Twenty-four-hour rhythms of plasma glucose and insulin secretion rate in regular night workers

The effect of altered sleep timing on glycaemic outcomes: Systematic review of human intervention studies

AIMS

Alterations in sleep timing can lead to disturbances in glycaemic control, although the evidence is inconsistent. Therefore, this systematic review summarizes results from human intervention studies of altered sleep timing on glycaemic outcomes.

MATERIALS AND METHODS

As part of a broader search on the effect of altering timing of sleep, physical activity and dietary intake, Medline and Embase were searched from inception to February 2023, and subsequent reference searches were done. With the help of a machine learning-aided program 'ASReview', we selected any type of intervention study in the general adult population, which acutely delayed sleep by ≥2 h for at least one night, while the total time in bed was the same between early and late sleep. Quality assessment was done using the quality assessment tool for quantitative studies.

RESULTS

In total, 14 studies (159 adults with normal or increased weight) were identified. Methodological quality was high (n = 4), moderate (n = 7) or low (n = 3). Acute delays of sleep onset showed unfavourable effects in 10 out of 27 measured glycaemic outcomes (one-six studies reported on each outcome) with outcomes mostly measured in the postprandial period, compared to (early) nighttime sleep.

CONCLUSIONS

Acutely delaying sleep timing might have unfavourable effects on glycaemic outcomes, compared to (early) nighttime sleep. Future research does however need better controlled trials, also measuring and controlling sleep quantity, sleep quality, physical activity and dietary intake, with longer follow-up periods, consistent outcomes and designs and more diverse populations to provide targeted advice regarding the optimal timing for sleep.

PROTOCOL REGISTRATION

This review is part of a larger search 'The effect of altering timing of physical activity, sleep and energy intake on glycaemia and Type 2 Diabetes risk in humans', of which the protocol was registered in the PROSPERO database on 27 November 2021 under number: CRD42021287828.

Cell-specific resetting of mouse islet cellular clocks by glucagon, glucagon-like peptide 1 and somatostatin

AIM

Molecular clocks, operative in pancreatic islet cells, represent an intrinsic mechanism regulating intracellular metabolism and hormone secretion. Glucagon, somatostatin and glucagon-like peptide 1 (GLP-1) are essential coordinators of islet physiology. Here, we assess the synchronizing capacity of glucagon, somatostatin and GLP-1 on pancreatic α- and β-cell circadian clocks.

METHODS

Triple transgenic mice, expressing a circadian PER2::luciferase (luc) reporter combined with α- and β-cell-specific fluorescent reporters, were employed. Isolated pancreatic islets and fluorescence-activated cell sorting-separated α- and β-cells were synchronized with glucagon, somatostatin analogue or GLP-1 mimetics, with subsequent real-time PER2::luc bioluminescence recording. Gene expression of Gcgr, Sstr2, Sstr3 and Glp1r in islet cells was assessed by RNA sequencing and RT-qPCR.

RESULTS

Glucagon and GLP-1 mimetics (liraglutide and exenatide) induced high-amplitude rhythmic expression of the PER2::luc reporter in β-cells, but not in α-cells, while the somatostatin analogue octreotide generated a significant phase shift between α- and β-cells. Enrichment of Gcgr and Glp1r transcripts was detected in β-cells compared to their α-cell counterparts. The synchronizing effect of glucagon was dose-dependent and mediated by the adenylate cyclase signalling cascade, as it was diminished by adenylate cyclase inhibitor.

CONCLUSION

We conclude that proglucagon-derived peptides and somatostatin exhibit receptor-mediated cell-specific synchronizing effects for mouse α- and β-cell oscillators. Differential islet cell clock modulation by glucagon and somatostatin may represent a physiological mechanism underlying paracrine regulation of rhythmic glucagon and insulin secretion. The reported here strong synchronizing properties of GLP-1 mimetics, widely used for treatment of type 2 diabetes, are of high clinical relevance.

Low glucose availability stimulates progesterone production by mouse ovaries in vitro

Steroid production by the ovary is primarily stimulated by gonadotropins but can also be affected by biological cues that provide information about energy status and environmental stress. To further understand which metabolic cues the ovary can respond to, we exposed gonadotropin-stimulated mouse ovaries in vitro to glucose metabolism inhibitors and measured steroid accumulation in media. Gonadotropin-stimulated ovaries exposed to 2-deoxy-d-glucose increased progesterone production and steroidogenic acute regulatory protein mRNA levels. However, oocytes and granulosa cells in antral follicles do not independently mediate this response because targeted treatment of these cell types with a different inhibitor of glucose metabolism (bromopyruvic acid) did not affect progesterone production. Elevated progesterone production is consistent with the homeostatic role of progesterone in glucose regulation in mammals. It also may regulate follicle growth and/or atresia within the ovary. These results suggest that ovaries can regulate glucose homeostasis in addition to their primary role in reproductive activity.

Glucose metabolism during rotational shift-work in healthcare workers

AIMS/HYPOTHESIS

Shift-work is associated with circadian rhythm disruption and an increased risk of obesity and type 2 diabetes. We sought to determine the effect of rotational shift-work on glucose metabolism in humans.

METHODS

We studied 12 otherwise healthy nurses performing rotational shift-work using a randomised crossover study design. On each occasion, participants underwent an isotope-labelled mixed meal test during a simulated day shift and a simulated night shift, enabling simultaneous measurement of glucose flux and beta cell function using the oral minimal model. We sought to determine differences in fasting and postprandial glucose metabolism during the day shift vs the night shift.

RESULTS

Postprandial glycaemic excursion was higher during the night shift (381±33 vs 580±48 mmol/l per 5 h, p<0.01). The time to peak insulin and C-peptide and nadir glucagon suppression in response to meal ingestion was also delayed during the night shift. While insulin action did not differ between study days, the beta cell responsivity to glucose (59±5 vs 44±4 × 10^-9 min^-1; p<0.001) and disposition index were decreased during the night shift.

CONCLUSIONS/INTERPRETATION

Impaired beta cell function during the night shift may result from normal circadian variation, the effect of rotational shift-work or a combination of both. As a consequence, higher postprandial glucose concentrations are observed during the night shift.

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.

Epigenetic alterations in the suprachiasmatic nucleus and hippocampus contribute to age-related cognitive decline

Circadian rhythm dysfunction and cognitive decline, specifically memory loss, frequently accompany natural aging. Circadian rhythms and memory are intertwined, as circadian rhythms influence memory formation and recall in young and old rodents. Although, the precise relationship between circadian rhythms and memory is still largely unknown, it is hypothesized that circadian rhythm disruption, which occurs during aging, contributes to age-associated cognitive decline, specifically memory loss. While there are a variety of mechanisms that could mediate this effect, changes in the epigenome that occur during aging has been proposed as a potential candidate. Interestingly, epigenetic mechanisms, such as DNA methylation and sirtuin1 (SIRT1) are necessary for both circadian rhythms and memory. During aging, similar alterations of epigenetic mechanisms occur in the suprachiasmatic nucleus (SCN) and hippocampus, which are necessary for circadian rhythm generation and memory, respectively. Recently, circadian rhythms have been linked to epigenetic function in the hippocampus, as some of these epigenetic mechanisms oscillate in the hippocampus and are disrupted by clock gene deletion. The current paper will review how circadian rhythms and memory change with age, and will suggest how epigenetic changes in these processes might contribute to age-related cognitive decline.

Rotating shift work and menstrual characteristics in a cohort of Chinese nurses

Background

Shift work disrupts the circadian rhythm and may cause menstruation disorders. This study assessed the impact of shift work on menstrual cycle in a population of Chinese nurses.

Methods

Questionnaires on menstrual characteristics and shift schedules were sent to female nurses of the First Affiliated Hospital of Sun Yat-sen University (FAHSYSU) and Guanghua Hospital of Stomatology (GHHS), affiliated to Sun Yat-sen University. Part I was a cross-sectional study and included 139 nurses in GHHS who had regular 8:00–17:30 working (non-shift group), and 334 nurses from FAHSYSU who worked shifts, a response rate of 67.5 % and 59.6 %, respectively (age ≤ 50 years). Menstrual patterns were compared and age-adjusted relative risks of shift work were analyzed. Part II was a nested case–control study. Cases were nurses in Part I who had regular cycle with mean cycle length (MCL) of 25–31 days and but at least 3 days variation in MCL after starting shift work (n = 45). Controls consisted of 67 nurses with matching shift patterns and age, but no MCL changes. A control non-shift age-matched group consisted of 30 GHHS nurses with no MCL changes. A follow-up second questionnaire was sent 2 years later.

Results

In Part I, the shift group had a significantly higher proportion of nurses with menstrual cycle irregularity. The proportion of nurses with a cycle of 25–31 days decreased from 81.7 to 67.8 % after changing to shift work. Logistic regression analysis showed that night shift frequency was the only risk factor associated with cycle shortening. After adjusting for age, MCL was shorter when night work was performed > 7 times per month. In Part II, the mean change in MCL in the case group, including prolongation or shortening, was 4.115 ± 2.084 days after shift working. In the 2 years’ follow-up, the MCL of the study group did not recover to the original length.

Conclusions

Rotating shift work can increase the prevalence of menstrual cycle irregularity. Night shift frequency was the only risk factor associated with cycle reduced. Changes in MCL did not show recovery over a follow-up period of 2 years.

Electronic supplementary material

The online version of this article (doi:10.1186/s12905-016-0301-y) contains supplementary material, which is available to authorized users.

Dietary and lifestyle habits and the associated health risks in shift workers

Traditionally only a small proportion of the workforce was engaged in shift work. Changing economic pressures have resulted in increased engagement in shift work, with approximately 17 % of the workforce in Europe engaged in this work pattern. The present narrative review aimed to summarise the data on the effects of shift work on the diet, lifestyle and health of employees, while addressing the barriers to, and opportunities for, improving health among shift workers. Shift work can result in low-quality diet and irregular eating patterns. Adverse health behaviours are also reported; particularly increased smoking and poor sleep patterns. These altered lifestyle habits, in conjunction with disruption to circadian rhythms, can create an unfavourable metabolic phenotype which facilitates the development and progression of chronic disease. Although the data are inconclusive due to issues such as poor study design and inadequate control for confounding factors; shift workers appear to be at increased mental and physical health risk, particularly with regard to non-communicable diseases. Information is lacking on the obstacles to leading a healthier lifestyle while working shifts, and where opportunities lie for intervention and health promotion among this group. In order to provide an informed evidence base to assist shift workers in overcoming associated occupational hazards, this gap must be addressed. This review highlights the unique nutritional issues faced by shift workers, and the subsequent effect on health. In societies already burdened with increased incidence of non-communicable chronic diseases, there is a clear need for education and behaviour change interventions among this group.

A Unifying Organ Model of Pancreatic Insulin Secretion

The secretion of insulin by the pancreas has been the object of much attention over the past several decades. Insulin is known to be secreted by pancreatic β-cells in response to hyperglycemia: its blood concentrations however exhibit both high-frequency (period approx. 10 minutes) and low-frequency oscillations (period approx. 1.5 hours). Furthermore, characteristic insulin secretory response to challenge maneuvers have been described, such as frequency entrainment upon sinusoidal glycemic stimulation; substantial insulin peaks following minimal glucose administration; progressively strengthened insulin secretion response after repeated administration of the same amount of glucose; insulin and glucose characteristic curves after Intra-Venous administration of glucose boli in healthy and pre-diabetic subjects as well as in Type 2 Diabetes Mellitus. Previous modeling of β-cell physiology has been mainly directed to the intracellular chain of events giving rise to single-cell or cell-cluster hormone release oscillations, but the large size, long period and complex morphology of the diverse responses to whole-body glucose stimuli has not yet been coherently explained. Starting with the seminal work of Grodsky it was hypothesized that the population of pancreatic β-cells, possibly functionally aggregated in islets of Langerhans, could be viewed as a set of independent, similar, but not identical controllers (firing units) with distributed functional parameters. The present work shows how a single model based on a population of independent islet controllers can reproduce very closely a diverse array of actually observed experimental results, with the same set of working parameters. The model's success in reproducing a diverse array of experiments implies that, in order to understand the macroscopic behaviour of the endocrine pancreas in regulating glycemia, there is no need to hypothesize intrapancreatic pacemakers, influences between different islets of Langerhans, glycolitic-induced oscillations or β-cell sensitivity to the rate of change of glycemia.

Serotonin, a possible intermediate between disturbed circadian rhythms and metabolic disease

It is evident that eating in misalignment with the biological clock (such as in shift work, eating late at night and skipping breakfast) is associated with increased risk for obesity and diabetes. The biological clock located in the suprachiasmatic nucleus dictates energy balance including feeding behavior and glucose metabolism. Besides eating and sleeping patterns, glucose metabolism also exhibits clear diurnal variations with higher blood glucose concentrations, glucose tolerance and insulin sensitivity prior to waking up. The daily variation in plasma glucose concentrations in rats, is independent of the rhythm in feeding behavior. On the other hand, feeding itself has profound effects on glucose metabolism, but differential effects occur depending on the time of the day. We here review data showing that a disturbed diurnal eating pattern results in alterations in glucose metabolism induced by a disrupted circadian clock. We first describe the role of central serotonin on feeding behavior and glucose metabolism and subsequently describe the effects of central serotonin on the circadian system. We next explore the interaction between the serotonergic system and the circadian clock in conditions of disrupted diurnal rhythms in feeding and how this might be involved in the metabolic dysregulation that occurs with chronodisruption.

Circadian Clocks and the Interaction between Stress Axis and Adipose Function

Many physiological processes and most endocrine functions show fluctuations over the course of the day. These so-called circadian rhythms are governed by an endogenous network of cellular clocks and serve as an adaptation to daily and, thus, predictable changes in the organism's environment. Circadian clocks have been described in several tissues of the stress axis and in adipose cells where they regulate the rhythmic and stimulated release of stress hormones, such as glucocorticoids, and various adipokine factors. Recent work suggests that both adipose and stress axis clock systems reciprocally influence each other and adrenal-adipose rhythms may be key players in the development and therapy of metabolic disorders. In this review, we summarize our current understanding of adrenal and adipose tissue rhythms and clocks and how they might interact to regulate energy homoeostasis and stress responses under physiological conditions. Potential chronotherapeutic strategies for the treatment of metabolic and stress disorders are discussed.

Determination of personalized diabetes treatment plans using a two-delay model

Diabetes cases worldwide have risen steadily over the past few decades, lending urgency to the search for more efficient, effective, and personalized ways to treat the disease. Current treatment strategies, however, may fail to maintain oscillations in blood glucose concentration that naturally occur multiple times per day, an important element of normal human physiology. Building upon recent successes in mathematical modeling of the human glucose-insulin system, we show that both food intake and insulin therapy likely demand increasingly precise control over insulin sensitivity if oscillations at a healthy average glucose concentration are to be maintained. We then model and describe personalized treatment options for patients with diabetes that maintain these oscillations. We predict that for a person with type II diabetes, both blood glucose levels can be controlled and healthy oscillations maintained when the patient gets an hour of daily exercise and is placed on a combination of Metformin and sulfonylurea drugs. We note that insulin therapy and an additional hour of exercise will reduce the patient׳s need for sulfonylureas. Results of a modeling analysis suggest that, with constant nutrition and controlled exercise, the blood glucose levels of a person with type I diabetes can be properly controlled with insulin infusion between 0.45 and 0.7μU/mlmin. Lastly, we note that all suggested strategies rely on existing clinical techniques and established treatment measures, and so could potentially be of immediate use in the design of an artificial pancreas.

Linking neuronal brain activity to the glucose metabolism

BACKGROUND

Energy homeostasis ensures the functionality of the entire organism. The human brain as a missing link in the global regulation of the complex whole body energy metabolism is subject to recent investigation. The goal of this study is to gain insight into the influence of neuronal brain activity on cerebral and peripheral energy metabolism. In particular, the tight link between brain energy supply and metabolic responses of the organism is of interest. We aim to identifying regulatory elements of the human brain in the whole body energy homeostasis.

METHODS

First, we introduce a general mathematical model describing the human whole body energy metabolism. It takes into account the two central roles of the brain in terms of energy metabolism. The brain is considered as energy consumer as well as regulatory instance. Secondly, we validate our mathematical model by experimental data. Cerebral high-energy phosphate content and peripheral glucose metabolism are measured in healthy men upon neuronal activation induced by transcranial direct current stimulation versus sham stimulation. By parameter estimation we identify model parameters that provide insight into underlying neurophysiological processes. Identified parameters reveal effects of neuronal activity on regulatory mechanisms of systemic glucose metabolism.

RESULTS

Our examinations support the view that the brain increases its glucose supply upon neuronal activation. The results indicate that the brain supplies itself with energy according to its needs, and preeminence of cerebral energy supply is reflected. This mechanism ensures balanced cerebral energy homeostasis.

CONCLUSIONS

The hypothesis of the central role of the brain in whole body energy homeostasis as active controller is supported.

Dynamic insulin on board: incorporation of circadian insulin sensitivity variation

BACKGROUND

Insulin-on-board (IOB) estimation is used in modern insulin therapy with continuous subcutaneous insulin infusion (CSII) as well as different automatic glucose-regulating strategies (i.e., artificial pancreas products) to prevent insulin stacking that may lead to hypoglycemia. However, most of the IOB calculations are static IOB (sIOB): they are based only on approximated insulin decay and do not take into account diurnal changes in insulin sensitivity.

METHODS

A dynamic IOB (dIOB) that takes into account diurnal insulin sensitivity variation is suggested in this work and used to adjust the sIOB estimations. The dIOB function is used to correct the dosage of insulin boluses in light of this circadian variation.

RESULTS

Basal-bolus as applied by pump users and model predictive control therapy with and without dIOB were evaluated using the University of Virginia/Padova metabolic simulator. Three protocols with four meals of 1 g carbohydrate/kg body weight were evaluated: a nominal scenario and two robustness scenarios, one in which insulin sensitivity was 15% greater than estimated and the other where the lunch is 30% less than announced. In the nominal and robustness scenarios, respectively, the dIOB led to 6% and 24% and 40% less hypoglycemia episodes than approaches without IOB. The new approach was also compared with the sIOB to evaluate the improvements with respect to the previous approach.

CONCLUSIONS

Improved glucose regulation was demonstrated using the dIOB where circadian insulin sensitivity is used to adjust IOB estimation. Use of diurnal variations of insulin sensitivity appears to promote effective and safe insulin therapy using CSII or artificial pancreas. Clinical trials are warranted to determine whether nocturnal hypoglycemia can be reduced using the dIOB approach.

Total antioxidant capacity and malondialdehyde in depressive rotational shift workers

Shift work is associated with sleep deprivation, occupational stress, and increased risk of depression. Depressed patients show increased oxidative stress. During excessive oxidative stress, Malondialdehyde (MDA) increases and total antioxidant capacity (TAC) decreases in body. This cross-sectional study was conducted to determine the serum level of TAC and MDA among depressed rotational shift workers in Shahid Tondooyan Tehran Oil Refinery. 21-item Beck Depression Inventory was used to measure depression level. The level of TAC and MDA was measured by 8 mL fasting blood sample. MDA was determined by thiobarbituric acid reaction. Serum total antioxidants were measured using the ABTS. Results of this study showed that TAC mean and standard deviation concentration was 2.451 (±0.536) mg/dL and MDA was 3.725 (±1.098) mic·mol/L, and mean and standard deviation of depression score and BMI were 14.07 (±3.84) and 24.92 (±3.65) kg/m², respectively. Depression score had a positive correlation with rotational shift work experience and work experience (r = 0.218 and r = 0.212), respectively, (P < 0.05).

Effect of omega-3 and ascorbic acid on inflammation markers in depressed shift workers in Shahid Tondgoyan Oil Refinery, Iran: a randomized double-blind placebo-controlled study

The present study aimed to assess the effect of supplementation of omega-3 and/or vitamin C on serum interleukin-6 and high sensitivity C-reactive protein concentration and depression scores among shift workers in Shahid Tondgoyan oil refinery. The study design was randomized, double-blind, placebo-controlled, parallel trial. Totally 136 shift workers with a depression score ≥10 in 21-item Beck Depression Rating Scale were randomly assigned to receive omega-3 (180 mg eicosapentaenoate acid and 120 mg docosahexaenoic acid) or/and vitamin C 250 mg or placebo twice daily (with the same taste and shape as omega-3 and vitamin C) for 60 days in four groups. Depression score, interleukin-6 and high sensitivity C-reactive protein were measured at baseline and after 60 days. This study showed that supplementation of omega-3 plus vitamin C is associated with a decrease in depression score (p<0.05). Supplementation of omega-3 without vitamin C, is associated with a reduction in depression score (p<0.0001) and high sensitivity C-reactive protein concentration (p<0.01). Therefore omega-3 supplementation showed a better effect on reducing depression score and high sensitivity C-reactive protein, but supplementation of vitamin C along with omega-3 did not have significant effect on change in C-reactive protein level compared to omega-3 alone. (Registration number: IRCT201202189056N1)

CNS control of glucose metabolism: response to environmental challenges

Over the last 15 years, considerable work has accumulated to support the role of the CNS in regulating postprandial glucose levels. As discussed in the first section of this review, the CNS receives and integrates information from afferent neurons, circulating hormones, and postprandially generated nutrients to subsequently direct changes in glucose output by the liver and glucose uptake by peripheral tissues. The second major component of this review focuses on the effects of external pressures, including high fat diet and changes to the light:dark cycle on CNS-regulating glucose homeostasis. We also discuss the interaction between these different pressures and how they contribute to the multifaceted mechanisms that we hypothesize contribute to the dysregulation of glucose in type 2 diabetes mellitus (T2DM). We argue that while current peripheral therapies serve to delay the progression of T2DM, generating combined obesity and T2DM therapies targeted at the CNS, the primary site of dysfunction for both diseases, would lead to a more profound impact on the progression of both diseases.

Noisy and individual, but doable: shift-work research in humans

Working around the clock is common for many occupations, as diverse as nurses, truck drivers, physicians, steel workers, and pilots. Each shift-work profession is individual in more aspects than just work hours and individual work scenarios, each posing a different impact on the health of workers. Related health problems in shift workers, therefore, are also diverse and encompass sleep problems, metabolic and cardiovascular system disturbances, as well as cancer. Little is known about how all these individual factors influence a shift worker's health status, partly because many shift-work studies show inconsistent results. In addition, these individual factors create many methodological difficulties for researchers who investigate such work scenarios. This chapter presents examples from our laboratory and field studies of shift workers, which emphasize the importance of taking individual circumstances into account. Both study approaches, laboratory and field based, are needed to fully account for the difficulties that shift-work studies pose on both workers and researchers. Finally, understanding the mechanisms that underpin interindividual differences in response to shift work will advance our understanding of how to design better and healthier shift-work schedules in the future.

The role of the circadian clock system in nutrition and metabolism

Mammals have an endogenous timing system in the suprachiasmatic nuclei (SCN) of the hypothalamic region of the brain. This internal clock system is composed of an intracellular feedback loop that drives the expression of molecular components and their constitutive protein products to oscillate over a period of about 24 h (hence the term 'circadian'). These circadian oscillations bring about rhythmic changes in downstream molecular pathways and physiological processes such as those involved in nutrition and metabolism. It is now emerging that the molecular components of the clock system are also found within the cells of peripheral tissues, including the gastrointestinal tract, liver and pancreas. The present review examines their role in regulating nutritional and metabolic processes. In turn, metabolic status and feeding cycles are able to feed back onto the circadian clock in the SCN and in peripheral tissues. This feedback mechanism maintains the integrity and temporal coordination between various components of the circadian clock system. Thus, alterations in environmental cues could disrupt normal clock function, which may have profound effects on the health and well-being of an individual.

[Sleep duration and overweight]

Sleep deprivation is considered a serious risk factor with a relevant public health impact due to its multiple effects, which we are just beginning to understand. This is underlined by the growing number of studies in recent decades that have investigated the association between sleep duration and overweight. This review gives an overview of the current state of research and potential biological mechanisms. Evidence from epidemiological studies suggests an association between short sleep duration and overweight in children as well as in adults. Different biological mechanisms have been discussed in this context with a special focus on hormonal changes as the potential mediator. To include sleep in a comprehensive strategy to prevent overweight, interventional studies that analyze the potentially protective effect of prolonged sleep duration are necessary.

Circadian feeding drive of metabolic activity in adipose tissue and not hyperphagia triggers overweight in mice: is there a role of the pentose-phosphate pathway?

High-fat (HF) diets trigger an increase in adipose tissue and body weight (BW) and disordered eating behavior. Our study deals with the hypothesis that circadian distribution of energy intake is more relevant for BW dynamics than diet composition. Four-week-old mice were exposed for 8 wk to a HF diet and compared with animals receiving control chow. HF mice progressively increased BW, decreased the amount of nocturnal (1800-0900 h) calories (energy or food intake) (30%) and increased diurnal (0900-1800 h) caloric intake (energy or food intake), although total daily intake was identical between groups. Animals were killed at 3-h intervals and plasma insulin, leptin, corticosterone, glucose, and fatty acid levels quantified. Adipose tissue was weighed, and enzymatic activities integral to the pentose phosphate pathway (PPP) assayed in lumbar adipose tissue. Phosphorylated AMP-dependent protein kinase and fatty acid synthase were quantified by Western blotting. In HF mice, there was a shift in the circadian oscillations of plasma parameters together with an inhibition of PPP activity and a decrease in phosphorylated AMP-dependent protein kinase and fatty acid synthase. In a second experiment, HF mice were forced to adhere to a circadian pattern of food intake similar to that in control animals. In this case, BW, adipose tissue, morning plasma parameters and PPP activity appeared to be normal. These data indicate that disordered feeding behavior can trigger BW gain independently of food composition and daily energy intake. Because PPP is the main source of reduced nicotinamide adenine dinucleotide phosphate, we suggest that PPP inhibition might be an early marker of adipose dysfunction in diet-induced obesity.

Paradoxical post-exercise responses of acylated ghrelin and leptin during a simulated night shift

Approximately 10% of employees undertake night work, which is a significant predictor of weight gain, possibly because responses to activity and eating are altered at night. It is known that the appetite-related hormone, acylated ghrelin, is suppressed after an acute bout of exercise during the day, but no researcher has explored whether evening exercise alters acylated ghrelin and other appetite-related outcomes during a subsequent night shift. Six healthy men (mean +/- SD: age 30 +/- 8 yrs, body mass index 23.1 +/- 1.1 kg/m(2)) completed two crossover trials (control and exercise) in random order. Participants fasted from 10:00 h, consumed a test meal at 18:00 h, and then cycled at 50% peak oxygen uptake or rested between 19:00-20:00 h. Participants then completed light activities during a simulated night shift which ended at 05:00 h. Two small isocaloric meals were consumed at 22:00 and 02:00 h. Venous blood samples were drawn via cannulation at 1 h intervals between 19:00-05:00 h for the determination of acylated ghrelin, leptin, insulin, glucose, triglyceride, and non-esterified fatty acids concentrations. Perceived hunger and wrist actimetry were also recorded. During the simulated night shift, mean +/- SD acylated ghrelin concentration was 86.5 +/- 40.8 pg/ml following exercise compared with 71.7 +/- 37.7 pg/ml without prior exercise (p = 0.015). Throughout the night shift, leptin concentration was 263 +/- 242 pg/ml following exercise compared with 187 +/- 221 pg/ml without prior exercise (p = 0.017). Mean levels of insulin, triglyceride, non-esterified fatty acids, and wrist actimetry level were also higher during the night shift that followed exercise (p < 0.05). These data indicate that prior exercise increases acylated ghrelin and leptin concentrations during a subsequent simulated night shift. These findings differ from the known effects of exercise on acylated ghrelin and leptin during the day, and therefore have implications for energy balance during night work.

Compact energy metabolism model: brain controlled energy supply

The regulation of the energy metabolism is crucial to ensure the functionality of the entire organism. Deregulations may lead to severe pathologies such as obesity and type 2 diabetes mellitus. The decisive role of the brain as the active controller and heavy consumer in the complex whole body energy metabolism is the matter of recent research. Latest studies suggest that the brain's energy supply has the highest priority while all organs in the organism compete for the available energy resources. In our novel mathematical model, we address these new findings. We integrate energy fluxes and their control signals such as glucose fluxes, insulin signals as well as the ingestion momentum in our new dynamical system. As a novel characteristic, the hormone insulin is regarded as central feedback signal of the brain. Hereby, our model particularly contains the competition for energy between brain and body periphery. The analytical investigation of the presented dynamical system shows a stable long-term behavior of the entire energy metabolism while short time observations demonstrate the typical oscillating blood glucose variations as a consequence of food intake. Our simulation results demonstrate a realistic behavior even in situations like exercise or exhaustion, and key elements like the brain's preeminence are reflected. The presented dynamical system is a step towards a systemic understanding of the human energy metabolism and thus may shed light to defects causing diseases based on deregulations in the energy metabolism.

Food intake during the normal activity phase prevents obesity and circadian desynchrony in a rat model of night work

Shift work or night work is associated with hypertension, metabolic syndrome, cancer, and other diseases. The cause for these pathologies is proposed to be the dissociation between the temporal signals from the biological clock and the sleep/activity schedule of the night worker. We investigated the mechanisms promoting metabolic desynchrony in a model for night work in rats, based on daily 8-h activity schedules during the resting phase. We demonstrate that the major alterations leading to internal desynchrony induced by this working protocol, flattened glucose and locomotor rhythms and the development of abdominal obesity, were caused by food intake during the rest phase. Shifting food intake to the normal activity phase prevented body weight increase and reverted metabolic and rhythmic disturbances of the shift work animals to control ranges. These observations demonstrate that feeding habits may prevent or induce internal desynchrony and obesity.

Obesity and shift work: chronobiological aspects

The present review has the objective of summarising chronobiological aspects of shift work and obesity. There was a systematic search in PubMed databases, using the following descriptors: shift work; obesity; biological clock. Shift work is extremely frequent in several services and industries, in order to systematise the needs for flexibility of the workforce, necessary to optimise productivity and business competitiveness. In developing countries, this population represents a considerable contingent workforce. Recently, studies showed that overweight and obesity are more prevalent in shift workers than day workers. In addition, the literature shows that shift workers seem to gain weight more often than those workers submitted to a usual work day. In conclusion, there is considerable epidemiological evidence that shift work is associated with increased risk for obesity, diabetes and CVD, perhaps as a result of physiological maladaptation to chronically sleeping and eating at abnormal circadian times. The impact of shift work on metabolism supports a possible pathway to the development of obesity and its co-morbities. The present review demonstrated the adverse cardiometabolic implications of circadian misalignment, as occurs chronically with shift workers.

Internal desynchronization in a model of night-work by forced activity in rats

Individuals engaged in shift- or night-work show disturbed diurnal rhythms, out of phase with temporal signals associated to the light/dark (LD) cycle, resulting in internal desynchronization. The mechanisms underlying internal desynchrony have been mainly investigated in experimental animals with protocols that induce phase shifts of the LD cycle and thus modify the activity of the suprachiasmatic nucleus (SCN). In this study we developed an animal model of night-work in which the light-day cycle remained stable and rats were required to be active in a rotating wheel for 8 h daily during their sleeping phase (W-SP). This group was compared with rats that were working in the wheel during their activity phase (W-AP) and with undisturbed rats (C). We provide evidence that forced activity during the sleeping phase (W-SP group) alters not only activity, but also the temporal pattern of food intake. In consequence W-SP rats showed a loss of glucose rhythmicity and a reversed rhythm of triacylglycerols. In contrast W-AP rats did not show such changes and exhibited metabolic rhythms similar to those of the controls. The three groups exhibited the nocturnal corticosterone increase, in addition the W-SP and W-AP groups showed increase of plasma corticosterone associated with the start of the working session. Forced activity during the sleep phase did not modify SCN activity characterized by the temporal patterns of PER1 and PER2 proteins, which remained in phase with the LD cycle. These observations indicate that a working regimen during the sleeping period elicits internal desynchronization in which activity combined with feeding uncouples metabolic functions from the biological clock which remains fixed to the LD cycle. The present data suggest that in the night worker the combination of work and eating during working hours may be the cause of internal desynchronization.

Working on atypical schedules

Shift work has been associated with a number of health problems including cardiovascular disease, impaired glucose and lipid metabolism, gastrointestinal discomfort, reproductive difficulties, and breast cancer. The specific contributions of disturbed physiological rhythms, circadian misalignment, and sleep debt to the various medical problems encountered by shift workers remain to be clarified. Fatigue can be caused by extended on-duty and/or waking periods, inadequate sleep quantity, sleep disturbances, disruption of circadian rhythms, and difficult work and familial conditions. Fatigue-related accidents raise a safety concern for shift workers, especially at the end of the night when the circadian nadir of alertness interacts with increased time awake. Individuals vary greatly in their capacity to adjust to atypical work schedules and their tolerance to circadian misalignment. Predisposing individual and domestic factors have been identified, such as increasing age, being a single woman in charge of children, and split sleep patterns, all of which can affect the ability to adjust to atypical schedules. However, prior studies indicate that predisposing individual and social determinants are generally poor predictors of shift work tolerance in a given individual. In this manuscript, we review several countermeasures to improve adaptation to shift work.

Circadian rhythm of blood glucose values in critically ill patients

OBJECTIVE

To test whether there is a circadian rhythm of blood glucose control in critically ill patients and whether morning blood glucose is an accurate surrogate of overall blood glucose control.

DESIGN

Retrospective multiple-center observational study.

SETTING

Intensive care units of three tertiary hospitals and one affiliated private hospital.

PATIENTS

Cohort of 8,307 consecutive critically ill patients.

INTERVENTIONS

Extraction of blood glucose values from electronically stored measurements. Extraction of demographic and outcome data from unit and hospital databases. Statistical assessment of variations in blood glucose control over each 24-hr cycle.

MEASUREMENTS AND MAIN RESULTS

We studied 208,362 blood glucose measurements in 8,307 patients (5.5 measurements/day/person). In each hospital, there was a circadian rhythm of blood glucose control (p<.0001). The differences between highest and lowest blood glucose concentration in different time periods in each hospital were 0.27, 0.28, 0.95, and 0.22 mmol/L. There was also significant variation in the incidence and notional duration of hyperglycemia. The differences between the lowest and highest incidence of hyperglycemia in different time periods were 3.3, 2.7, 9.9, and 2.6% in each hospital. In all four hospitals, the average blood glucose value from 5:30 am to 6:30 am was significantly lower than the 24-hr average.

CONCLUSIONS

Blood glucose values and the incidence of hyperglycemia have a circadian rhythm in critically ill patients. Morning blood glucose may not be an accurate surrogate of blood glucose control over the daily cycle.

Combined effects of high-fat feeding and circadian desynchronization

OBJECTIVE

To assess whether circadian desynchronization leads to metabolic alterations capable of promoting dietary obesity and/or impairing glucose tolerance.

DESIGN

Rats fed either with chow pellets (i.e., low-fat diet with 4% mass of fat) or high-fat diet (34% mass of fat). Half of each diet group was exposed to a fixed light-dark cycle or to a 10-h weekly shift in the light-dark cycle from Thursday to Sunday (20 shifts). To enforce the shifted animals to be active at unusual times of the day, food was available only during the daily dark period for all groups.

RESULTS

Shifting the light-dark cycle on a weekly basis was efficient to induce circadian desynchronization, as evidenced by strong disturbances in the daily expression of locomotor activity. Shifted rats fed with a nocturnal low-fat diet had lower plasma insulin and similar blood glucose compared to rats fed with the same diet under a fixed light-dark cycle. Nocturnal high-fat feeding led to an abdominal fat overload associated with increased plasma leptin and basal glucose. These metabolic changes were not significantly modified by circadian desynchronization.

CONCLUSION

Chronic desynchronization with low-fat diet impaired insulin regulation. Metabolic changes induced by the high-fat diet were not aggravated by chronic desynchronization.

Complex systems model of fatigue: integrative homoeostatic control of peripheral physiological systems during exercise in humans

Fatigue is hypothesised as being the result of the complex interaction of multiple peripheral physiological systems and the brain. In this new model, all changes in peripheral physiological systems such as substrate depletion or metabolite accumulation act as afferent signallers which modulate control processes in the brain in a dynamic, non-linear, integrative manner.