Abstract: Human beings exhibit wide variation in their timing of daily behavior. We and others have suggested previously that such differences might arise because of alterations in the period length of the endogenous human circadian oscillator. Using dermal fibroblast cells from skin biopsies of 28 subjects of early and late chronotype (11 "larks" and 17 "owls"), we have studied the circadian period lengths of these two groups, as well as their ability to phase-shift and entrain to environmental and chemical signals. We find not only period length differences between the two classes, but also significant changes in the amplitude and phase-shifting properties of the circadian oscillator among individuals with identical "normal" period lengths. Mathematical modeling shows that these alterations could also account for the extreme behavioral phenotypes of these subjects. We conclude that human chronotype may be influenced not only by the period length of the circadian oscillator, but also by cellular components that affect its amplitude and phase. In many instances, these changes can be studied at the molecular level in primary dermal cells.

Abstract: Previous research suggests a possible link between eveningness and general difficulties with self-regulation (e.g., evening types are more likely than other chronotypes to have irregular sleep schedules and social rhythms and use substances). Our study investigated the relationship between eveningness and self-regulation by using two standardized measures of self-regulation: the Self-Control Scale and the Procrastination Scale. We predicted that an eveningness preference would be associated with poorer self-control and greater procrastination than would an intermediate or morningness preference. Participants were 308 psychology students (mean age=19.92 yrs) at a small Canadian college. Students completed the self-regulation questionnaires and Morningness/Eveningness Questionnaire (MEQ) online. The mean MEQ score was 46.69 (SD=8.20), which is intermediate between morningness and eveningness. MEQ scores ranged from definite morningness to definite eveningness, but the dispersion of scores was skewed toward more eveningness. Pearson and partial correlations (controlling for age) were used to assess the relationship between MEQ score and the Self-Control Scale (global score and 5 subscale scores) and Procrastination Scale (global score). All correlations were significant. The magnitude of the effects was medium for all measures except one of the Self-Control subscales, which was small. A multiple regression analysis to predict MEQ score using the Self-Control Scale (global score), Procrastination Scale, and age as predictors indicated the Self-Control Scale was a significant predictor (accounting for 20% of the variance). A multiple regression analysis to predict MEQ scores using the five subscales of the Self-Control Scale and age as predictors showed the subscales for reliability and work ethic were significant predictors (accounting for 33% of the variance). Our study showed a relationship between eveningness and low self-control, but it did not address whether the relationship is a causal one.

Abstract: Very few studies have focused on differences in chronotype around the globe. In this study, morningness-eveningness was measured using the Composite Scale of Morningness and the midpoint of sleep on free days (corrected for sleep debt, MSF(sc)) in adolescents of sixteen German schools abroad, dispersed all over the world, and for comparison pupils attending German schools. There was no influence of duration of residence in the respective locality, and sex differences were weak. Age correlated negatively with morningness. A significant influence of climate zone (temperate, subtropics, tropics) was found, with adolescents in the subtropics being the latest chronotypes, and an interaction of agexclimate zone was identified. Significant associations between chronotype and longitude and latitude were identified within the time zone of central Europe. Adolescents were found to be more morning oriented both toward the East and North. The results indicate that climate, longitude, and latitude contribute to chronotype.

Abstract: Light and dark patterns are the major synchronizer of circadian rhythms to the 24-hour solar day. Disruption of circadian rhythms has been associated with a variety of maladies. Ecological studies of human exposures to light are virtually nonexistent, however, making it difficult to determine if, in fact, light-induced circadian disruption directly affects human health. A newly developed field measurement device recorded circadian light exposures and activity from day-shift and rotating-shift nurses. Circadian disruption defined in terms of behavioral entrainment was quantified for these two groups using phasor analyses of the circular cross-correlations between light exposure and activity. Circadian disruption also was determined for rats subjected to a consistent 12-hour light/12-hour dark pattern (12L:12D) and ones subjected to a "jet-lagged" schedule. Day-shift nurses and rats exposed to the consistent light-dark pattern exhibited pronounced similarities in their circular cross-correlation functions and 24-hour phasor representations except for an approximate 12-hour phase difference between species. The phase difference reflects the diurnal versus nocturnal behavior of humans versus rodents. Phase differences within species likely reflect chronotype differences among individuals. Rotating-shift nurses and rats subjected to the "jet-lagged" schedule exhibited significant reductions in phasor magnitudes compared to the day-shift nurses and the 12L:12D rats. The reductions in the 24-hour phasor magnitudes indicate a loss of behavioral entrainment compared to the nurses and the rats with regular light-dark exposure patterns. This paper provides a quantitative foundation for systematically studying the impact of light-induced circadian disruption in humans and in animal models. Ecological light and activity data are needed to develop the essential insights into circadian entrainment/disruption actually experienced by modern people. These data can now be obtained and analyzed to reveal the interrelationship between actual light exposures and markers of circadian rhythm such as rest-activity patterns, core body temperature, and melatonin synthesis. Moreover, it should now be possible to bridge ecological studies of circadian disruption in humans to parametric studies of the relationships between circadian disruption and health outcomes using animal models.

Abstract: Objectives: Although a broad range of circadian rhythm variations have been reported, analyses of chronotypes in bipolar I disorder (BDI) and schizophrenia are rare. The present study aimed to investigate specific chronotype patterns in BDI and schizophrenia compared with healthy subjects. Methods: All patients were clinically stable and recruited from the outpatient clinics of Seoul National University Hospital. They were diagnosed according to DSM-IV criteria. ‘Morningness/eveningness’, a chronotype correlated with circadian phase, was evaluated using the Composite Scale of Morningness (CSM) among 92 patients with BDI, 113 patients with schizophrenia (SZ), and 95 control individuals. Results: The CSM scores were significantly correlated with age in the control group, but a significant correlation with age was not observed in the schizophrenia or BDI groups. After controlling for age, there were significant differences between the BDI and control groups. The SZ patients did not differ from the BDI or control groups. Conclusions: In this Korean sample, patients with BDI showed a significantly greater preference for ‘eveningness’ (including delayed sleep timing) than control individuals. The influences of pharmacotherapy or clinical status on CSM scores need to be clarified.

Abstract: Sleep inertia is the impaired cognitive performance immediately upon awakening, which decays over tens of minutes. This phenomenon has relevance to people who need to make important decisions soon after awakening, such as on-call emergency workers. Such awakenings can occur at varied times of day or night, so the objective of the study was to determine whether or not the magnitude of sleep inertia varies according to the phase of the endogenous circadian cycle. Twelve adults (mean, 24 years; 7 men) with no medical disorders other than mild asthma were studied. Following 2 baseline days and nights, subjects underwent a forced desynchrony protocol composed of seven 28-h sleep/wake cycles, while maintaining a sleep/wakefulness ratio of 1:2 throughout. Subjects were awakened by a standardized auditory stimulus 3 times each sleep period for sleep inertia assessments. The magnitude of sleep inertia was quantified as the change in cognitive performance (number of correct additions in a 2-min serial addition test) across the first 20 min of wakefulness. Circadian phase was estimated from core body temperature (fitted temperature minimum assigned 0°). Data were segregated according to: (1) circadian phase (60° bins); (2) sleep stage; and (3) 3rd of the night after which awakenings occurred (i.e., tertiary 1, 2, or 3). To control for any effect of sleep stage, the circadian rhythm of sleep inertia was initially assessed following awakenings from Stage 2 (62% of awakening occurred from this stage; n = 110). This revealed a significant circadian rhythm in the sleep inertia of cognitive performance (p = 0.007), which was 3.6 times larger during the biological night (circadian bin 300°, ~2300–0300 h in these subjects) than during the biological day (bin 180°, ~1500–1900 h). The circadian rhythm in sleep inertia was still present when awakenings from all sleep stages were included (p = 0.004), and this rhythm could not be explained by changes in underlying sleep drive prior to awakening (changes in sleep efficiency across circadian phase or across the tertiaries), or by the proportion of the varied sleep stages prior to awakenings. This robust endogenous circadian rhythm in sleep inertia may have important implications for people who need to be alert soon after awakening.

Abstract: Only a few studies focus on comparisons to reveal differences in sleep and circadian preferences in adolescents. This study used the same instrument to compare adolescents in Eastern and Western Germany. In all, 674 pupils between 11 and 16 yrs participated. The questionnaire asked questions about wake times and bed times (on weekend and weekdays), and the Composite Scale of Morningness (CSM) was completed to assess diurnal preferences. Locality (East/West Germany) had no effect on rise and bed times during the week and on sleep length on the weekend. Western pupils rose later on weekends and went to bed later on weekends. While sleep length on weekdays was shorter in West Germany, weekend oversleep was longer and misalignment was higher. Diurnal preferences (CSM scores) suggested a higher eveningness in West Germany. As interaction effects were insignificant, changes throughout adolescence seem similar in East and West Germany. These data suggest that given similar school start times, Western German pupi...

Abstract: Inter-individual differences in the phase of the endogenous circadian rhythms have been established. Individuals with early circadian phase are called morning types; those with late circadian phase are evening types. The Horne and Ostberg Morningness-Eveningness Questionnaire (MEQ) is the most frequently used to assess individual chronotype. The distribution of MEQ scores is likely to be biased by several fact, ors, such as gender, age, genetic background, latitude, and social habits. The objective of the present study was to determine the effect of different social synchronizers on the sleep/wake cycle of persons with different chronotypes. Volunteers were selected from a total of 1232 UFPR undergraduate students who completed the MEQ. Thirty-two subjects completed the study, including 8 morning types, 8 evening types and 16 intermediate types. Sleep schedules were recorded by actigraphy for 1 week on two occasions: during the school term and during vacation. Sleep onset and offset times, sleep duration, and mid-sleep time for each chronotype group were compared by the Mann-Whitney U-test separately for school term and vacation. School term and vacation data were compared by the Wilcoxon matched-pair test. Morning types showed earlier sleep times and longer sleep duration compared with evening types (23:00 ± 44 and 508.9 ± 50.27 vs 01:08 ± 61.95 and 456.44 ± 59.08, for the weekdays during vacation). During vacation, the subjects showed later sleep times, except for the morning types, who did not exhibit differences for sleep onset times. The results support the idea that social schedules have an impact on the expression of circadian rhythmicity but this impact depends on the individual chronotype.

Abstract: During adolescence and early adulthood, most humans are predisposed developmentally, both biologically and socially, toward evening/night activity. The morningness-eveningness (M-E) tendency to be an evening-preferring (E-type) rather than a morning-preferring (M-type) or intermediate/neither (N-type) "chronotype" may affect athletic performance at various times of day. This study evaluated M-E effects on rowing performance of an intact, experienced, university club crew with near-daily early morning (0500-0700 hours) and late afternoon (1630-1800 hours) training schedules. The hypothesis tested was that chronotype would modify circadian effects during morning and afternoon performances. Eight men and eight women (mean age 19.6 +/- 1.5 years) were tested in a randomized, counterbalanced design. A standard qualifying 2000-m ergometer rowing sprint and a nonroutine standing broad jump task were measured during early morning and late afternoon, separated by 3 days of rest. Each subject's chronotype was determined using two standard self-rating M-E scales, resulting in eight E-type (three women/five men), four M-type (two women/two men), and four N-type (three women/one man) subjects. The rowing results show that E-type and N-type subjects did not differ between morning and afternoon rowing performances, whereas M-type subjects rowed significantly faster in the morning. In contrast, the standing broad jump showed no consistent time-of-day or chronotype effect. These findings suggest that basic performance timing in young athletes is determined to some extent by naturally occurring M-E predispositions. Further, modification of time-of-day influences may be possible by routine practice at the same time each day, as was suggested here by the absence of evening superiority in performances. Understanding their personal M-E tendencies could allow young athletes to arrange training schedules at specific times of day to help counteract any natural circadian influences that might work against their performance.

Abstract: Some factors influencing smoking habits have been identified, but chronotype or the misalignment of social and biological time has rarely been examined. In this study, chronotype was examined and it was found that on average, smokers consumed more coffee, more alcohol, and had less developed breakfast habits compared with nonsmokers. Smokers tended towards eveningness and had a greater difference in wake-up times between week days and free days. Using a canonical discriminant function it was found that breakfast habits, coffee and alcohol consumption, total Composite Scale of Morningness (CSM) score, and weekend sleep-in separated smokers from nonsmokers, while age and gender were excluded. As late chronotypes do not fit well into conventional social and working schedules, this relationship can be interpreted as a behavioral symptom of the inability to cope with social demands.

Abstract: Evidence suggests that advancing currently limited knowledge about self-reported sleep onset latency is important for better understanding insomnia. Relatively little research has been conducted to understand factors that affect sleep onset latency. The current study tested a hypothesized role of factors of the global anxiety sensitivity (AS) construct as moderators of the relation between sleep onset latency and physical and cognitive components of sleep anticipatory anxiety. As hypothesized, AS-Physical Concerns moderated the relation between the physical component of sleep anticipatory anxiety and sleep onset latency, even after controlling for age, gender, negative affect, and substance use variables. However, in contrast to prediction, AS-Mental Incapacitation Concerns did not appear to moderate the relation between sleep onset latency and the cognitive component of sleep anticipatory anxiety. These findings are discussed in terms of extant research on sleep onset latency, and future directions for research to advance this body of knowledge are considered. Keywords: sleep onset latency; sleep anticipatory anxiety; anxiety sensitivity ; moderator Latency to sleep onset and problems maintaining sleep are two of the main components of primary insomnia, which affects 30%-35% of adults in the United States (Breslau, Roth, Rosenthal, & Andreski, 1996). Factors related to sleep maintenance have been the focus of several empirical studies (see Benca, 2005 for a review); however, relatively less is known about sleep onset latency. This relatively underdeveloped literature is noteworthy as subjective reports (cf., objective measurement) of sleep onset latency have been linked to primary insomnia. For instance, in a relatively early study in this domain, individuals with insomnia overestimated sleep onset by an average of 36 minutes, in comparison to normal sleepers who overestimated their sleep onset by 0.5 minutes (Moore, Bonnet, & Warm 1981 ). Indeed, overestimation of sleep onset latency is associated with increased rates of primary insomnia (Downey & Bonnet, 1992). This research suggests that examination of factors associated with self-reported sleep onset latency may advance understanding of insomnia and related sleep problems. One factor that has been the subject of empirical attention in terms of sleep onset latency is anxiety. Broadly, anxiety occurring upon entering bed may be associated with self-reported delayed sleep onset, which may further increase presleep anxiety, increasing reported sleep onset latency, and so on. This vicious cycle may ultimately culminate in primary insomnia. In fact, anxious cognitive arousal, such as worry (Borkovec, 1982), and anxiety about obtaining insufficient sleep (Lundh, 1998) have been linked to the maintenance of insomnia (Harvey, 2002). In a related way, physiological arousal at sleep onset has been linked to problems with sleep onset and sleep maintenance (Frankel, Buchbinder, Coursey, & Snyder, 1973; Monroe, 1967). In a series of experiments, Tang and Harvey (2004) manipulated cognitive and physiological arousal and measured the effects on perception of several sleep-related factors, including sleep onset latency and total sleep time. In one experiment, participants were instructed to take a nap in a sleep research laboratory. They also were told that upon waking they would have to either give a speech (anxious cognitive arousal), complete an essay (neutral cognitive arousal), or neither. In a separate experiment, physiological arousal was manipulated via administration of 5 mg/kg of caffeine as compared to a placebo control, prior to instructions to nap. Measures of sleep onset latency and total sleep time were obtained through actigraph measures as well as self-report upon awakening. Participants in both the anxious cognitive and increased physiological arousal groups had greater self-reported sleep onset latency in comparison to the nonmanipulated group. …

Abstract: Insomnia and excessive sleepiness are common in the investigation of sleep-disordered breathing. Circadian rhythm sleep disorders are perhaps the most often overlooked conditions in the differential diagnosis of these symptoms. Circadian rhythm sleep disorders manifest as misalignment between the sleep period and the physical/social 24-h environmental cycle. The two most prevalent circadian rhythm sleep disorders are delayed sleep phase (common in adolescents) and advanced sleep phase (common in the elderly), situations in which the sleep period is displaced to a later or earlier time, respectively. It is important to keep these two disorders in mind, since they can be confused with insomnia and excessive sleepiness. However, there are nine possible diagnoses, and all nine are of clinical interest. Since light is the principal cue used in synchronizing the biological clock, blind individuals and night-shift/swing-shift workers are more prone to develop circadian rhythm sleep disorders. In this article, the new international classification of circadian rhythm sleep disorders is reviewed.

Abstract: Summary Objective Adults patients previously treated for craniopharyngioma have increased general and physical fatigue compared to healthy controls. This could be related to disturbed sleep patterns. The aim of this study was to compare sleepiness and sleep patterns in those patients to healthy controls and to patients treated for nonfunctioning macroadenomas (NFMA) of the pituitary. Design Case-control study. Patients and methods Sleepiness and sleep patterns were assessed in 27 adult patients [14 men, 8 patients diagnosed at childhood age, mean age of 53 years (range 27–80 year)] after long-term follow-up and compared to 50 healthy controls and 38 age-, gender- and BMI-matched patients with NFMA. We used two validated questionnaires for sleep parameters (Epworth sleepiness score and Munchener Chronotype Questionnaire). Results Sleep patterns (onset, sleep timing, duration and rise time) were not statistically different between the three groups. However, daytime sleepiness scores were increased in patients treated for craniopharyngioma compared to healthy controls, but not different from patients with NFMA. Thirty-three percent of patients with craniopharyngiomas had ESS scores above 10 compared to 8% of healthy controls (P = 0·005), indicating severe daytime hypersomnolence. Neither type of surgery, previous radiotherapy, or age at diagnosis influenced the sleepiness scores in patients with craniopharyngioma. Conclusion Patients treated for craniopharyngioma or NFMA have increased daytime somnolence despite normal sleep patterns, compared to healthy subjects. The results indicate that increased daytime somnolence is a general consequence of large tumours, and/or their treatment, in the hypothalamic/pituitary region, rather than a specific feature of craniopharyngiomas per se.

Abstract: The relationship between actigraphy- or diary-based sleep parameters and salivary melatonin-based dim light melatonin onsets (DLMOs) was examined in 21 adolescents with a history of substance abuse and current complaints of sleep difficulties. The adolescents displayed relationships between diary-based sleep times and DLMO that were of comparable strength with those reported for adult insomniacs and healthy adolescents during the school year, but weaker than those observed in healthy adults and healthy adolescents on summer vacation. When the sample was divided into adolescents with late and early DLMOs, the 2 groups had significantly different phase angles between DLMO and sleep variables but no other significant differences in sleep parameters. However, circadian phase and its relationship to sleep may have sleep and behavioral consequences.

Abstract: Studies have shown a time-of-day of training effect on long-term explicit memory with a greater effect being shown in the afternoon than in the morning. However, these studies did not control the chronotype variable. Therefore, the purpose of this study was to assess if the time-of-day effect on explicit memory would continue if this variable were controlled, in addition to identifying the occurrence of a possible synchronic effect. A total of 68 undergraduates were classified as morning, intermediate, or afternoon types. The subjects listened to a list of 10 words during the training phase and immediately performed a recognition task, a procedure which they repeated twice. One week later, they underwent an unannounced recognition test. The target list and the distractor words were the same in all series. The subjects were allocated to two groups according to acquisition time: a morning group (N = 32), and an afternoon group (N = 36). One week later, some of the subjects in each of these groups were subjected to a test in the morning (N = 35) or in the afternoon (N = 33). The groups had similar chronotypes. Long-term explicit memory performance was not affected by test time-of-day or by chronotype. However, there was a training time-of-day effect [F (1,56) = 53.667; P = 0.009] with better performance for those who trained in the afternoon. Our data indicated that the advantage of training in the afternoon for long-term memory performance does not depend on chronotype and also that this performance is not affected by the synchronic effect.

Abstract: Individual differences in circadian rhythm have been studied since the past century. Chronotypes are a chronobiology classification based on the preferential times for beginning and ending activities throughout the day. Chronotypes can be classified as definitely morning, moderately morning, indifferent, moderately evening, and definitely evening. We aim to assess the distribution of chronotypes in asthmatics and the relationship of chronotype to the presence of nocturnal symptoms. Two hundred subjects were evaluated, 100 asthmatics and 100 non-asthmatics. The Morningness/Eveningness questionnaire was applied for chronotype determination. The asthmatics were subdivided according to the presence or absence of nocturnal symptoms. The chronotype distribution did not differ significantly between asthmatics and non-asthmatics. Thirty-five percent of the asthma group reported nocturnal symptoms. There was a significant difference in chronotype distribution between asthmatics with and without nocturnal worsening...

Abstract: Studies of morning–evening preference consistently recognize age differences, but gender differences are rarely reported. To examine these differences, we applied the Sleep–Wake Pattern Assessment Questionnaire (Putilov 1990, 2000, 2007). Unlike unidimensional morningness–eveningness scales, it includes separate scales for the measurement of Evening and Morning lateness (E and M, respectively). Data on 7465 adolescents and adults suggest that both E and M scores significantly decline with age. Genders are identical on the sum of E and M scores, but female respondents of any age cohort have a lower E score and a higher M score. Differences between these scores remain constant across ages. Analysis of a subset of data on 2016 adults revealed significant correlation of this difference with self-reported sleep length which, however, is not significantly longer in women. We concluded that, compared to men, women prefer earlier bedtimes and later wake up times and can desire but not obtain more sleep.

Abstract: Humor research has focused on relationships between humor and various personality traits. As personality and morningness-eveningness, as well as personality and humor, are related based on genetics and neurobehavioral function, one might also expect a relationship between humor and chronotype. 197 students responded to the Composite Scale of Morningness as a measure of chronotype, the Sense of Humor Questionnaire and a 10-item version of the Big Five Inventory. Individuals scoring as evening types reported a greater sense of humor than morning individuals, with higher morningness scores. In a stepwise linear regression, Extraversion, Agreeableness, Openness, and Chronotype each accounted for a significant amount of variance in sense of Humor scores. That is, the relationship between scores on Sense of Humor and evening orientation was significant after controlling for personality dimensions. Eveningness was related to sense of Humor scores in women but not in men. Social but not cognitive humor was predicted by eveningness.

Abstract: Background Somatostatin analogs induce alterations in sleep in healthy adults. Presently, it is unknown whether somatostatin analog treatment affects sleep parameters in patients with acromegaly. Design Case-control study. Patients and measurements We assessed sleepiness and sleep patterns in 62 adult patients (32 men, age 61 years (33–88 years) controlled by surgery alone or postoperative radiotherapy (69%), and/or somatostatin analogs (31%). We used two validated sleep questionnaires (Epworth sleepiness score and Munchener chronotype questionnaire). Patient outcomes were compared to controls. Results Sleep duration and timing of sleep were not different in patients compared to controls. However, sleepiness score was increased in all patients compared to controls: 6 (1–20) vs. 4 (0–14), P =0.014 (median (range)), reflecting increased daytime sleepiness. Snoring was reported in 68% of both patients and controls ( P =0.996), observed apnoea's and restless legs in 23% and 37% of patients compared to 12% and 21% of controls ( P =0.062 and P =0.031, resp.). In addition, sleep latency was increased in patients treated by somatostatin analogs compared to patients cured by surgery and/ or radiotherapy (52±48min vs. 26±40min, P =0.005), resulting in a delayed sleep onset (24:08±1:26h vs. 23:25±0:43h, P =0.053). Sleep duration was unaffected. Conclusions Daytime sleepiness is increased in a homogeneous cohort of patients in long-term remission from acromegaly. In addition, somatostatin analog treatment increases sleep latency and delays sleep onset in patients with long-term biochemical control of growth hormone overproduction without altering total sleep duration.

Abstract: This study examined college students' beliefs about effective ways to relieve sleepiness to see whether they were related to their circadian preferences (i.e. evening types or morning types). Students rated the effectiveness of 10 methods used to cope with sleepiness. Evening types were more likely to endorse methods that are maladaptive to good night-time sleep, compared to intermediate types (i.e. neither morning nor evening types). Evening types rated exercise as relatively less effective and caffeine consumption as relatively more effective at relieving sleepiness than did intermediate types. The methods students use to relieve sleepiness may be one of the factors that contribute to the higher rate of sleep problems among evening types compared to other chronotypes. Further research is needed to see how student beliefs about effective ways to relieve sleepiness relate to what they actually do when they are sleepy.

Abstract: To the Editor The hypothalamic ventrolateral preoptic (VLPO) nucleus has multiple functions, temperature regulation and NREM sleep control being salient. Sleep active VLPO neurons show increased activity during NREM [1, 2]. They also show increased c-fos expression in animals killed after a bout of increased NREM produced after sleep deprivation [2] and use gamma-amino butyric acid and galanin to inhibit the wake-on hypothalamic hypocretinergic and histaminergic neurons, as well as the pontomesencephalic serotonergic and noradrenergic ones [3]. The VLPO neuronal pool is heterogeneous in cats and rats, with warm sensitive neurons (WSN), cold sensitive neurons (CSN), and temperature independent neurons [1, 2]. Sleep active VLPO neurons have been mostly observed in nocturnal animals such as rats, mice and cats, but also in diurnal degus, in diurnal and nocturnal monkeys and probably in humans too [4]. However, the thermal properties of the VLPO neurons have not been explored in diurnal animals. Most rats and cats WSN are sleep-on, unlike CSN, which increase their firing rate during wakefulness [1, 2], which contradicts the increase in sleep propensity observed in humans during the descending part of the circadian cycle of body temperature [5]. To solve this contradiction, a formal mathematical model based on the interaction between the hypothalamic temperature and the set-point, which in turn is modulated by a circadian oscillator controlling the temperature rhythm and another additional oscillator modulating the activity of WSN, with the two oscillators being separated by a fixed phase difference, was developed [6]. However, the model is complex and somehow counterintuitive, as both WSN and CSN readily fire in response to warming and cooling, respectively and their response promoting sleep or waking is also straightforward [1, 2]. Other researchers argued that the thermal responses of WSN reflect in fact the increase in skin temperature [7]. However, thermosensitivity has been observed in hypothalamic slices devoid of synaptic input, indicating that their thermal response is intrinsic [2]. Thus, an explanation for the contradictory results seems to be needed. The aim of this note is to propose a simple hypothesis explaining all the known facts relating the thermoregulatory and sleep controlling properties of VLPO in different species, and to suggest additional experiments which would support (or falsify) the proposed hypothesis. The hormone of the dark, melatonin, is the main effector of the biological clock, controlling the thermoregulatory and rest-activity cycles [8], amongst many others. Although the involvement of melatonin in the control of the VLPO activity was initially taken into account, it was soon dismissed due to the lack of a clear hypnotic activity [9]. However, a second look into the similarities and differences of the effects of melatonin in different animals could shed light on its role. The hypothermic effects of melatonin are well recognized, having been documented in diurnal [10], nocturnal [11] and even in poikilothermic animals [12]. However, the behavioral effects of melatonin vary between species, depending on the chronotype, as it enhances activity in nocturnal animals, but shows a mild hypnotic effect and causes a reduction in the activity of diurnal species [13]. Summarizing, melatonin promotes the behavior typical of the species during the dark period of the circadian cycle [14]. We propose here that the differential effects of melatonin in the behavior of nocturnal and diurnal species could be correlated with the activity of VLPO warm and cold sensitive neurons as well as with their respective sleep-on and wake-on properties. Agreeing with the experiments studying the wake-on activity of CSN, melatonin causes body cooling and enhances wakefulness in cats and rats. Conversely, the diurnal low levels of melatonin occur during the warm phase of their circadian cycle of body temperature. Hence, they sleep, which again fits with the sleep-on properties of the VLPO WSN neurons [1, 2, 9]. Turning now to diurnal species (humans, for instance), the pattern must be exactly opposite. The nocturnal melatonin secretion should produce body cooling and sleep, while body warming and wakefulness should appear in the absence of melatonin, i.e., during light time. Thus, the opposite results observed in nocturnal cats and rats and diurnal humans can be easily explained. Cats and rats sleep when their hypothalamic temperature is increased, while diurnal humans show increased sleep propensity when the circadian temperature cycle is in its descending phase. The crucial experiments to test this hypothesis have not been performed as they involve recording the activity of VLPO warm and cold sensitive neurons in diurnal animals. Additional experiments would consist in recording the levels of wake promoting substances, histamine, hypocretins, serotonin and NA after localized hypothalamic warming and cooling in diurnal animals. Recording the EEG signs of sleep and waking and sleep latencies in the same animals might also provide evidence. Similar tests applied to other animal groups may show the importance of the chronotype in the whole animal kingdom. Ectothermic reptiles are essentially diurnal and it is unthinkable that brain cooling would enhance wakefulness in this group. Hence, their thermal dependence must be opposite to that of cats and rats. Also, experiments J. Pineal Res. 2008; 45:225–226