. Specifically, the nonvisual effects on the natural light environment in the ward throughout the year were simulated and analyzed from the aspects of equivalent melanopic lux (EML), stimulus frequency (Stim.freq), and circadian effective area (CEA). During the study, natural light in the ward was measured on site, and the evaluation tool and workflow of nonvisual effects were created with the aid of Grasshopper modeling platform, Ladybug+Honeybee, and VB script editor. The results show that the nonvisual effects of natural light on the body of parturients gradually weaken as they further go inside the ward. What is worse, in the most unfavorable all-overcast condition, daylighting on beds far away from the window does not meet the stimulus of human circadian rhythm from April to August. Therefore, additional light is required. The wards have the best nonvisual natural light environment when they are south facing and have a window to floor ratio of 0.3.

Bright Light Increases Alertness and Not Cortisol in Healthy Men: A Forced Desynchrony Study Under Dim and Bright Light (I)

Light-induced improvements in alertness are more prominent during nighttime than during the day, suggesting that alerting effects of light may depend on internal clock time or wake duration. Relative contributions of both factors can be quantified using a forced desynchrony (FD) designs. FD designs have only been conducted under dim light conditions (<10 lux) since light above this amount can induce non-uniform phase progression of the circadian pacemaker (also called relative coordination). This complicates the mathematical separation of circadian clock phase from homeostatic sleep pressure effects. Here we investigate alerting effects of light in a novel 4 × 18 h FD protocol (5 h sleep, 13 h wake) under dim (6 lux) and bright light (1300 lux) conditions. Hourly saliva samples (melatonin and cortisol assessment) and 2-hourly test sessions were used to assess effects of bright light on subjective and objective alertness (electroencephalography and performance). Results reveal (1) stable free-running cortisol rhythms with uniform phase progression under both light conditions, suggesting that FD designs can be conducted under bright light conditions (1300 lux), (2) subjective alerting effects of light depend on elapsed time awake but not circadian clock phase, while (3) light consistently improves objective alertness independent of time awake or circadian clock phase. Reconstructing the daily time course by combining circadian clock phase and wake duration effects indicates that performance is improved during daytime, while subjective alertness remains unchanged. This suggests that high-intensity indoor lighting during the regular day might be beneficial for mental performance, even though this may not be perceived as such.

Light exposure during sleep impairs cardiometabolic function

SignificanceAmbient nighttime light exposure is implicated as a risk factor for adverse health outcomes, including cardiometabolic disease. However, the effects of nighttime light exposure during sleep on cardiometabolic outcomes and the related mechanisms are unclear. This laboratory study shows that, in healthy adults, one night of moderate (100 lx) light exposure during sleep increases nighttime heart rate, decreases heart rate variability (higher sympathovagal balance), and increases next-morning insulin resistance when compared to sleep in a dimly lit (<3 lx) environment. Moreover, a positive relationship between higher sympathovagal balance and insulin levels suggests that sympathetic activation may play a role in the observed light-induced changes in insulin sensitivity.

Daytime Exposure to Blue Light Alters Cardiovascular Circadian Rhythms, Electrolyte Excretion and Melatonin Production

Artificial light is characterized by certain features of its impact on the body in terms of its spectral distribution of power, duration of exposure and intensity. Short waves, perceived as blue light, are the strongest synchronizing agent for the circadian system. In the present work, we investigated the features of the circadian rhythms of blood pressure (BP), heart rate (HR), the excretion of electrolytes and the secretion of melatonin in normotensive (Wistar–Kyoto) and hypertensive (SHR) rats under the action of monochromatic blue light in the daytime period. It was found that the exposure of Wistar–Kyoto rats to monochromatic blue light was accompanied by a significant decrease in nighttime and 24 h systolic BP. The most remarkable changes are characteristic of the HR in SHR rats under monochromatic light. A significant decrease in HR in each time period was found, but the predominance of nighttime over daytime values remained in SHR animals. There was also a significant increase in the mesor of the HR in SHR rats. Additionally, the amplitude of diastolic BP and HR, as well as the range of oscillations in HR, were significantly increased compared with the standard light pattern. In contrast to SHR rats, the regulation of the circadian rhythms in Wistar–Kyoto rats was more flexible and presented more changes, which may be aimed at the adaptation of the body to environmental conditions. For Wistar–Kyoto rats, an increase in the level of excreted electrolytes was observed under the action of monochromatic light, but no similar changes were found in SHR rats. For Wistar–Kyoto rats, a significant decrease in the urine concentration of aMT6s in the daytime and nighttime periods is characteristic, which results in the loss of the circadian rhythm. In SHR rats, there was a significant decrease in the nighttime content of aMT6s in the urine, while the daytime concentration, on the contrary, increased. The obtained data demonstrate that prolonged exposure to monochromatic blue light in the daytime period affects the circadian structure of the rhythms of the cardiovascular system, the rhythm of electrolyte excretion and the production of epiphyseal melatonin in wild-type and hypertensive animals. In SHR rats, the rhythms of BP and HR exhibit a more rigid pattern.

Influence of daytime blue-enriched bright light on heart rate variability in healthy subjects

Heart rate variability (HRV), the indicator of the autonomic nervous system-induced modulation of heart rate, is a focal topic in psychophysiological research. The effect of indoor light on HRV may be related to various psychophysiological functions. The current study (N = 20) examined the response of the autonomic nervous system (ANS) to bright vs. dim blue-enriched light (1200 lx or 200 lx at eye level, 6500 K) exposure for five hours in the afternoon among healthy young adults. The results revealed a significant main effect of light condition on the time-domain indicators, with the significantly higher HRV (SDNN and RMSSD) under 200 lx versus 1200 lx condition, and the same case was revealed for the standard deviations of the Poincaré plot in non-linear effects. Conversely, no significant effects were revealed for the frequency- domain indicators of HRV measured with the subjects' eyes open. These findings suggested that the autonomic nervous system modulation of HRV was stronger under bright light conditions.

Time Restricted Feeding to the Light Cycle Dissociates Canonical Circadian Clocks and Physiological Rhythms in Heart Rate

Circadian rhythms are approximate 24-h biological cycles that optimize molecular and physiological functions to predictable daily environmental changes in order to maintain internal and organismal homeostasis. Environmental stimuli (light, feeding, activity) capable of altering the phase of molecular rhythms are important tools employed by circadian biologists to increase understanding of the synchronization of circadian rhythms to the environment and to each other within multicellular systems. The central circadian clock, located in the suprachiasmatic nucleus (SCN) of the hypothalamus is largely responsive to light and is thought to entrain the phase of peripheral clocks via neurohumoral signals. Mice are nocturnal and consume most of their food during the dark cycle. Early studies demonstrated that altered metabolic cues in the form of time restricted feeding, specifically, feeding mice during the light cycle, resulted in an uncoupling of molecular clocks in peripheral tissues with those from the SCN. These studies showed as much as a 12-h shift in gene expression in some peripheral tissues but not others. The shifts occurred without corresponding changes in the central clock in the brain. More recent studies have demonstrated that changes in cardiac physiology (heart rate, MAP) in response to time of food intake occur independent of the cardiac molecular clock. Understanding differences in the physiology/function and gene expression in other organs both independently and in relation to the heart in response to altered feeding will be important in dissecting the roles of the various clocks throughout the body, as well as, understanding their links to cardiovascular pathology.

Chrono-communication and cardiometabolic health: The intrinsic relationship and therapeutic nutritional promises

Circadian rhythm, an innate 24-h biological clock, regulates several mammalian physiological activities anticipating daily environmental variations and optimizing available energetic resources. The circadian machinery is a complex neuronal and endocrinological network primarily organized into a central clock, suprachiasmatic nucleus (SCN), and peripheral clocks. Several small molecules generate daily circadian fluctuations ensuring inter-organ communication and coordination between external stimuli, i.e., light, food, and exercise, and body metabolism. As an orchestra, this complex network can be out of tone. Circadian disruption is often associated with obesity development and, above all, with diabetes and cardiovascular disease onset. Moreover, accumulating data highlight a bidirectional relationship between circadian misalignment and cardiometabolic disease severity. Food intake abnormalities, especially timing and composition of meal, are crucial cause of circadian disruption, but evidence from preclinical and clinical studies has shown that food could represent a unique therapeutic approach to promote circadian resynchronization. In this review, we briefly summarize the structure of circadian system and discuss the role playing by different molecules [from leptin to ghrelin, incretins, fibroblast growth factor 21 (FGF-21), growth differentiation factor 15 (GDF15)] to guarantee circadian homeostasis. Based on the recent data, we discuss the innovative nutritional interventions aimed at circadian re-synchronization and, consequently, improvement of cardiometabolic health.

Understanding Circadian Mechanisms of Sudden Cardiac Death: A Report From the National Heart, Lung, and Blood Institute Workshop, Part 1: Basic and Translational Aspects

Sudden cardiac death (SCD), the unexpected death due to acquired or genetic cardiovascular disease, follows distinct 24-hour patterns in occurrence. These 24-hour patterns likely reflect daily changes in arrhythmogenic triggers and the myocardial substrate caused by day/night rhythms in behavior, the environment, and endogenous circadian mechanisms. To better address fundamental questions regarding the circadian mechanisms, the National Heart, Lung, and Blood Institute convened a workshop, Understanding Circadian Mechanisms of Sudden Cardiac Death. We present a 2-part report of findings from this workshop. Part 1 summarizes the workshop and serves to identify research gaps and opportunities in the areas of basic and translational research. Among the gaps was the lack of standardization in animal studies for reporting environmental conditions (eg, timing of experiments relative to the light dark cycle or animal housing temperatures) that can impair rigor and reproducibility. Workshop participants also pointed to uncertainty regarding the importance of maintaining normal circadian rhythmic synchrony and the potential pathological impact of desynchrony on SCD risk. One related question raised was whether circadian mechanisms can be targeted to reduce SCD risk. Finally, the experts underscored the need for studies aimed at determining the physiological importance of circadian clocks in the many different cell types important to normal heart function and SCD. Addressing these gaps could lead to new therapeutic approaches/molecular targets that can mitigate the risk of SCD not only at certain times but over the entire 24-hour period.

Lighting and Alzheimer's Disease and Related Dementias: Spotlight on Sleep and Depression

Alzheimer's disease and related dementias is the collective term for a progressive neurodegenerative disease for which there is presently no cure. This paper focuses on two symptoms of the disease, sleep disturbances and depression, and discusses how light can be used as a non-pharmacological intervention to mitigate their negative effects. Bright days and dark nights are needed for health and well-being, but the present components of the built environment, especially those places where older adults spend most of their days, are too dimly illuminated during the day and too bright at night. To be effective light needs to be correctly specified, implemented, and measured. Yet without the appropriate specification and measurement of the stimulus, researchers will not be able to successfully demonstrate positive results in the field, nor will lighting designers and specifiers have the confidence to implement lighting solutions for promoting better sleep and mood in this population.

Diurnal rhythms of urine volume and electrolyte excretion in healthy young men under differing intensities of daytime light exposure

In humans, most renal functions, including urine volume and electrolyte excretions, have a circadian rhythm. Light is a strong circadian entrainment factor and daytime-light exposure is known to affect the circadian rhythm of rectal temperature (RT). The effects of daytime-light exposure on the diurnal rhythm of urinary excretion have yet to be clarified. The aim of this study was to clarify whether and how daytime exposure to bright-light affects urinary excretions. Twenty-one healthy men (21–27 years old) participated in a 4-day study involving daytime (08:00–18:00 h) exposure to two light conditions, Dim (< 50 lx) and Bright (~ 2500 lx), in a random order. During the experiment, RT was measured continuously. Urine samples were collected every 3 ~ 4 h. Compared to the Dim condition, under the Bright condition, the RT nadir time was 45 min earlier (p = 0.017) and sodium (Na), chloride (Cl), and uric acid (UA) excretion and urine volumes were greater (all p < 0.001), from 11:00 h to 13:00 h without a difference in total daily urine volume. The present results suggest that daytime bright light exposure can induce a phase shift advance in urine volume and urinary Na, Cl, and UA excretion rhythms.

Clocks, Rhythms, Sex, and Hearts: How Disrupted Circadian Rhythms, Time-of-Day, and Sex Influence Cardiovascular Health

Cardiovascular diseases are the top cause of mortality in the United States, and ischemic heart disease accounts for 16% of all deaths around the world. Modifiable risk factors such as diet and exercise have often been primary targets in addressing these conditions. However, mounting evidence suggests that environmental factors that disrupt physiological rhythms might contribute to the development of these diseases, as well as contribute to increasing other risk factors that are typically associated with cardiovascular disease. Exposure to light at night, transmeridian travel, and social jetlag disrupt endogenous circadian rhythms, which, in turn, alter carefully orchestrated bodily functioning, and elevate the risk of disease and injury. Research into how disrupted circadian rhythms affect physiology and behavior has begun to reveal the intricacies of how seemingly innocuous environmental and social factors have dramatic consequences on mammalian physiology and behavior. Despite the new focus on the importance of circadian rhythms, and how disrupted circadian rhythms contribute to cardiovascular diseases, many questions in this field remain unanswered. Further, neither time-of-day nor sex as a biological variable have been consistently and thoroughly taken into account in previous studies of circadian rhythm disruption and cardiovascular disease. In this review, we will first discuss biological rhythms and the master temporal regulator that controls these rhythms, focusing on the cardiovascular system, its rhythms, and the pathology associated with its disruption, while emphasizing the importance of the time-of-day as a variable that directly affects outcomes in controlled studies, and how temporal data will inform clinical practice and influence personalized medicine. Finally, we will discuss evidence supporting the existence of sex differences in cardiovascular function and outcomes following an injury, and highlight the need for consistent inclusion of both sexes in studies that aim to understand cardiovascular function and improve cardiovascular health.

Increase in cortisol concentration due to standardized bright and blue light exposure on saliva cortisol in the morning following sleep laboratory

Research studies on LED light exposure and cortisol are inconsistent and not comparable due to different types of light, exposure times, and sample sizes. Therefore, one hour of standardized exposure LED light at different intensities and the spectral composition during the post-awakening phase at 7:30 were compared. A sample of 23 (Study 1) and 26 (Study 2) healthy males were randomly assigned to: 1) bright white light (414 lux) and 2) dim darkened light (<2 lux) as well as 3) red light (235 lux) and 4) blue light (201 lux) exposure conditions. Results from repeated measures ANOVA confirm that light exposure affects the cortisol concentration. Study 1 revealed an increase in the saliva cortisol concentration after bright light exposure compared to dim light. An increase in the cortisol concentration of blue light compared to red light (Study 2) and dim light was found. This study shows that bright light and blue light affect the cortisol response in contrast to dim light and red light conditions. The HPA axis showed a stimulatory effect by bright versus dim light and different wavelengths of light exposure.Lay summaryThe effects of LED light exposure on the stress hormone cortisol were investigated. The light exposure took place during the hours people would start working at the office. The results showed that after one hour of exposure to bright light or blue light the stress hormones increase in contrast to dim light and red light conditions. Thus, stress hormones can be altered by the types of light people are exposed to.

Effect of artificial dawn light on cardiovascular function, alertness, and balance in middle-aged and older adults

STUDY OBJECTIVES

When arising in the morning, many older people experience dizziness and difficulty maintaining proper balance, as the cardiovascular system is not able to compensate to the postural shift (standing) and maintain sufficient blood flow to the brain. Such changes in cardiovascular function are observed in young individuals exposed to a dawn simulation light. In this study, we examined whether exposure to a dawn simulation light could impact cardiovascular function and consequent changes in balance in middle-aged and older adults.

METHODS

Twenty-three participants (67.3 ± 8.8 y), 12 of whom reported a history of dizziness in the morning, underwent two overnight stays in our laboratory. During both nights, they slept in complete darkness, except for the last 30 minutes of one of the nights during which a dawn simulation light was used. Continuous blood pressure (BP) and heart rate (HR) were monitored. Subjective and objective alertness, salivary cortisol, and mobile and standing balance were examined upon arising.

RESULTS

Dawn simulation light decreased (33%) the amount of sleep before morning awakening, lowered BP (6.24 mmHg), and increased HR (0.93 bpm). Despite these changes in physiology, there was no significant impact of dawn simulation on subjective or objective alertness, measures of standing or ambulatory balance, morning cortisol awakening response, or cardiovascular function after awakening.

CONCLUSION

While the dawn simulation did cause an increase in wake and a change in cardiovascular function prior to morning arousal in older adults, we could find no evidence of a functional change in either cardiovascular function or balance upon standing.

CLINICAL TRIAL

Registered on Clinicaltrials.gov, #NCT02632318, https://clinicaltrials.gov/ct2/show/NCT02632318.

Metabolic Implications of Exposure to Light at Night: Lessons from Animal and Human Studies

Lately, the incidence of overweight, obesity, and type 2 diabetes has shown a staggering increase. To prevent and treat these conditions, one must look at their etiology. As life on earth has evolved under the conditions of nature's 24-hour light/dark cycle, it seems likely that exposure to artificial light at night (LAN) would affect physiology. Indeed, ample evidence has shown that LAN impacts many metabolic parameters, at least partly via the biological clock in the suprachiasmatic nucleus of the hypothalamus. This review focuses on the impact of chronic and acute effects of LAN of different wavelengths on locomotor activity, food intake, the sleep/wake cycle, body temperature, melatonin, glucocorticoids, and glucose and lipid metabolism. While chronic LAN disturbs daily rhythms in these parameters, experiments using short-term LAN exposure also have shown acute negative effects in metabolically active peripheral tissues. Experiments using LAN of different wavelengths not only have indicated an important role for melanopsin, the photopigment found in intrinsically photosensitive retinal ganglion cells, but also provided evidence that each wavelength may have a specific impact on energy metabolism. Importantly, exposure to LAN has been shown to impact glucose homeostasis also in humans and to be associated with an increased incidence of overweight, obesity, and atherosclerosis.

Characterizing the temporal Dynamics of Melatonin and Cortisol Changes in Response to Nocturnal Light Exposure

We studied the dynamics of melatonin suppression and changes in cortisol levels in humans in response to light exposure at night using high-frequency blood sampling. Twenty-one young healthy participants were randomized to receive either intermittent bright (~9,500 lux) light (IBL), continuous bright light (CBL) or continuous dim (~1 lux) light (VDL) for 6.5 h during the biological night (n = 7 per condition). Melatonin suppression occurred rapidly within the first 5 min and continued until the end of each IBL stimuli (t1/2 = ~13 min). Melatonin recovery occurred more slowly between IBL stimuli (half-maximal recovery rate of ~46 min). Mean melatonin suppression (~40%) and recovery (~50%) were similar across IBL stimuli. Suppression dynamics under CBL were also rapid (t1/2 = ~18 min), with no recovery until the light exposure ended. There was a significant linear increase of cortisol levels between the start and end of each IBL stimulus. Under CBL conditions cortisol showed trimodal changes with an initial linear activating phase, followed by an exponential inhibitory phase, and a final exponential recovery phase. These results show that light exposure at night affects circadian driven hormones differently and that outcomes are influenced by the duration and pattern of light exposure.

Indoor light pollution and progression of carotid atherosclerosis: A longitudinal study of the HEIJO-KYO cohort

Exposure to light at inappropriate times in relation to the solar cycle can disturb circadian endocrine and metabolic rhythms. Previous studies have suggested an association between light exposure at night (LAN) and obesity, an important risk factor of atherosclerosis, although it remains unclear whether LAN associates with progression of atherosclerosis. To evaluate the longitudinal association between bedroom LAN intensity and progression of carotid atherosclerosis, light intensity in the bedroom at baseline and carotid artery intima-media thickness (IMT) at baseline and follow-up were measured in 989 elderly participants (945 at baseline and 780 at a median follow-up time of 34 months). The mean age of the participants was 71.4 ± 6.9 years. The average mean and maximal carotid IMT at baseline were 0.88 ± 0.15 and 1.10 ± 0.32 mm, respectively. The median intensity of bedroom LAN was 0.68 lx (IQR, 0.07-3.29). In multivariable analysis adjusted for potential confounders, the highest LAN group exhibited a significant increase in mean carotid IMT (adjusted β, 0.028; 95% CI, 0.005-0.052; P = 0.019) compared with the lowest LAN quartile group. A similar relationship was found between LAN and maximal carotid IMT (adjusted β, 0.083; 95% CI, 0.037-0.129; P < 0.001). In conclusion, we found a clear and significant association between bedroom LAN intensity and progression of subclinical carotid atherosclerosis, which was independent of known risk factors for atherosclerosis, including age, obesity, smoking, economic status, hypertension, and diabetes. Indoor light pollution may be a potential risk factor for atherosclerosis in the general population.

Fluorescent Light Incites a Conserved Immune and Inflammatory Genetic Response within Vertebrate Organs (Danio Rerio, Oryzias Latipes and Mus Musculus)

Fluorescent light (FL) has been utilized for ≈60 years and has become a common artificial light source under which animals, including humans, spend increasing amounts of time. Although the solar spectrum is quite dissimilar in both wavelengths and intensities, the genetic consequences of FL exposure have not been investigated. Herein, we present comparative RNA-Seq results that establish expression patterns within skin, brain, and liver for Danio rerio, Oryzias latipes, and the hairless mouse (Mus musculus) after exposure to FL. These animals represent diurnal and nocturnal lifestyles, and ≈450 million years of evolutionary divergence. In all three organisms, FL induced transcriptional changes of the acute phase response signaling pathway and modulated inflammation and innate immune responses. Our pathway and gene clustering analyses suggest cellular perception of oxidative stress is promoting induction of primary up-stream regulators IL1B and TNF. The skin and brain of the three animals as well as the liver of both fish models all exhibit increased inflammation and immune responses; however, the mouse liver suppressed the same pathways. Overall, the conserved nature of the genetic responses observed after FL exposure, among fishes and a mammal, suggest the presence of light responsive genetic circuitry deeply embedded in the vertebrate genome.

Dynamics of Non-visual Responses in Humans: As Fast as Lightning?

The eye drives non-visual (NV) responses to light, including circadian resetting, pupillary reflex and alerting effects. Initially thought to depend on melanopsin-expressing retinal ganglion cells (ipRGCs), classical photopigments play a modulatory role in some of these responses. As most studies have investigated only a limited number of NV functions, generally under conditions of relatively high light levels and long duration of exposure, whether NV functions share similar irradiance sensitivities and response dynamics during light exposure is unknown. We addressed this issue using light exposure paradigms spectrally and spatially tuned to target mainly cones or ipRGCs, and by measuring longitudinally (50 min) several NV responses in 28 men. We demonstrate that the response dynamics of NV functions are faster than previously thought. We find that the brain, the heart, and thermoregulation are activated within 1 to 5 min of light exposure. Further, we show that NV functions do not share the same response sensitivities. While the half-maximum response is only ∼48 s for the tonic pupil diameter, it is ∼12 min for EEG gamma activity. Most NV responses seem to be saturated by low light levels, as low as 90 melanopic lux. Our results also reveal that it is possible to maintain optimal visual performance while modulating NV responses. Our findings have real-life implications. On one hand, light therapy paradigms should be re-evaluated with lower intensities and shorter durations, with the potential of improving patients' compliance. On the other hand, the significant impact of low intensity and short duration light exposures on NV physiology should make us reconsider the potential health consequences of light exposure before bedtime, in particular on sleep and circadian physiology.

Functional Organization of the Sympathetic Pathways Controlling the Pupil: Light-Inhibited and Light-Stimulated Pathways

Pupil dilation is mediated by a sympathetic output acting in opposition to parasympathetically mediated pupil constriction. While light stimulates the parasympathetic output, giving rise to the light reflex, it can both inhibit and stimulate the sympathetic output. Light-inhibited sympathetic pathways originate in retina-receptive neurones of the pretectum and the suprachiasmatic nucleus (SCN): by attenuating sympathetic activity, they allow unimpeded operation of the light reflex. Light stimulates the noradrenergic and serotonergic pathways. The hub of the noradrenergic pathway is the locus coeruleus (LC) containing both excitatory sympathetic premotor neurones (SympPN) projecting to preganglionic neurones in the spinal cord, and inhibitory parasympathetic premotor neurones (ParaPN) projecting to preganglionic neurones in the Edinger-Westphal nucleus (EWN). SympPN receive inputs from the SCN via the dorsomedial hypothalamus, orexinergic neurones of the latero-posterior hypothalamus, wake- and sleep-promoting neurones of the hypothalamus and brain stem, nociceptive collaterals of the spinothalamic tract, whereas ParaPN receive inputs from the amygdala, sleep/arousal network, nociceptive spinothalamic collaterals. The activity of LC neurones is regulated by inhibitory α₂-adrenoceptors. There is a species difference in the function of the preautonomic LC. In diurnal animals, the α₂-adrenoceptor agonist clonidine stimulates mainly autoreceptors on SymPN, causing miosis, whereas in nocturnal animals it stimulates postsynaptic α₂-arenoceptors in the EWN, causing mydriasis. Noxious stimulation activates SympPN in diurnal animals and ParaPN in nocturnal animals, leading to pupil dilation via sympathoexcitation and parasympathetic inhibition, respectively. These differences may be attributed to increased activity of excitatory LC neurones due to stimulation by light in diurnal animals. This may also underlie the wake-promoting effect of light in diurnal animals, in contrast to its sleep-promoting effect in nocturnal species. The hub of the serotonergic pathway is the dorsal raphe nucleus that is light-sensitive, both directly and indirectly (via an orexinergic input). The light-stimulated pathways mediate a latent mydriatic effect of light on the pupil that can be unmasked by drugs that either inhibit or stimulate SympPN in these pathways. The noradrenergic pathway has widespread connections to neural networks controlling a variety of functions, such as sleep/arousal, pain, and fear/anxiety. Many physiological and psychological variables modulate pupil function via this pathway.

The transcriptional response of skin to fluorescent light exposure in viviparous (Xiphophorus) and oviparous (Danio, Oryzias) fishes

Differences in light sources are common in animal facilities and potentially can impact experimental results. Here, the potential impact of lighting differences on skin transcriptomes has been tested in three aquatic animal models commonly utilized in biomedical research, (Xiphophorus maculatus (platyfish), Oryzias latipes (medaka) and Danio rerio (zebrafish). Analysis of replicate comparative RNA-Seq data showed the transcriptional response to commonly utilized 4100K or "cool white" fluorescent light (FL) is much greater in platyfish and medaka than in zebrafish. FL induces genes associated with inflammatory and immune responses in both medaka and zebrafish; however, the platyfish exhibit suppression of genes involved with immune/inflammation, as well as genes associated with cell cycle progression. Furthermore, comparative analyses of gene expression data from platyfish UVB exposures, with medaka and zebrafish after exposure to 4100K FL, show comparable effects on the same stress pathways. We suggest the response to light is conserved, but that long-term adaptation to species specific environmental niches has resulted in a shifting of the wavelengths required to incite similar "genetic" responses in skin. We forward the hypothesis that the "genetic perception" of light may have evolved differently than ocular perception and suggest that light type (i.e., wavelengths emitted) is an important parameter to consider in experimental design.

Temporal partitioning of cardiac metabolism by the cardiomyocyte circadian clock

NEW FINDINGS

What is the topic of this review? This review highlights temporal partitioning of cardiac metabolism by the cardiomyocyte circadian clock. What advances does it highlight? Advances include: 1) cardiac glucose utilization peaks during the active period to meet increased energetic demands at this time; 2) synthesis of glycogen and triglyceride peak in the heart during the latter half of the active period, likely in anticipation of the upcoming sleep/fasting period; and 3) protein turnover increases in the heart at the beginning of the sleep phase, probably to promote growth and repair at this time. Cell-autonomous circadian clocks have emerged as crucial mediators of 24 h rhythms in cellular processes. In doing so, these molecular timekeepers confer the selective advantage of anticipation, allowing cells and organs to prepare for stimuli and stresses before their onset. The heart is subjected to dramatic fluctuations in energetic demand and nutrient supply in association with sleep-wake and fasting-feeding cycles. Recent studies suggest that the cardiomyocyte circadian clock orchestrates daily rhythms in both oxidative and non-oxidative glucose and fatty acid metabolism, as well as protein turnover. Here, I review this evidence and discuss whether disruption of these rhythms can contribute to cardiovascular disease.

In a Heartbeat: Light and Cardiovascular Physiology

Light impinging on the retina fulfils a dual function: it serves for vision and it is required for proper entrainment of the endogenous circadian timing system to the 24-h day, thus influencing behaviors that promote health and optimal quality of life but are independent of image formation. The circadian pacemaker located in the suprachiasmatic nuclei modulates the cardiovascular system with an intrinsic ability to anticipate morning solar time and with a circadian nature of adverse cardiovascular events. Here, we infer that light exposure might affect cardiovascular function and provide evidence from existing research. Findings show a time-of-day dependent increase in relative sympathetic tone associated with bright light in the morning but not in the evening hours. Furthermore, dynamic light in the early morning hours can reduce the deleterious sleep-to-wake evoked transition on cardiac modulation. On the contrary, effects of numerous light parameters, such as illuminance level and wavelength of monochromatic light, on cardiac function are mixed. Therefore, in future research studies, light modalities, such as timing, duration, and its wavelength composition, should be taken in to account when testing the potential of light as a non-invasive countermeasure for adverse cardiovascular events.

Seasonal and time-of-day variations in acute non-image forming effects of illuminance level on performance, physiology, and subjective well-being

This study investigated seasonal and time-of-day dependent moderations in the strength and direction of acute diurnal non-image forming (NIF) effects of illuminance level on performance, physiology, and subjective well-being. Even though there are indications for temporal variations in NIF-responsiveness to bright light, scientific insights into potential moderations by season are scarce. We employed a 2 (Light: 165 versus 1700 lx at the eye level, within) × 2 (Season: autumn/winter versus spring, between) × 2 (Time of day: morning versus afternoon, between) mixed-model design. During each of the two 90-min experimental sessions, participants (autumn/winter: N = 34; spring: N = 39) completed four measurement blocks (incl. one baseline block of 120 lx at the eye level) each consisting of a Psychomotor Vigilance Task (PVT) and a Backwards Digit-Span Task (BDST) including easy trials (4-6 digits) and difficult trials (7-8 digits). Heart rate (HR) and skin conductance level (SCL) were measured continuously. At the end of each lighting condition, subjective sleepiness, vitality, and mood were measured. The results revealed a clear indication for significant Light * Season interaction effects on both subjective sleepiness and vitality, which appeared only during the morning sessions. Participants felt significantly more vital and less sleepy in winter, but not in spring during bright light exposure in the morning. In line with these subjective parameters, participants also showed significantly better PVT performance in the morning in autumn/winter, but not in spring upon bright light exposure. Surprisingly, for difficult working memory performance, the opposite was found, namely worse performance during bright light exposure in winter, but better performance when exposed to bright light in spring. The effects of bright versus regular light exposure on physiology were quite subtle and largely nonsignificant. Overall, it can be concluded that acute illuminance-induced NIF effects on subjective alertness and vitality as well as objectively measured vigilance in the morning are significantly moderated by season. Possibly, these greater illuminance-induced benefits during the morning sessions in autumn/winter compared to spring occurred due to increased responsiveness to bright light exposure as a function of a relatively low prior light dose in autumn/winter.

Association between depressive symptoms and morningness-eveningness, sleep duration and rotating shift work in Japanese nurses

Higher depressive symptoms have been reported in rotating shift workers compared with day workers. Depressive symptoms in adults who do not engage in night work have also been shown to be associated with chronotype and sleep duration. This study examines associations between depressive symptoms, morningness-eveningness (i.e. the degree to which people prefer to be active in the morning or the evening), sleep duration and rotating shift work. Japanese nurses (1252 day workers and 1780 rotating shift workers, aged 20-59) were studied using a self-administered questionnaire. The questionnaire covered depressive symptoms, morningness-eveningness, sleep habits and demographic characteristics of the participants. The Center for Epidemiologic Studies Depression Scale (CES-D) was used to determine the levels of depressive symptoms. A Japanese version of the Morningness-Eveningness Questionnaire (MEQ) was used to measure morningness-eveningness. The CES-D score of shift workers was significantly (p < 0.05) higher than that of day workers. The MEQ score was significantly (p < 0.05) lower (i.e. greater eveningness) in shift workers than in day workers. Sleep duration on the day shift was significantly (p < 0.05) shorter in shift workers than in day workers. Simple linear regression revealed that the MEQ score, sleep duration on the day shift and current work shift (i.e. rotating shift work) were significantly (p < 0.05) associated with the CES-D score. Multivariate linear regression indicated that greater eveningness and shorter sleep duration were independently associated with higher CES-D scores, while rotating shift work was not. These associations between the MEQ score, the sleep duration and the CES-D score were also confirmed in both day workers and shift workers when the groups were analyzed separately. These results suggest that greater eveningness and shorter sleep duration on the day shift were independently associated with higher levels of depressive symptoms, which may explain associations between rotating shift workers and depressive symptoms. These findings have important implications for the development of novel strategies for preventing poor mental health in day workers and rotating shift workers.

Circadian variation in acute myocardial infarct size assessed by cardiovascular magnetic resonance in reperfused STEMI patients

OBJECTIVE

Clinical studies using serum cardiac biomarkers to investigate a circadian variation in acute myocardial infarct (MI) size in ST-segment elevation myocardial infarction (STEMI) patients reperfused by primary percutaneous coronary intervention (PPCI) have produced mixed results. We aimed to investigate this phenomenon using acute MI size measured by cardiovascular magnetic resonance (CMR).

METHODS

Patient-level data was obtained from 4 randomized controlled trials investigating the MI-limiting effects of cardioprotective therapies in this pooled analysis. The primary analysis was performed in those patients with no pre-infarct angina; duration of ischemia >60min and <360min; Thrombolysis In Myocardial Infarction (TIMI) flow pre-PPCI ≤1; TIMI flow post-PPCI 3; and no collateral flow.

RESULTS

169 out of 376 patients with CMR data met the inclusion criteria for the primary analysis. A 24-hour circadian variation in acute MI size as a % of the area-at-risk (%AAR), after adjusting for confounders, was observed with a peak and nadir MI size in patients with symptom onset between 00:00 and 01:00 and between 12:00 and 13:00 respectively (difference from the average MI size 5.2%, 95%CI 1.1-9.4%; p=0.013). This was associated with a non-significant circadian variation in left ventricular ejection fraction (LVEF) (difference from the average LVEF 5.9%, 95%CI -0.6-2.2%, p=0.073). There was no circadian variation in MI size or LVEF in the whole cohort.

CONCLUSIONS

We report a circadian variation in acute MI size assessed by CMR in a subset of STEMI patients treated by PPCI, with the largest and smallest MI size occurring in patients with symptom onset between 00:00 and 01:00 and between 12:00 and 13:00 respectively.

Day-night dependence of gene expression and inflammatory responses in the remodeling murine heart post-myocardial infarction

Diurnal or circadian rhythms are fundamentally important for healthy cardiovascular physiology and play a role in timing of onset and tolerance to myocardial infarction (MI) in patients. Whether time of day of MI triggers different molecular and cellular responses that can influence myocardial remodeling is not known. This study was designed to test whether time of day of MI triggers different gene expression, humoral, and innate inflammatory responses that contribute to cardiac repair after MI. Mice were infarcted by left anterior descending coronary artery ligation (MI model) within a 2-h time window either shortly after lights on or lights off, and the early remodeling responses at 8 h postinfarction were examined. We found that sleep-MI preferentially triggers early expression of genes associated with inflammatory responses, whereas wake-MI triggers more genes associated with metabolic pathways and transcription/translation, by microarray analyses. Homozygous clock mutant mice exhibit altered diurnal gene expression profiles, consistent with their cycling before onset of MI. In the first 8 h, crucial for innate immune responses to MI, there are also significant differences in sleep-MI and wake-MI serum cytokine responses and in neutrophil infiltration to infarcted myocardium. By 1-wk post-MI, there are differences in survivorship between the sleep and wake MI mice that could be explained by the different molecular and cellular responses. Our whole body physiology, tissues, and cells exhibit endogenous daily rhythms, and understanding their role in triggering effective responses after MI could lead to new strategies to benefit patients with cardiovascular disease.

Acute effects of different light spectra on simulated night-shift work without circadian alignment

Short-wavelength and short-wavelength-enhanced light have a strong impact on night-time working performance, subjective feelings of alertness and circadian physiology. In the present study, we investigated acute effects of white light sources with varied reduced portions of short wavelengths on cognitive and visual performance, mood and cardiac output.Thirty-one healthy subjects were investigated in a balanced cross-over design under three light spectra in a simulated night-shift paradigm without circadian adaptation.Exposure to the light spectrum with the largest attenuation of short wavelengths reduced heart rate and increased vagal cardiac parameters during the night compared to the other two light spectra without deleterious effects on sustained attention, working memory and subjective alertness. In addition, colour discrimination capability was significantly decreased under this light source.To our knowledge, the present study for the first time demonstrates that polychromatic white light with reduced short wavelengths, fulfilling current lighting standards for indoor illumination, may have a positive impact on cardiac physiology of night-shift workers without detrimental consequences for cognitive performance and alertness.

Nutrition in the spotlight: metabolic effects of environmental light

Use of artificial light resulted in relative independence from the natural light-dark (LD) cycle, allowing human subjects to shift the timing of food intake and work to convenient times. However, the increase in artificial light exposure parallels the increase in obesity prevalence. Light is the dominant Zeitgeber for the central circadian clock, which resides within the hypothalamic suprachiasmatic nucleus, and coordinates daily rhythm in feeding behaviour and metabolism. Eating during inappropriate light conditions may result in metabolic disease via changes in the biological clock. In this review, we describe the physiological role of light in the circadian timing system and explore the interaction between the circadian timing system and metabolism. Furthermore, we discuss the acute and chronic effects of artificial light exposure on food intake and energy metabolism in animals and human subjects. We propose that living in synchrony with the natural daily LD cycle promotes metabolic health and increased exposure to artificial light at inappropriate times of day has adverse effects on metabolism, feeding behaviour and body weight regulation. Reducing the negative side effects of the extensive use of artificial light in human subjects might be useful in the prevention of metabolic disease.

Optimizing indoor illumination quality and energy efficiency using a spectrally tunable lighting system to augment natural daylight

This article demonstrates the benefits of complementing a daylight-lit environment with a spectrally tunable illumination system. The spectral components of daylight present in the room are measured by a low-cost miniature spectrophotometer and processed through a number of optimization algorithms, carefully trading color fidelity for energy efficiency. Spectrally-tunable luminaires provide only those wavelengths that ensure that either the final illumination spectrum inside the room is kept constant or carefully follows the dynamic spectral pattern of natural daylight. Analyzing the measured data proves that such a hybrid illumination system brings both unprecendented illumination quality and significant energy savings.

Circadian rhythmicity of synapses in mouse somatosensory cortex

The circadian rhythmicity displayed by motor behavior of mice: activity at night and rest during the day; and the associated changes in the sensory input are reflected by cyclic synaptic plasticity in the whisker representations located in the somatosensory (barrel) cortex. It was not clear whether diurnal rhythmic changes in synapse density previously observed in the barrel cortex resulted from changes in the activity of the animals, from daily light/dark (LD) rhythm or are driven by an endogenous clock. These changes were investigated in the barrel cortex of C57BL/6 mouse strain kept under LD 12 : 12 h conditions and in constant darkness (DD). Stereological analysis of serial electron microscopic sections was used to assess numerical density of synapses. In mice kept under LD conditions, the total density of synapses and the density of excitatory synapses located on dendritic spines was higher during the light period (rest phase). In contrast, the density of inhibitory synapses located on dendritic spines increased during the dark period (activity phase). Under DD conditions, the upregulation of the inhibitory synapses during the activity phase was retained, but the cyclic changes in the density of excitatory synapses were not observed. The results show that the circadian plasticity concerns only synapses located on spines (and not those on dendritic shafts), and that excitatory and inhibitory synapses are differently regulated during the 24 h cycle: the excitatory synapses are influenced by light, whilst the inhibitory synapses are driven by the endogenous circadian clock.

Influence of the Cardiomyocyte Circadian Clock on Cardiac Physiology and Pathophysiology

Cardiac function and dysfunction exhibit striking time-of-day-dependent oscillations. Disturbances in both daily rhythms and sleep are associated with increased risk of heart disease, adverse cardiovascular events, and worsening outcomes. For example, the importance of maintaining normal daily rhythms is highlighted by epidemiologic observations that night shift workers present with increased incidence of cardiovascular disease. Rhythmicity in cardiac processes is mediated by a complex interaction between extracardiac (e.g., behaviors and associated neural and humoral fluctuations) and intracardiac influences. Over the course of the day, the intrinsic properties of the myocardium vary at the levels of gene and protein expression, metabolism, responsiveness to extracellular stimuli/stresses, and ion homeostasis, all of which affect contractility (e.g., heart rate and force generation). Over the past decade, the circadian clock within the cardiomyocyte has emerged as an essential mechanism responsible for modulating the intrinsic properties of the heart. Moreover, the critical role of this mechanism is underscored by reports that disruption, through genetic manipulation, results in development of cardiac disease and premature mortality in mice. These findings, in combination with reports that numerous cardiovascular risk factors (e.g., diet, diabetes, aging) distinctly affect the clock in the heart, have led to the hypothesis that aberrant regulation of this mechanism contributes to the etiology of cardiac dysfunction and disease. Here, we provide a comprehensive review on current knowledge regarding known roles of the heart clock and discuss the potential for using these insights for the future development of innovative strategies for the treatment of cardiovascular disease.

Plastic oscillators and fixed rhythms: changes in the phase of clock-gene rhythms in the PVN are not reflected in the phase of the melatonin rhythm of grass rats

The same clock-genes, including Period (PER) 1 and 2, that show rhythmic expression in the suprachiasmatic nucleus (SCN) are also rhythmically expressed in other brain regions that serve as extra-SCN oscillators. Outside the hypothalamus, the phase of these extra-SCN oscillators appears to be reversed when diurnal and nocturnal mammals are compared. Based on mRNA data, PER1 protein is expected to peak in the late night in the paraventricular nucleus of the hypothalamus (PVN) of nocturnal laboratory rats, but comparable data are not available for a diurnal species. Here we use the diurnal grass rat (Arvicanthis niloticus) to describe rhythms of PER1 and 2 proteins in the PVN of animals that either show the species-typical day-active (DA) profile, or that adopt a night-active (NA) profile when given access to running wheels. For DA animals housed with or without wheels, significant rhythms of PER1 or PER2 protein expression featured peaks in the late morning; NA animals showed patterns similar to those expected from nocturnal laboratory rats. Since the PVN is part of the circuit that controls pineal rhythms, we also measured circulating levels of melatonin during the day and night in DA animals with and without wheels and in NA wheel runners. All three groups showed elevated levels of melatonin at night, with higher levels during both the day and night being associated with the levels of activity displayed by each group. The differential phase of rhythms in the clock-gene protein in the PVN of diurnal and nocturnal animals presents a possible mechanism for explaining species differences in the phase of autonomic rhythms controlled, in part, by the PVN. The present study suggests that the phase of the oscillator of the PVN does not determine that of the melatonin rhythm in diurnal and nocturnal species or in diurnal and nocturnal chronotypes within a species.