Daytime melatonin and temazepam in young adult humans: equivalent effects on sleep latency and body temperatures

Melatonin-induced symptomatic bradycardia in an otherwise healthy male: a case report

Melatonin, a pineal gland hormone closely associated with the circadian rhythm, has been trending over the past years as an over-the-counter medication to aid with sleep disturbances. Although generally believed to be safe, recent studies show negative inotropic and chronotropic effects on the heart rate and blood pressure in humans. Several studies suggested that melatonin induces cardiac vagal tone and affects heart rate and mean arterial pressure. Limited literature is currently available on the effects of melatonin beyond its sleep function. We present a case of a healthy 22-year-old male who visited the emergency department reporting palpitations and dizziness following the ingestion of 20 mg of melatonin. Subsequent examinations revealed marked bradycardia. Fortunately, the patient experienced spontaneous resolution of the bradycardia without necessitating intervention after a few hours of observation, and he was observed and discharged.

Effect of melatonin intake on postural balance, functional mobility and fall risk in persons with multiple sclerosis: a pilot study

AIM

To assess the safety and the effect of a nocturnal melatonin (MEL) ingestion on postural balance, functional mobility and fall risk the following morning in adults with multiple sclerosis (MS).

METHODS

Fourteen adults with relapsing-remitting MS (RR-MS) (28.36 ± 6.81 years) were evaluated before and after nocturnal ingestion of MEL (6 mg) or placebo (PLA). Evaluations included a posturographic test of static bipedal postural balance with dual-task in eyes open (EO) and eyes closed conditions, and a clinical test of unipedal balance. The physical performance tests were: Timed Up and Go test (TUGT) (mobility), Four Square Step Test (FSST) (fall risk), and Timed 25-foot walk test (T25FWT) (walking speed). Cognitive performance [Montreal Cognitive Assessment (MoCA) and Simple Reaction Time (SRT) tests] and sleep quality [Spiegel's sleep questionnaire (SSQ)] were also assessed.

RESULTS

In EO condition, MEL decreased the posturographic parameters [center of pressure (CoP) sway area (CoPAr), CoP path length (CoPL) and CoPL in the mediolateral axis (CoPLX)] more than PLA by 15.82% (p = 0.0006), 12.48% (p  = 0.0004) and 14.25% (p = 0.0002), respectively. Durations of TUGT and FSST decreased following MEL session more than the PLA one by 14.52% (p = 0.017) and 19.85% (p = 0.0006), respectively. MEL increased the unipedal stance time, SSQ and MoCA scores more than PLA by 49.81% (p = 0.04), 32.21% (p = 0.004) and 11.87% (p = 0.008), respectively.

CONCLUSION

This pilot study showed that acute nocturnal MEL ingestion seems to be safe for enhancing postural balance, fun mobility and fall risk in RR-MS adults probably through improving sleep quality and cognitive function.

Circadian aspects in the aetiology and pathophysiology of insomnia

Because the endogenous circadian pacemaker is a very strong determinant of alertness/sleep propensity across the 24 h period, its mistiming may contribute to symptoms of insomnia (e.g., difficulties initiating sleep and maintaining sleep) and to the development of insomnia disorder. Despite the separation of insomnia and circadian rhythm disorders in diagnostic nosology implying independent pathophysiology, there is considerable evidence of co-morbidity and interaction between them. Sleep onset insomnia is associated with later timed circadian rhythms and can be treated with morning bright light to shift rhythms to an earlier timing. It is also possible that the causal link may go in both directions and that having a delayed circadian rhythm can result in enough experiences of delayed sleep onset to lead to some conditioned insomnia or insomnia disorder further exacerbating a delayed circadian rhythm. Early morning awakening insomnia is associated with an advanced circadian phase (early timing) and can be treated with evening bright light resulting in a delay of rhythms and an improved ability to sleep later in the morning and to obtain more sleep. There is some evidence suggesting that sleep maintenance insomnia is associated with a blunted amplitude of circadian rhythm that may be treated with increased regularity of sleep and light exposure timing. However, this is an insomnia phenotype that requires considerably more circadian research as well as further insomnia clinical research with the other insomnia phenotypes incorporating circadian timing measures and treatments.

Effects of 90- and 30-min naps or a 120-min nap on alertness and performance: reanalysis of an existing pilot study

The aim of this study was to investigate alertness and cognitive performance immediately after and until the end of the night shift after taking a 120-min monophasic nap (One-nap) or a split 90-min and 30-min nap (Two-nap) during a 16-h simulated night shift, and the relationship between sleep quality and both alertness and performance. This study was performed in 41 females. Among them, 15 participants were included in the No-nap group, 14 in the One-nap group (22:00-00:00), and 12 in the Two-nap group (22:30-00:00 and 02:30-03:00). Participants were tested every hour from 16:00 to 09:00 for performance on the Uchida-Kraepelin test, as well as for subjective feelings of fatigue and drowsiness, body temperature, and heart rate variability. The shorter the sleep latency, the worse the alertness immediately after the 90-min nap. The 120-min and 30-min naps also revealed that prolonged total sleep time led to increased fatigue and drowsiness upon awakening. From 04:00 to 09:00, in the No-nap and One-nap groups, fatigue was higher than in the Two-nap group. The One-nap and Two-nap groups did not show improved morning performance. These results suggest that a split nap might improve drowsiness and fatigue during a long night shift.

Melatonin as a Chronobiotic with Sleep-promoting Properties

The use of exogenous melatonin (exo-MEL) as a sleep-promoting drug has been under extensive debate due to the lack of consistency of its described effects. In this study, we conduct a systematic and comprehensive review of the literature on the chronobiotic, sleep-inducing, and overall sleep-promoting properties of exo-MEL. To this aim, we first describe the possible pharmacological mechanisms involved in the sleep-promoting properties and then report the corresponding effects of exo-MEL administration on clinical outcomes in: a) healthy subjects, b) circadian rhythm sleep disorders, c) primary insomnia. Timing of administration and doses of exo-MEL received particular attention in this work. The exo-MEL pharmacological effects are hereby interpreted in view of changes in the physiological properties and rhythmicity of endogenous melatonin. Finally, we discuss some translational implications for the personalized use of exo-MEL in the clinical practice.

What do we really know about the safety and efficacy of melatonin for sleep disorders?

Melatonin is a hormonal product of the pineal gland, a fact that is often forgotten. Instead it is promoted as a dietary supplement that will overcome insomnia, as an antioxidant and as a prescription only drug in most countries outside the United States of America and Canada. The aim of this review is to step back and highlight what we know about melatonin following its discovery 60 years ago. What is the role of endogenous melatonin; what does melatonin do to sleep, body temperature, circadian rhythms, the cardiovascular system, reproductive system, endocrine system and metabolism when administered to healthy subjects? When used as a drug/dietary supplement, what safety studies have been conducted? Can we really say melatonin is safe when it has not been systematically studied and many studies show interactions with a wide range of physiological processes? Finally the results of studies investigating the efficacy of melatonin as a drug to alleviate insomnia are critically evaluated. In summary, melatonin is an endogenous pineal gland hormone with specific physiological functions in animals and humans, with its primary role in humans to maintain synchrony of sleep with the day/night cycle. When administered as a drug it affects a wide range of physiological systems and has clinically important drug interactions. With respect to efficacy for treating sleep disorders, melatonin can advance the time of sleep onset but the effect is modest and variable. In children with neurodevelopmental disabilities melatonin appears to have the greatest impact on sleep onset but little effect on sleep efficiency.

Skin Temperatures of Back or Neck Are Better Than Abdomen for Indication of Average Proximal Skin Temperature During Sleep of School-Aged Children

PURPOSE

The tight association between sleep, body temperature regulation, and patterns of skin temperature change highlights the necessity for accurate and valid assessment of skin temperatures during sleep. With increased interest in this functional relationship in infants and children, it is important to identify where to best measure proximal skin temperature and whether it is possible to reduce the number of sites of measures, in order to limit the experimental effects in natural settings. Thus, the aim of this study was to determine the most suitable single skin temperature sites for representation of average proximal skin temperature during sleep of school aged children.

METHODS

Statistical analyses were applied to skin temperature data of 22 children, aged 6 to 12 years, measured over four consecutive school nights in their home settings, to compare single site measures of abdomen, back, neck, forehead and subclavicular skin temperatures (local temperatures) with average proximal skin temperatures.

RESULTS

Abdomen and forehead skin temperatures were significantly different (respectively higher and lower) to the other local proximal temperatures and to average proximal skin temperatures. Moreover, the time pattern of forehead temperature was very different from that of the other local temperatures.

CONCLUSIONS

Local forehead and abdomen skin temperatures are least suitable as single site representations of average proximal skin temperatures in school aged children when considering both the level and the time course pattern of the temperature across the night. Conversely, back and neck temperatures provide most fitting representation of average proximal skin temperatures.

Current and emerging pharmacological approaches for treating diarrhea-predominant irritable bowel syndrome

Introduction: Irritable bowel syndrome with diarrhea (IBS-D) is among the most common functional gastrointestinal (GI) disorders and is associated with impaired quality of life, increased health-care utilization, and significant costs to patients and society. The treatment of IBS is typically hierarchal with initial therapies consisting of dietary and lifestyle modifications. Pharmacotherapy with over-the-counter and prescription medications is also commonly used for symptomatic control in the course of therapy.Areas covered: Three medications are approved by the United States Food and Drug Administration (FDA) for IBS-D, with all of them demonstrating efficacy in randomized, placebo-controlled trials. In this review, the authors discuss the clinical trial data applicable to the current FDA approved IBS-D therapies as well as review data related to new and emerging therapies for this condition.Expert opinion: Clinicians should be familiar with emerging therapies for IBS-D as they may provide benefit to some IBS-D patients. The exact mechanisms of action of many of the emerging agents for IBS-D remain unknown. Despite substantial differences and limitations in the design and quality of supporting studies, there is an increasing body of evidence suggesting that emerging agents may promote meaningful symptom improvement in patients with IBS-D.

Effects of Melatonin Ingestion Before Nocturnal Sleep on Postural Balance and Subjective Sleep Quality in Older Adults

The present study aimed to investigate the effect of acute nocturnal melatonin (MEL) ingestion on sleep quality, cognitive performance, and postural balance in older adults. A total of 12 older men (58 ± 5.74 years) volunteered to participate in this study. The experimental protocol consisted in two testing sessions after nocturnal MEL (10 mg) or placebo ingestion the night before the tests. During each session, sleep quality tests, cognitive tests, and postural balance protocol were conducted. Static and dynamic postural control was assessed using a force platform. Most of the sleep parameters have been improved following nocturnal MEL ingestion without any effect on cognitive performance. Likewise, measurements related to the center of pressure (CoP) have been significantly decreased with MEL compared with placebo. In conclusion, postural control has been improved the morning following nocturnal MEL ingestion in older adults. This trend could be explained by the potential effect of MEL on sleep quality and cerebellum.

Melatonin MT1 and MT2 Receptors Exhibit Distinct Effects in the Modulation of Body Temperature across the Light/Dark Cycle

Melatonin (MLT) is a neurohormone that regulates many physiological functions including sleep, pain, thermoregulation, and circadian rhythms. MLT acts mainly through two G-protein-coupled receptors named MT₁ and MT₂, but also through an MLT type-3 receptor (MT₃). However, the role of MLT receptor subtypes in thermoregulation is still unknown. We have thus investigated the effects of selective and non-selective MLT receptor agonists/antagonists on body temperature (T_b) in rats across the 12/12-h light-dark cycle. Rectal temperature was measured every 15 min from 4:00 a.m. to 9:30 a.m. and from 4:00 p.m. to 9:30 p.m., following subcutaneous injection of each compound at either 5:00 a.m. or 5:00 p.m. MLT (40 mg/kg) had no effect when injected at 5 a.m., whereas it decreased T_b during the light phase only when injected at 5:00 p.m. This effect was blocked by the selective MT₂ receptor antagonist 4P-PDOT and the non-selective MT₁/MT₂ receptor antagonist, luzindole, but not by the α₁/MT₃ receptors antagonist prazosin. However, unlike MLT, neither the selective MT₁ receptor partial agonist UCM871 (14 mg/kg) nor the selective MT₂ partial agonist UCM924 (40 mg/kg) altered T_b during the light phase. In contrast, UCM871 injected at 5:00 p.m. increased T_b at the beginning of the dark phase, whereas UCM924 injected at 5:00 a.m. decreased T_b at the end of the dark phase. These effects were blocked by luzindole and 4P-PDOT, respectively. The MT₃ receptor agonist GR135531 (10 mg/kg) did not affect T_b. These data suggest that the simultaneous activation of both MT₁ and MT₂ receptors is necessary to regulate T_b during the light phase, whereas in a complex but yet unknown manner, they regulate T_b differently during the dark phase. Overall, MT₁ and MT₂ receptors display complementary but also distinct roles in modulating circadian fluctuations of T_b.

The effects of a 120-minute nap on sleepiness, fatigue, and performance during 16-hour night shifts: A pilot study

Objective

To investigate sleepiness, fatigue, and performance following a 120‐minute nap during simulated 16‐hour night shifts based on subjective and objective assessments.

Methods

Fourteen females participated in this crossover comparative study. Three experimental nap conditions were used: naps from 22:00 to 00:00 (22‐NAP), 00:00 to 02:00 (00‐NAP), and 02:00 to 04:00 (02‐NAP), respectively. Measurement items were sleep parameters, sublingual temperature, a Visual Analog Scale for sleepiness and fatigue, a single‐digit mental arithmetic task (for 10 minutes), and heart rate variability. Participants wore an ActiGraph to estimate their sleep state.

Results

There was no difference in the sleep parameters at the time of naps among the three conditions. Immediately following a 120‐minute nap, sleepiness and fatigue increased, and the number of calculations performed in the single‐digit mental arithmetic task decreased in any of the conditions. In particular, immediately after the 02‐NAP, fatigue and high‐frequency power (HF) were higher than after the 22‐NAP. In the early morning (from 05:00 to 09:00), in the 22‐NAP, sleepiness and fatigue increased, and performance and sublingual temperature decreased more than in the 00‐NAP and 02‐NAP. Furthermore, the ratio of errors was significantly lower in the 00‐NAP than in the 22‐NAP in the early morning.

Conclusions

A 120‐minute nap taken from 22:00 to 02:00 may cause temporary sleepiness after waking, increase fatigue and reduce performance. Greater attention should be given to naps taken at a later time (ie, 02‐NAP). In addition, taking a nap starting at 00:00 might decrease the risks of errors in the morning.

Melatonin ingestion after exhaustive late-evening exercise improves sleep quality and quantity, and short-term performances in teenage athletes

The present study aimed to explore the effects of a single 10-mg dose of melatonin (MEL) administration after exhaustive late-evening exercise on sleep quality and quantity, and short-term physical and cognitive performances in healthy teenagers. Ten male adolescent athletes (mean ± SD, age = 15.4 ± 0.3 years, body-mass = 60.68 ± 5.7 kg, height = 167.9 ± 6.9 cm and BMI = 21.21 ± 2.5) performed two test sessions separated by at least one week. During each session, participants completed the Yo-Yo intermittent-recovery-test level-1 (YYIRT-1) at ~20:00 h. Then, sleep polysomnography was recorded from 22:15 min to 07:00 h, after a double blind randomized order administration of a single 10-mg tablet of MEL (MEL-10 mg) or Placebo (PLA). The following morning, Hooper wellness index was administered and the participants performed the Choice Reaction Time (CRT) test, the Zazzo test and some short-term physical exercises (YYIRT-1, vertical and horizontal Jumps (VJ; HJ), Hand grip strength (HG), and five-jump test (5-JT)). Evening total distance covered in the YYIRT-1 did not change during the two conditions (p > 0.05). Total sleep time (Δ = 24.55 mn; p < 0.001), sleep efficiency (Δ = 4.47%; p < 0.001), stage-3 sleep (N3 sleep) (Δ = 1.73%; p < 0.05) and rapid-eye-movement sleep (Δ = 2.15%; p < 0.001) were significantly higher with MEL in comparison with PLA. Moreover, sleep-onset-latency (Δ = -8.45mn; p < 0.001), total time of nocturnal awakenings after sleep-onset (NA) (Δ = -11 mn; p < 0.001), stage-1 sleep (N1 sleep) (Δ = -1.7%; p < 0.001) and stage-2 sleep (N2 sleep) (Δ = -1.9%; p < 0.05) durations were lower with MEL. The Hooper index showed a better subjective sleep quality, a decrease of the subjective perception of fatigue and a reduced level of muscle soreness with MEL. Moreover, MEL improved speed and performance but not inaccuracy during the Zazzo test. CRT was faster with MEL. Morning YYIRT-1 (Δ = 82 m; p < 0.001) and 5-JT (Δ = 0.08 m; p < 0.05) performances were significantly higher with MEL in comparison with PLA. In contrast, HG, VJ and HJ performances did not change during the two conditions (p > 0.05). The administration of a single dose of MEL-10 mg after strenuous late-evening exercise improved sleep quality and quantity, selective attention, subjective assessment of the general wellness state, and some short-term physical performances the following morning in healthy teenagers.

The influence of light on thermal responses

Light is essential for vision and plays an important role in non-visual responses, thus affecting alertness, mood and circadian rhythms. Furthermore, light influences physiological processes, such as thermoregulation, and therefore may be expected to play a role in thermal comfort (TC) as well. A systematic literature search was performed for human studies exploring the relation between ocular light exposure, thermophysiology and TC. Experimental results show that light in the evening can reduce melatonin secretion, delay the natural decline in core body temperature (CBT) and slow down the increase in distal skin temperature. In the morning though, bright light can result in a faster decline in melatonin levels, thus enabling a faster increase in CBT. Moreover, the colour of light can affect temperature perception of the environment. Light with colour tones towards the red end of the visual spectrum leads to a warmer perception compared to more bluish light tones. It should be noted, however, that many results of light on thermal responses are inconclusive, and a theoretical framework is largely lacking. In conclusion, light is capable of evoking thermophysiological responses and visual input can alter perception of the thermal environment. Therefore, lighting conditions should be taken into consideration during thermophysiological research and in the design of indoor climates.

Involvement of the circadian rhythm and inflammatory cytokines in the pathogenesis of rheumatoid arthritis

Among the symptoms of patients with rheumatoid arthritis (RA), joint stiffness is influenced by diurnal rhythm and reaches peak in the morning, which is a common complaint and reflects the circadian nature of disease manifestation. In addition, inflammatory cytokines, which reach peak secretion early in the morning are major players causing the morning stiffness. In this review, we explore the link between the circadian clock and inflammation, focusing on the interactions of various clock genes with the immune-pathways underlying the pathology of rheumatoid arthritis.

Melatonin for the treatment of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common disorder characterized by recurrent abdominal pain or discomfort, in combination with disturbed bowel habits in the absence of identifiable organic cause. Melatonin (N-acetyl-5-methoxytryptamine) is a hormone produced by the pineal gland and also large number by enterochromaffin cells of the digestive mucosa. Melatonin plays an important part in gastrointestinal physiology which includes regulation of gastrointestinal motility, local anti-inflammatory reaction as well as moderation of visceral sensation. Melatonin is commonly given orally. It is categorized by the United States Food and Drug Administration as a dietary supplement. Melatonin treatment has an extremely wide margin of safety though it may cause minor adverse effects, such as headache, rash and nightmares. Melatonin was touted as a potential effective candidate for IBS treatment. Putative role of melatonin in IBS treatment include analgesic effects, regulator of gastrointestinal motility and sensation to sleep promoter. Placebo-controlled studies in melatonin suffered from heterogeneity in methodology. Most studies utilized 3 mg at bedtime as the standard dose of trial. However, all studies had consistently showed improvement in abdominal pain, some showed improvement in quality of life of IBS patients. Melatonin is a relatively safe drug that possesses potential in treating IBS. Future studies should focus on melatonin effect on gut mobility as well as its central nervous system effect to elucidate its role in IBS patients.

Effects of two 15-min naps on the subjective sleepiness, fatigue and heart rate variability of night shift nurses

The purpose of this study was to examine the effects of two 15-min naps on nurses who work at night in a three-shift system. Of the 15 nurses who were included as study subjects on a night shift, eight took two short naps (the Nap condition), and seven worked without taking a nap (the No-nap condition) during the night shift. We measured sublingual temperature and the bispectral index (BIS), obtained heart rate and heart rate variability measures from an electrocardiogram (ECG), and evaluated sleepiness and fatigue levels every hour using the Visual Analog Scale (VAS). Both subjective sleepiness and fatigue increased between 4:00 and 5:00, with no significant differences observed between the two groups. However, the low- to high-frequency ratio (LF/HF) in the Nap condition group was found to be significantly lower than in the No-nap condition group. Furthermore, a sudden, brief increase in HF values was observed in the No-nap condition group in the morning. The results of this study suggest that taking two short naps may effectively reduce tension and prevent a brief increase in HF values by suppressing sympathetic nervous activity in the morning.

Seasonal differences in melatonin concentrations and heart rates during sleep in obese subjects in Japan

During the past several decades, obesity has been increasing globally. In Japan, obesity is defined by a BMI of 25 kg/m² or over; 28.6 % of men and 20.6 % of women are obese. Obese people have an increased incidence of developing cardiovascular, renal, and hormonal diseases and sleep disorders. Obese people also have shortened sleep durations. We investigated seasonal differences in melatonin concentrations, heart rates, and heart rate variability during sleep in obese subjects in Japan. Five obese (BMI, 32.0 ± 4.9 kg/m²) and five non-obese (BMI, 23.2 ± 2.9 kg/m²) men participated in this study in the summer and winter. Electrocardiograms were measured continuously overnight in a climatic chamber at 26 °C with a relative humidity of 50 %. Saliva samples for melatonin were collected at 2300 hours, 0200 hours, and 0600 hours. We found that melatonin concentrations during sleep in obese subjects were significantly lower than those in non-obese subjects in the winter. Heart rate during sleep in winter was significantly higher than that in summer in both obese and non-obese subjects. Heart rate variability was not significantly different in the summer and winter in both obese and non-obese subjects. Our results show that decreased nocturnal melatonin concentrations during winter in obese men may be related to higher heart rates, and this may suggest that obese men are at an increased risk of a cardiovascular incident during sleep, especially in the winter.

Significance and application of melatonin in the regulation of brown adipose tissue metabolism: relation to human obesity

A worldwide increase in the incidence of obesity indicates the unsuccessful battle against this disorder. Obesity and the associated health problems urgently require effective strategies of treatment. The new discovery that a substantial amount of functional brown adipose tissue (BAT) is retained in adult humans provides a potential target for treatment of human obesity. BAT is active metabolically and disposes of extra energy via generation of heat through uncoupling oxidative phosphorylation in mitochondria. The physiology of BAT is readily regulated by melatonin, which not only increases recruitment of brown adipocytes but also elevates their metabolic activity in mammals. It is speculated that the hypertrophic effect and functional activation of BAT induced by melatonin may likely apply to the human. Thus, melatonin, a naturally occurring substance with no reported toxicity, may serve as a novel approach for treatment of obesity. Conversely, because of the availability of artificial light sources, excessive light exposure after darkness onset in modern societies should be considered a potential contributory factor to human obesity as light at night dramatically reduces endogenous melatonin production. In the current article, the potential associations of melatonin, BAT, obesity and the medical implications are discussed.

Effects of the melatonin MT-1/MT-2 agonist ramelteon on daytime body temperature and sleep

STUDY OBJECTIVES

A reduction in core temperature and an increase in the distal-proximal skin gradient (DPG) are reported to be associated with shorter sleep onset latencies (SOL) and better sleep quality. Ramelteon is a melatonin MT-1/MT-2 agonist approved for the treatment of insomnia. At night, ramelteon has been reported to shorten SOL. In the present study we tested the hypothesis that ramelteon would reduce core temperature, increase the DPG, as well as shorten SOL, reduce wakefulness after sleep onset (WASO), and increase total sleep time (TST) during a daytime sleep opportunity.

DESIGN

Randomized, double-blind, placebo-controlled, cross-over design. Eight mg ramelteon or placebo was administered 2 h prior to a 4-h daytime sleep opportunity.

SETTING

Sleep and chronobiology laboratory.

PARTICIPANTS

Fourteen healthy adults (5 females), aged (23.2 +/- 4.2 y).

MEASUREMENTS AND RESULTS

Primary outcome measures included core body temperature, the DPG and sleep physiology (minutes of total sleep time [TST], wake after sleep onset [WASO], and SOL). We also assessed as secondary outcomes, proximal and distal skin temperatures, sleep staging and subjective TST. Repeated measures ANOVA revealed ramelteon significantly reduced core temperature and increased the DPG (both P < 0.05). Furthermore, ramelteon reduced WASO and increased TST, and stages 1 and 2 sleep (all P < 0.05). The change in the DPG was negatively correlated with SOL in the ramelteon condition.

CONCLUSIONS

Ramelteon improved daytime sleep, perhaps mechanistically in part by reducing core temperature and modulating skin temperature. These findings suggest that ramelteon may have promise for the treatment of insomnia associated with circadian misalignment due to circadian sleep disorders.

Hypothermic effects of hops are antagonized with the competitive melatonin receptor antagonist luzindole in mice

Hops (Humulus lupulus, Cannabinaceae) has been used in traditional European medicine as a mild sedative for the treatment of anxiety, nervousness, and insomnia. However, there has been little information available about the underlying sleep inducing mechanism of hops. We have investigated the effects of a hops extract on the rectal body temperature in mice. Hops extract (250 mg kg−1) significantly decreased body temperature in male BL6/C57J mice (ΔT −0.75 ± 0.07°C) 2 h after oral administration. The effects of the plant extract were comparable with melatonin (50 mg kg−1; ΔT −0.66 ± 0.06°C; 2 h after i.p. injection). The hypothermic effects of melatonin and hops extract were antagonized with the competitive melatonin receptor antagonist luzindole. Thus, our data suggests that the hypothermic-and therefore the sleep-inducing-effects of hops extract are possibly mediated through activation of melatonin receptors.

The relationship between insomnia and body temperatures

Sleepiness and sleep propensity are strongly influenced by our circadian clock as indicated by many circadian rhythms, most commonly by that of core body temperature. Sleep is most conducive in the temperature minimum phase, but is inhibited in a "wake maintenance zone" before the minimum phase, and is disrupted in a zone following that phase. Different types of insomnia symptoms have been associated with abnormalities of the body temperature rhythm. Sleep onset insomnia is associated with a delayed temperature rhythm presumably, at least partly, because sleep is attempted during a delayed evening wake maintenance zone. Morning bright light has been used to phase advance circadian rhythms and successfully treat sleep onset insomnia. Conversely, early morning awakening insomnia has been associated with a phase advanced temperature rhythm and has been successfully treated with the phase delaying effects of evening bright light. Sleep maintenance insomnia has been associated not with a circadian rhythm timing abnormality, but with nocturnally elevated core body temperature. Combination of sleep onset and maintenance insomnia has been associated with a 24-h elevation of core body temperature supporting the chronic hyper-arousal model of insomnia. The possibility that these last two types of insomnia may be related to impaired thermoregulation, particularly a reduced ability to dissipate body heat from distal skin areas, has not been consistently supported in laboratory studies. Further studies of thermoregulation are needed in the typical home environment in which the insomnia is most evident.

Pharmacognostic and pharmacological profile of Humulus lupulus L

The present review describes the morphological, phytochemical and ethnopharmacological aspects of Humulus lupulus L. (Cannabinaceae) and summarizes the most interesting findings obtained in the preclinical and clinical research related to the plant. The female inflorescences of Humulus lupulus (hops), well-known as bittering agent in brewing industry, have long been used in traditional medicine mainly to treat sleep disturbances. However the sedative activity is still under investigation in order to recognize the active principles responsible for the neuropharmacological effects observed in laboratory animals, and their mechanism of action. Here we report the data from our experiments as well as those obtained from other researchers, focusing on the variability of the results. Other traditional applications of hops as stomachic, antibacterial and antifungal remedy have been supported by in vivo and/or in vitro investigations. In recent years some prenylated chalcones present in hops have received much attention for their biological effects: in particular, xanthohumol has been shown to exert cancer chemopreventive activity in in vitro experiments, while 8-prenylnaringenin has been characterized as one of the most potent phytoestrogens isolated until now. Nevertheless much additional work is needed to open up new biomedical application of these compounds.

Exogenous melatonin administration modifies cutaneous vasoconstrictor response to whole body skin cooling in humans

Humans and other diurnal species experience a fall in internal temperature (T(int)) at night, accompanied by increased melatonin and altered thermoregulatory control of skin blood flow (SkBF). Also, exogenous melatonin induces a fall in T(int), an increase in distal skin temperatures and altered control of the cutaneous active vasodilator system, suggesting an effect of melatonin on the control of SkBF. To test whether exogenous melatonin also affects the more tonically active vasoconstrictor system in glabrous and nonglabrous skin during cooling, healthy males (n = 9) underwent afternoon sessions of whole body skin temperature (T(sk)) cooling (water-perfused suits) after oral melatonin (Mel; 3 mg) or placebo (Cont). Cutaneous vascular conductance (CVC) was calculated from SkBF (laser Doppler flowmetry) and non-invasive blood pressure. Baseline T(int) was lower in Mel than in Cont (P < 0.01). During progressive reduction of T(sk) from 35 degrees C to 32 degrees C, forearm CVC was first significantly reduced at T(sk) of 34.33 +/- 0.01 degrees C (P < 0.05) in Cont. In contrast, CVC in Mel was not significantly reduced until T(sk) reached 33.33 +/- 0.01 degrees C (P < 0.01). The decrease in forearm CVC in Mel was significantly less than in Cont at T(sk) of 32.66 +/- 0.01 degrees C and lower (P < 0.05). In Mel, palmar CVC was significantly higher than in Cont above T(sk) of 33.33 +/- 0.01 degrees C, but not below. Thus exogenous melatonin blunts reflex vasoconstriction in nonglabrous skin and shifts vasoconstrictor system control to lower T(int). It provokes vasodilation in glabrous skin but does not suppress the sensitivity to falling T(sk). These findings suggest that by affecting the vasoconstrictor system, melatonin has a causal role in the nocturnal changes in body temperature and its control.

The thermophysiological cascade leading to sleep initiation in relation to phase of entrainment

This article reviews circadian thermoregulation in relation to sleep induction and phase of entrainment in the light of the comprehensive thermophysiological and chronobiological concepts of Jürgen Aschoff. The idea that temperature and sleep are interrelated is based on evolutionary history. Mammalian sleep developed in association with endothermy, and all species, independent of temporal niche, usually sleep during the circadian trough of their core body temperature (CBT) rhythm. The circadian pattern of CBT results from the balance between heat production and heat loss, the latter being relevant for sleep induction. Sleep under entrained conditions is typically initiated on the declining portion of the CBT curve when its rate of change and body heat loss is maximal. Body heat loss before lights off, via selective vasodilatation of distal skin regions, promotes sleepiness and the rapid onset of sleep. This thermophysiological effect represents the cement between the circadian clock and the sleep-wake cycle, and in turn determines phase of entrainment (Psi) and sleep onset latency (SOL). These interrelationships have been recently studied in a particular subset of the general population, mainly women, who suffer from cold hands and feet (the so-called vasospastic syndrome, VS). Women with VS exhibit not only a lower capacity to lose heat during the daytime but also a prolonged SOL, a disturbed Psi of the circadian clock with respect to the sleep-wake cycle and psychologically, a disposition to turn experienced anger inwards. This naturalistic model leads us to a more general conclusion that regulation of distal skin blood flow may have clinical relevance for insomnia, in particular sleep onset insomnia.

Ontogênese do sistema de temporização: a construção e as reformas dos ritmos biológicos ao longo da vida humana

Nesta revisão apresentamos fatos e comentários sobre a evolução dos sistemas de temporização ("relógios biológicos") na espécie humana. Na Introdução definimos alguns conceitos básicos da Cronobiologia que serão utilizados ao longo do artigo. Nas quatro seções subseqüentes, discutimos fatos marcantes que caracterizam a ritmicidade biológica em distintas etapas da ontogênese: bebês, adolescentes, adultos e idosos. Concluímos o artigo com um convite à reflexão sobre as perspectivas que se abrem com esse novo campo do conhecimento.

The human sleep–wake cycle reconsidered from a thermoregulatory point of view

Sleep is typically initiated on the declining portion of the circadian rhythm of core body temperature (CBT) when its rate of change, and body heat loss, is maximal. Distal vasodilatation plays a primary role in the circadian regulation of body heat loss and is strongly associated with sleepiness and sleep induction. In contrast, sleep (i.e. non-REM sleep and slow-wave activity, SWA) has no or only a minor thermoregulatory function. Two lines of evidence support this statement. First, detailed analyses of thermoregulatory changes before and after lights off show clearly that they start before stage 2 sleep begins. Second, accumulation of sleep pressure with increasing time awake, increases subjective sleepiness and SWA during the succeeding recovery night, but does not influence the thermoregulatory system. Taken together, the circadian modulation of sleepiness and sleep induction is clearly associated with thermoregulatory changes, but the thermoregulatory system seems to be independent of the sleepiness/sleep regulatory system. A simplified model is presented which attempts to explain the relationship between these two systems. It is based on the main hypothesis that all thermoregulatory effects which lead to an increase in the core/shell ratio (e.g. a reduced shell by increased distal skin temperature) lead to increased sleepiness and, as a consequence, to increased sleep propensity. However, the sleepiness/sleep regulatory system feeds back onto the thermoregulatory system only indirectly via sleep-related behaviors (e.g. relaxation, lying down).

Modification of cutaneous vasodilator response to heat stress by daytime exogenous melatonin administration

In humans, the nocturnal fall in internal temperature is associated with increased endogenous melatonin and with a shift in the thermoregulatory control of skin blood flow (SkBF), suggesting a role for melatonin in the control of SkBF. The purpose of this study was to test whether daytime exogenous melatonin would shift control of SkBF to lower internal temperatures during heat stress, as is seen at night. Healthy male subjects ( n = 8) underwent body heating with melatonin administration (Mel) or without (control), in random order at least 1 wk apart. SkBF was monitored at sites pretreated with bretylium to block vasoconstrictor nerve function and at untreated sites. Cutaneous vascular conductance, calculated from SkBF and arterial pressure, sweating rate (SR), and heart rate (HR) were monitored. Skin temperature was elevated to 38°C for 35–50 min. Baseline esophageal temperature (Tes) was lower in Mel than in control ( P < 0.01). The Tes threshold for cutaneous vasodilation and the slope of cutaneous vascular conductance with respect to Tes were also lower in Mel at both untreated and bretylium-treated sites ( P < 0.05). The Tes threshold for the onset of sweating and the Tes for a standard HR were reduced in Mel. The slope of the relationship of HR, but not SR, to Tes was lower in Mel ( P < 0.05). These findings suggest that melatonin affects the thermoregulatory control of SkBF during hyperthermia via the cutaneous active vasodilator system. Because control of SR and HR are also modified, a central action of melatonin is suggested.

The relationship between the rate of melatonin excretion and sleep consolidation for locomotive engineers in natural sleep settings

Background

The aim of the study was to examine the role that melatonin production plays in the regulation of sleep consolidation in a population of shiftworkers working and sleeping in their natural environments.

Methods

253 locomotive engineers (249 male, 4 female, mean age = 39.7 years) participated in the study for a 2-week period whilst working their normal roster patterns. Participants recorded details for all sleep periods in a sleep diary and collected urine samples during each day's main sleep period. The samples were subsequently assayed for the metabolite of melatonin in urine, 6-sulphatoxymelatonin (aMT6s), and the rate of excretion during main sleep periods was calculated.

Results

Separate one-way factorial ANOVAs revealed a significant effect of time of sleep onset on aMT6s excretion rate, sleep duration, and subjective sleep quality. Generally, the rate of aMT6s excretion was lower, sleep duration was shorter, and sleep quality was lower for sleeps initiated during the daytime than for sleeps initiated at night.

Conclusion

Combined with previous studies linking melatonin production and sleep propensity, and others demonstrating the relationship between sleep consolidation and melatonin production in forced desynchrony protocols, the current results indicate that low production of melatonin may play a role in the poor consolidation of daytime sleep in natural sleep settings.

Thermoregulatory effects of melatonin in relation to sleepiness

Thermoregulatory processes have long been implicated in the initiation of human sleep. In this paper, we review our own studies conducted over the last decade showing a crucial role for melatonin as a mediator between the thermoregulatory and arousal system in humans. Distal heat loss, via increased skin temperature, seems to be intimately coupled with increased sleepiness and sleep induction. Exogenous melatonin administration during the day when melatonin is essentially absent mimics the endogenous thermophysiological processes occurring in the evening and induces sleepiness. Using a cold thermic challenge test, it was shown that melatonin-induced sleepiness occurs in parallel with reduction in the thermoregulatory set-point (threshold); thus, melatonin may act as a circadian modulator of the thermoregulatory set-point. In addition, an orthostatic challenge can partially block the melatonin-induced effects, suggesting an important role of the sympathetic nervous system as a link between the thermoregulatory and arousal systems. A topographical analysis of finger skin temperature with infrared thermometry revealed that the most distal parts of the fingers, i.e., fingertips, represent the important skin regions for heat loss regulation, most probably via opening the arteriovenous anastomoses, and this is clearly potentiated by melatonin. Taken together, melatonin is involved in the fine-tuning of vascular tone in selective vascular beds, as circulating melatonin levels rise and fall throughout the night. Besides the role of melatonin as "nature's soporific", it can also serve as nature's nocturnal vascular modulator.

Caffeine: implications for alertness in athletes

Caffeine is one of the most widely consumed drugs in the world, taken socially and for its alertness- and performance-promoting actions. Extensive reports assert that caffeine increases alertness and cognitive performance levels and, when taken before exercise, demonstrates ergogenic properties. Caffeine ingestion has been associated with increased performance during endurance submaximal, and acute, high-intensity exercise. The exact mechanism of action for the performance effects of caffeine is unknown, although several physiologically and psychologically based theories exist as to how caffeine achieves increased performance capabilities. This paper outlines the known sites of caffeine activity in the body,and discusses these with respect to the effects of caffeine observed during performance assessments.

Time-of-day-dependent effects of bright light exposure on human psychophysiology: comparison of daytime and nighttime exposure

Bright light can influence human psychophysiology instantaneously by inducing endocrine (suppression of melatonin, increasing cortisol levels), other physiological changes (enhancement of core body temperature), and psychological changes (reduction of sleepiness, increase of alertness). Its broad range of action is reflected in the wide field of applications, ranging from optimizing a work environment to treating depressed patients. For optimally applying bright light and understanding its mechanism, it is crucial to know whether its effects depend on the time of day. In this paper, we report the effects of bright light given at two different times of day on psychological and physiological parameters. Twenty-four subjects participated in two experiments (n = 12 each). All subjects were nonsmoking, healthy young males (18-30 yr). In both experiments, subjects were exposed to either bright light (5,000 lux) or dim light <10 lux (control condition) either between 12:00 P.M. and 4:00 P.M. (experiment A) or between midnight and 4:00 A.M. (experiment B). Hourly measurements included salivary cortisol concentrations, electrocardiogram, sleepiness (Karolinska Sleepiness Scale), fatigue, and energy ratings (Visual Analog Scale). Core body temperature was measured continuously throughout the experiments. Bright light had a time-dependent effect on heart rate and core body temperature; i.e., bright light exposure at night, but not in daytime, increased heart rate and enhanced core body temperature. It had no significant effect at all on cortisol. The effect of bright light on the psychological variables was time independent, since nighttime and daytime bright light reduced sleepiness and fatigue significantly and similarly.

Melatonin and sleep in aging population

The neurohormone melatonin is released from the pineal gland in close association with the light-dark cycle. There is a temporal relationship between the nocturnal rise in melatonin secretion and the 'opening of the sleep gate' at night. This association, as well as the sleep promoting effect of exogenous melatonin, implicates the pineal product in the physiological regulation of sleep. Aging is associated with a significant reduction in sleep continuity and quality. A decreased production of melatonin with age is documented in a majority of studies. Diminished nocturnal melatonin secretion with severe disturbances in sleep/wake rhythm has been consistently reported in Alzheimer's disease (AD). A recent survey on the effects of melatonin in sleep disturbances, including all age groups, failed to document significant and clinically meaningful effects of exogenous melatonin on sleep quality, efficiency and latency. However, in clinical trials involving elderly insomniacs and AD patients suffering from sleep disturbances exogenous melatonin has repeatedly been found to be effective in improving sleep. The results indicate that exogenous melatonin is more effective to promote sleep in the presence of a diminished production of endogenous melatonin. A MT1/MT2 receptor analog of melatonin (ramelteon) has recently been introduced as a new type of hypnotics with no evidence of abuse or dependence.

Weak relationships between suppression of melatonin and suppression of sleepiness/fatigue in response to light exposure

In this paper we examine the relationship between melatonin suppression and reduction of sleepiness through light by comparing three different data sets. In total 36 subjects participated in three studies and received 4 h of bright light either from midnight till 4:00 hours (experiments A and B) or from noon till 16:00 hours (experiment C). In experiment A (night-time light, partial illumination of the retina, pupil dilated) subjects were exposed to either 100 lx of ocular light on the temporal, 100 lx on the nasal part of the retina, or <10 lx of dim light on the whole retina. In experiments B (night-time light, whole retina, pupil not dilated) and C (daytime light, whole retina, pupil not dilated) subjects were exposed either to bright (5000 lx) or to dim light (<10 lx). Subjective sleepiness/fatigue and melatonin concentrations in saliva were assessed hourly in all three experiments. For experiment A, a significant suppression of melatonin due to nasal and temporal illumination of the retina was found, that was not accompanied by a detectable reduction of subjective sleepiness/fatigue. For experiment B we found a suppression of melatonin that was paralleled with a significant reduction in subjective sleepiness, but not in fatigue. During experiment C we found no melatonin suppression but a reduction of subjective sleepiness, but also no effect on fatigue. From these data we conclude that the effects of light on sleepiness/fatigue are not mediated by melatonin and that the influence of endogenous melatonin concentration on sleepiness/fatigue is restricted.

Melatonin as a hypnotic: con

The physiological roles of melatonin are still unclear despite almost 50 years of research. Elevated melatonin levels from either endogenous nocturnal production or exogenous daytime administration are associated in humans with effects including increased sleepiness, reduced core temperature, increased heat loss and other generally anabolic physiological changes. This supports the idea that endogenous melatonin increases nocturnal sleep propensity, either directly or indirectly via physiological processes associated with sleep. The article "Melatonin as a hypnotic--Pro", also in this issue, presents evidence to support this viewpoint. We do not entirely disagree, but nevertheless feel this is an overly simplistic interpretation of the available data. Our interpretation is that melatonin is primarily a neuroendocrine transducer promoting an increased propensity for 'dark appropriate' behavior. Thus, it is our view that exogenous melatonin is only hypnotic in those species or individuals for which endogenous melatonin increases sleep propensity and is consequently a dark appropriate outcome. Evidence supporting this position is drawn primarily from studies of exogenous administration of melatonin and its varied effects on sleep/wake behavior based on dose, time of administration, age and other factors. From this perspective, it will be shown that melatonin can exert hypnotic-like effects but only under limited circumstances.

Cutaneous warming promotes sleep onset

Sleep occurs in close relation to changes in body temperature. Both the monophasic sleep period in humans and the polyphasic sleep periods in rodents tend to be initiated when core body temperature is declining. This decline is mainly due to an increase in skin blood flow and consequently skin warming and heat loss. We have proposed that these intrinsically occurring changes in core and skin temperatures could modulate neuronal activity in sleep-regulating brain areas (Van Someren EJW, Chronobiol Int 17: 313–54, 2000). We here provide results compatible with this hypothesis. We obtained 144 sleep-onset latencies while directly manipulating core and skin temperatures within the comfortable range in eight healthy subjects under controlled conditions. The induction of a proximal skin temperature difference of only 0.78 ± 0.03°C (mean ± SE) around a mean of 35.13 ± 0.11°C changed sleep-onset latency by 26%, i.e., by 3.09 minutes [95% confidence interval (CI), 1.91 to 4.28] around a mean of 11.85 min (CI, 9.74 to 14.41), with faster sleep onsets when the proximal skin was warmed. The reduction in sleep-onset latency occurred despite a small but significant decrease in subjective comfort during proximal skin warming. The induction of changes in core temperature (δ = 0.20 ± 0.02°C) and distal skin temperature (δ = 0.74 ± 0.05°C) were ineffective. Previous studies have demonstrated correlations between skin temperature and sleep-onset latency. Also, sleep disruption by ambient temperatures that activate thermoregulatory defense mechanisms has been shown. The present study is the first to experimentally demonstrate a causal contribution to sleep-onset latency of skin temperature manipulations within the normal nocturnal fluctuation range. Circadian and sleep-appetitive behavior-induced variations in skin temperature might act as an input signal to sleep-regulating systems.

Enhanced heat loss and age-related hypersensitivity to diazepam

Whether elderly people suffer from age-related changes in pharmacokinetics and/or pharmacodynamics with administration of benzodiazepines is still a matter of controversy. We investigated the course of brain function and thermoregulation after oral administration of a standard benzodiazepine, diazepam (DZP), in 8 healthy young men (mean age, 19.8 years; range, 18 to 23 years) and 8 healthy middle-aged and older men (mean age, 60.9 years; range, 53 to 71 years). Placebo or DZP was administered in a single-blind crossover manner to the young men (placebo, 5-mg, 10-mg DZP) and to the older men (placebo, 5-mg DZP), and plasma DZP concentration, choice reaction time, proximal body temperature, and distal body temperature were monitored with high time resolution under a modified constant routine condition to exclude masking effects. Whereas there was no evidence of age-related alterations in pharmacokinetics between the 2 groups, the older subjects, in comparison to the young subjects, showed a more delayed choice reaction time in response to the same plasma DZP level, suggesting that hypersensitivity is related to increased age. DZP at 5 mg in the older subjects induced acute and transient hypothermia to the same degree as that induced by DZP at 10 mg in the young subjects. The distal-proximal body temperature gradient (difference between distal body temperature and proximal body temperature), an indicator of blood flow in distal skin regions, showed strong positive correlation with the delay in choice reaction time in both groups. These findings suggest that hypersensitivity to benzodiazepine in older persons may be due, at least in part, to age-related changes in thermoregulation, especially in the heat loss process.

Relationships between the circadian rhythms of finger temperature, core temperature, sleep latency, and subjective sleepiness

Skin temperature circadian rhythms have been explored relatively recently. It has been suggested that distal and proximal skin temperature changes play a role in the regulation of the core temperature circadian rhythm and sleepiness. The authors investigated the circadian finger and core temperature rhythms in conjunction with the circadian rhythms of subjective and objective sleepiness. Fourteen healthy, young, good sleepers participated in a modified constant-routine procedure in which palmar finger temperature, rectal temperature, subjective sleepiness, and objective sleep latency were measured half-hourly across a 48-h period of enforced wakeful bed rest. Individual curves were adjusted to the group mean temperature minimum time of 0500 h and averaged to create the 4 mean curves. The 5 possible cross-correlation curves between these 4 measures were calculated for half-hourly phase lags from 12 h before to 12 h after the group mean core temperature minimum time. Maximum cross-correlations for each curve suggested that finger temperature preceded core temperature by 3 h (r = -0.22), and subjective sleepiness followed core temperature by 0.5 h (r = -0.33) and objective sleepiness by 2 h (r = 0.29). Although these data are correlational, they are consistent with the notion that finger temperature changes drive core temperature changes, which determine changes of subjective and objective sleepiness.

A single blind, placebo controlled, across groups dose escalation study of the safety, tolerability, pharmacokinetics and pharmacodynamics of the melatonin analog beta-methyl-6-chloromelatonin

Clinical investigation of melatonin agonists has been hampered by side effects such as hypothermia, hypotension and bradycardia. The availability of a melatonin agonist devoid of these side effects would improve our understanding of the mechanisms by which melatonin agonists affect sleep. This study investigated the pharmacokinetics, pharmacodynamics and safety of the melatonin agonist beta-methyl-6-chloromelatonin at doses up to 100 mg in healthy volunteers. The design was a single blind, across subjects, placebo controlled, group wise dose escalation using doses of 20, 35, 50 and 100 mg beta-methyl-6-chloromelatonin. Eight subjects received one dose of study drug or placebo. Pharmacokinetic analysis showed a consistent Tmax across all doses with a mean of 1.12 +/- 0.11 hr for all groups (mean +/- SD). The half-life was also consistent across dose, with a mean of 1.04 +/- 0.04 hr. Maximum plasma concentrations increased with increasing dose with values of 44.83 +/- 29.79, 100.3 +/- 41.08, 79.84 +/- 26.36 and 410.3 +/- 129.4 ng/ml at doses of 20, 35, 50 and 100 mg, respectively. Area under the curve showed similar increases. No consistent changes in vital signs occurred as a function of dose or time after study drug. The incidence of all adverse events, the severity of the event or the event's relationship to treatment did not increase with higher doses of beta-methyl-6-chloromelatonin. Sleepiness was reported after all doses of beta-methyl-6-chloromelatonin. beta-methyl-6-chloromelatonin appears safe and well tolerated at doses up to 100 mg. These doses are not associated with hypothermia, bradycardia or hypotension. A melatonin agonist lacking these side effects should allow investigation of the direct soporific effects of melatonin agonists.

Thermoregulation as a sleep signalling system

Temperature and sleep are interrelated processes. Under normal environmental conditions, the rhythms of core body temperature Tc and sleep propensity vary inversely across the day and night in healthy young adults. Although this relationship has drawn considerable interest, particularly in recent years, it is still not known whether this relationship is causative or merely coincidental. As somnogenic brain areas contain thermosensitive cells, it is possible that the sleep/wake cycle may be directly affected by thermoregulatory changes themselves. That is, that changes in temperature may trigger, either directly or indirectly, somnogenic brain areas to initiate sleep. There is now an emerging body of evidence from both physiological and neuroanatomical studies to indicate that this may indeed be the case. This paper will examine the literature relating to this relationship and propose a model where thermoregulatory changes provide an additional signal to the brain regions that regulate sleep and wakefulness. The model attempts to explain how temperature changes before and after sleep onset act in a positive feedback loop to maintain a consolidated sleep bout.

The relevance of melatonin to sports medicine and science

The pineal hormone, melatonin, has widespread effects on the body. The aim of this review is to consider the specific interactions between melatonin and human physiological functions associated with sport and exercise medicine. Separate researchers have reported that melatonin concentrations increase, decrease and remain unaffected by bouts of exercise. Such conflicting findings may be explained by inter-study differences in lighting conditions and the time of day the study participants have exercised. Age and fitness status have also been identified as intervening factors in exercise-mediated changes in melatonin concentration. The administration of exogenous melatonin leads to hypnotic and hypothermic responses in humans, which can be linked to immediate reductions in short-term mental and physical performance. Depending on the dose of melatonin, these effects may still be apparent 3-5 hours after administration for some types of cognitive performance, but effects on physical performance seem more short-lived. The hypothesis that the hypothermic effects of melatonin lead to improved endurance performance in hot environments is not supported by evidence from studies involving military recruits who exercised at relatively low intensities. Nevertheless, no research group has examined such a hypothesis with athletes as study participants and with the associated more intense levels of exercise. The fact that melatonin has also been found to preserve muscle and liver glycogen in exercised rats adds weight to the notion that melatonin might affect endurance exercise in humans. Melatonin has been successfully used to alleviate jet lag symptoms of travellers and there is also a smaller amount of evidence that the hormone helps shiftworkers adjust to nocturnal regimens. Nevertheless, the symptoms of jet lag and shiftwork problems have primarily included sleep characteristics rather than performance variables. The few studies that have involved athletes and performance-related symptoms have produced equivocal results. Melatonin has also been found to be useful for treating some sleeping disorders, but interactions between sleep, melatonin and exercise have not been studied extensively with trained study participants. It is unknown whether melatonin plays a role in some exercise training-related problems such as amenorrhoea and over-training syndrome.

Comparison of digital infrared thermal imaging (DITI) with contact thermometry: pilot data from a sleep research laboratory

Body temperature regulation is associated with changes in sleep propensity; therefore, sleep research often necessitates concomitant assessment of core and skin surface temperatures. Attachment to thermistors may limit the range of movement and comfort, introducing a potential confound that may prolong sleep initiation or increase wakefulness after sleep onset. It has been suggested that contact thermometry may artificially increase temperatures due to insulation. We report here on a method of remote sensing skin temperatures using a digital infrared thermal imaging (DITI) system, which can reduce these potential confounds. Using data from four healthy young adult volunteers (age = 26.8 +/- 2.2 years; mean +/- SEM), we compared measures of skin temperature using a DITI system with contact thermometry methods already in use in our sleep laboratory. A total of 416 skin temperature measurements (T(sk)) were collected from various sites, resulting in an overall correlation coefficient of R = 0.99 (p < 0.0001) between both methods. Regression analyses for individuals resulted in correlation coefficients between 0.80 and 0.97. These pilot results suggest that DITI can assess skin surface temperatures as accurately as contact thermometry, provided the interest is in relative and not absolute temperature changes. This and some other important limitations are discussed in more detail hereafter.

Heat loss, sleepiness, and impaired performance after diazepam administration in humans

In spite of the accumulation of knowledge regarding the neuropharmacological action of benzodiazepines (Bz), the physiological process by which their sedative/hypnotic effects are induced remains poorly understood. We conducted a single-blind, crossover trial to evaluate the role of the thermoregulatory process in sleepiness and impaired psychomotor performance induced by a standard Bz, diazepam (DZP). Each of the eight healthy young male volunteers (mean age, 19.75 years; range, 18-23 years) was given a single oral dose of either 5 or 10 mg of DZP or placebo 12 h after his average sleep onset time. Changes in plasma DZP concentration, proximal body temperature (p-BT), distal body temperature (d-BT), subjective sleepiness measured by the Visual Analog Scale and Stanford Sleepiness Scale, and psychomotor performance measured by Choice Reaction Time were monitored under a modified constant routine condition in which various factors affecting thermoregulation, alertness, and psychomotor performances were strictly controlled. Orally administered DZP induced a significant transient decrease in p-BT and psychomotor performance as well as an increase in d-BT and subjective sleepiness. Distal-p-BT gradient (DPG; difference between d-BT and p-BT), which is an indicator of blood flow in distal skin regions, showed a strong positive correlation with the plasma DZP concentration, indicating that DZP in clinical doses promotes heat loss in a dose-dependent manner. The DPG also correlated positively with the magnitude of subjective sleepiness and impaired psychomotor performance. These findings indicate that the sedative/hypnotic effects of Bz could be due, at least in part, to changes in thermoregulation, especially in the process of heat loss, in humans.

Melatonin: clinical relevance

This chapter reviews the neural connections between the retinas and the pineal gland and summarizes the role of the light:dark cycle and the biological clock, i.e. the suprachiasmatic nuclei, in regulating pineal melatonin synthesis and secretion. The cellular mechanisms governing the nocturnal production of melatonin are described together with the way in which the misuse of light interferes with the circadian melatonin cycle and the total quantity of the indole generated. The chapter describes the nature of the membrane melatonin receptors and their signal transduction mechanisms in peripheral organs. The clinical implications and potential uses of melatonin in terms of influencing the biological clock (e.g. sleep and jet lag), immune function, and cancer initiation and growth are noted. Additionally, the chapter includes a description of the newly discovered free radical scavenging and antioxidant activities of melatonin; it also includes a list of clinical situations in which melatonin has been used with beneficial effects.