Rest-activity Cycle Research Articles

Correlation between nocturnal scratch behavior assessed by actigraphy and subjective/objective parameters in patients with atopic dermatitis

Atopic dermatitis (AD) is a chronic relapsing inflammatory skin disorder caused by multiple factors. Pruritus, and consequent scratch behavior, is one of the major exacerbating factors of AD and is associated with the itch-scratch cycle [1]. Unconscious nocturnal scratch behavior induces sleep disturbance and symptomatic worsening, resulting in a decreased quality of life [2]. Actigraphy is a non-invasive method of monitoring the human rest-activity cycle. Although several studies have used actigraphy [...]

Rest-Activity Cycle Disturbances in the Acute Phase of Moderate to Severe Traumatic Brain Injury

Background. Sleep-wake disturbances are among the most persistent sequelae after traumatic brain injury (TBI) and probably arise during the hospital stay following TBI. These disturbances are characterized by difficulties sleeping at night and staying awake during the day. Objective. The aim of the present study was to document rest-activity cycle consolidation in acute moderate/severe TBI using actigraphy and to assess its association with injury severity and outcome. Methods. In all, 16 hospitalized patients (27.1 ± 11.3 years) with moderate/severe TBI wore actigraphs for 10 days, starting in the intensive care unit (ICU) when continuous sedation was discontinued and patients had reached medical stability. Activity counts were summed for daytime (7:00-21:59 hours) and nighttime periods (22:00-6:59 hours). The ratio of daytime period activity to total 24-hour activity was used to quantify rest-activity cycle consolidation. An analysis of variance was carried out to characterize the evolution of the daytime activity ratio over the recording period. Results. Rest-activity cycle was consolidated only 46.6% of all days; however, a significant linear trend of improvement was observed over time. Greater TBI severity and longer ICU and hospital lengths of stay were associated with poorer rest-activity cycle consolidation and evolution. Patients with more rapid return to consolidated rest-activity cycle were more likely to have cleared posttraumatic amnesia and have lower disability at hospital discharge. Conclusions. Patients with acute moderate/severe TBI had an altered rest-activity cycle, probably reflecting severe fragmentation of sleep and wake episodes, which globally improved over time. A faster return to rest-activity cycle consolidation may predict enhanced brain recovery.

Association between the consolidation of the rest-activity cycle and brain recovery in the acute phase of traumatic brain injury

Introduction Based on clinical observations, sleep–wake cycle disturbances, arise in the days following traumatic brain injury (TBI), and remain among the most persistent and debilitating symptoms. The aim of this study was to document the evolution of rest-activity cycle consolidation in the acute phase of moderate/severe TBI, and its association with injury severity and outcome, using actigraphy. Materials and methods Sixteen hospitalised patients (13 men; 27.1 ± 11.3 years) with moderate/severe TBI wore actigraphs for 10 days. Recordings began in the intensive care unit (ICU) when continuous sedation was discontinued and patients had reached medical stability. The ratio of daytime (7:00–21:59) activity to total 24-h activity was used to quantify the level of consolidation of a day-night rest-activity cycle, and a ratio ⩾80% was considered to reflect adequate consolidation. The Galveston Orientation and Amnesia Test and the Disability Rating Scale were used to assess posttraumatic amnesia (PTA) and functionality at discharge. An analysis of variance was used to characterize the evolution of the daytime activity ratio over the recording period. Pearson’s correlations were carried out to measure associations between actigraphy and injury severity and outcome. T-tests for independent samples were used to compare the rest-activity cycle of patients with and without PTA at discharge. Results Most patients showed a very low daytime activity ratio during the first 48-h of recording (mean ± SD = 70.8 ± 10.1%), with only 4 patients having a ratio ⩾80%. Although this consolidation threshold was reached in only 46.6% of all recording days, there was a significant linear trend of improvement over the 10 days of recording (p Conclusion TBI patients have severe rest-activity cycle disturbances during their hospital stay, but these disturbances globally improve over time. The absence of rest-activity cycle consolidation reflects severe fragmentation of sleep–wake patterns. A faster return to rest-activity cycle consolidation may predict enhanced brain recovery. Acknowledgements Research funded by the Canadian Institutes of Health Research (CIHR) and the Fonds de recherche du Quebec – Sante (FRQS).

Examination of polysomnography in hospitalized patients with acute moderate-severe traumatic brain injury

Introduction Traumatic brain injury (TBI) is a commonly occurring condition and a significant public health issue, as it is one of the leading causes of disability worldwide. Complaints of disturbed sleep, excessive daytime sleepiness and disorders of arousal have been well established as being among the most pervasive, enduring and common sequelae, and have been shown to compromise the recovery process if left untreated. Current evidence suggests that sleep wake disorders begin in the acute stage with altered rest-activity cycles as evidenced from actigraphy findings. For those in the chronic stage, studies involving polysomnography have identified a number of changes in sleep architecture including reductions in total sleep time, sleep efficiency, and N3 sleep, and increased stage N1 sleep. For those with severe TBI, hypersomnia and increased sleep need have been identified as being the most common sleep complaint. To date however, there has not yet been an examination of sleep architecture in the acute stage of TBI. The objective of this study was to examine the sleep architecture of adults with acute moderate-severe traumatic brain injury, and to compare these findings with the sleep architecture of age- and sex-matched healthy controls. Materials and methods Six adults (4 males; 25 ± 11.3 yrs), with moderate-severe TBI underwent 24-h polysomnography (PSG) on average 21 days (range 7–38 days) during their acute hospital stay. Results were compared with those of 11 healthy controls (7 males; 25 ± 10.5 yrs); who underwent in-laboratory PSG. Groups were compared for sleep architecture variables for the night period, determined as the longest ‘visually observable consecutive sleep period’ identified on the PSG recording in TBI patients. A Mann–Whitney test for non-parametric data was carried out to in order to compare PSG parameters between TBI patients and controls. Results Our results indicated that the TBI group (TBI) tended to have longer total sleep duration (TBI: 8.1 ± 2.1 h; Controls: 6.6 ± 1.7 h p = 0.12), greater total wake duration (TBI: 159 ± 104.7 min; Controls: 53.6 ± 47 min, p = 0.04), higher number of arousals and micro-arousals per hour of sleep ( p = 0.009), and greater number of sleep cycles ( p = 0.02). No group difference was found for the percentage of each sleep stage (N1: TBI: 46.1 ± 11.8 %, Controls 42.6 ± 16.2%; N2; TBI: 20.6 ± 11.4%, Controls 28.5 ± 11.5; N3; TBI: 29.8 ± 17.0%, Controls: 28.3 ± 11.0; REM: TBI: 17.2 ± 6.0%, Controls 16.8 ± 4.6%). Conclusion Although our sample size was small, these preliminary findings indicate that even in this very early stage, hospitalized patients with acute severe TBI experience a longer sleep duration, however with greater sleep fragmentation in comparison to control subjects. Additional research is needed to further elucidate and determine the course of these disturbances, and their relationship with clinical, neurological and functional outcomes. Acknowledgements We gratefully acknowledge the assistance of Ms. Helene Blais for her assistance with the data for this project.

M6A mRNA Methylation: A New Circadian Pacesetter

The purpose of m(6)A methylation of RNA, first observed in the 1970s, has been a longstanding mystery. Fustin et al. now show that it regulates RNA processing and determines the period and oscillatory stability of the mammalian circadian clockwork.

Infralow Frequencies and Ultradian Rhythms

Our brain activity demonstrates amazing stability across multiple time frames ranging from a few milliseconds to several hours. The longer cycles are commonly called ultradian rhythms and they correspond to infralow frequencies (ILFs) in the milli-Hz range (0.001 Hz). Ultradian rhythms between 90 minutes and 2 hours or longer are readily observed in our electroencephalogram, and they reflect periods of activity and rest, cycles of cortical excitability and plasticity followed by relative inactivity. Our nightly sleep is organized into similar stages (rapid eye movement and non-rapid eye movement sleep) as is our daily behavior (ie, the basic rest-activity cycle). Astrocytes often exhibit milli-Hz ILFs, and they play a major role in shaping neuronal plasticity and activity, and thus may organize or influence the basic rhythms of sleep and waking. The nature and importance of astrocytes in human brain functioning is subsequently reviewed.

Correction: Corrigendum: Deleterious effect of suboptimal diet on rest-activity cycle in Anastrepha ludens manifests itself with age

Activity patterns and sleep-wake cycles are among the physiological processes that change most prominently as animals age, and are often good indicators of healthspan. In this study, we used the video-based high-resolution Behavioral Monitoring System (BMS) to monitor the daily activity cycle of tephritid fruit flies Anastrepha ludens over their lifetime. Surprisingly, there was no dramatic change in activity profile with respect to age if flies were consistently fed with a nutritionally balanced diet. However, if flies were fed with sugar-only diet, their activity profile decreased in amplitude at old age, suggesting that suboptimal diet affected activity patterns, and its detrimental effect may not manifest itself until the animal ages. Moreover, by simulating different modes of behavior monitoring with a range of resolution and comparing the resulting conclusions, we confirmed the superior performance of video-based monitoring using high-resolution BMS in accurately representing activity patterns in an insect model.

Brown adipose tissue thermogenesis precedes food intake in genetically obese Zucker (fa/fa) rats

In Sprague–Dawley rats, brown adipose tissue (BAT) thermogenesis occurs in an episodic ultradian manner (BAT on-periods) as part of the basic rest–activity cycle (BRAC). Eating occurs approximately 15min after the onset of BAT on-periods. Zucker obese (fa/fa) rats eat larger less frequent meals than control rats. In chronically instrumented conscious unrestrained Zucker obese rats we examined ultradian fluctuations in BAT, body and brain temperatures, and the relation between BAT temperature and eating. The interval between BAT temperature peaks for the 12hour dark phase was 121±3 (mean±SE) min for Zucker obese rats and 91±3min for control lean rats (p<0.01). Corresponding values for the light phase were 148±6 and 118±4min (p<0.01). Mean BAT and body temperatures were lower in Zucker obese rats, in comparison with lean controls, during both BAT on-periods and BAT off-periods. Mean brain temperatures were lower during BAT off-periods. Amplitudes of the BRAC-related increases in all 3 temperatures were greater in the Zucker obese rats. Meal onset in Zucker obese rats commenced 15±1min after the onset of a BAT on-period, not significantly different for the delay observed in lean control rats (18±1min, p>0.05). Thus periods between eating are increased in the Zucker obese rats, but the action of leptin, absent in these animals, is not crucial for the timing of eating in relation to increases in BAT and body temperature. Lack of the normal excitatory action of leptin on brain-regulated BAT sympathetic discharge could also contribute to lower BAT thermogenesis in Zucker obese rats.

Deleterious effect of suboptimal diet on rest-activity cycle in Anastrepha ludens manifests itself with age

Activity patterns and sleep-wake cycles are among the physiological processes that change most prominently as animals age, and are often good indicators of healthspan. In this study, we used the video-based high-resolution Behavioral Monitoring System (BMS) to monitor the daily activity cycle of tephritid fruit flies Anastrepha ludens over their lifetime. Surprisingly, there was no dramatic change in activity profile with respect to age if flies were consistently fed with a nutritionally balanced diet. However, if flies were fed with sugar-only diet, their activity profile decreased in amplitude at old age, suggesting that suboptimal diet affected activity patterns, and its detrimental effect may not manifest itself until the animal ages. Moreover, by simulating different modes of behavior monitoring with a range of resolution and comparing the resulting conclusions, we confirmed the superior performance of video-based monitoring using high-resolution BMS in accurately representing activity patterns in an insect model.

Effects of Sleep Loss on the Rest–Activity Circadian Rhythm of Helpers Participating in Continuous Dogsled Races

The Finnmarksløpet dogsled race lasts up to 7 days. Helpers, who keep time, coach, and transport equipment, have to be alert though they have little and fragmented sleep. This study investigated disruptions of the rest-activity rhythm among helpers. 10 helpers were monitored by actigraph a week before, during, and after the race. Sleep logs, sleepiness rate, and self-reported quality of sleep were collected. Nonparametric circadian rhythm analysis showed significant differences between the pre- and postrace interdaily stability and amplitude of rhythm. Compared to prerace, sleepiness at bedtime was increased and number of nocturnal awakenings was reduced postrace, although the actigraphic outputs showed no improvement in sleep quality. Helpers who were engaged in the race for a longer span (5-6 days) had more difficulty recovering from the sleep loss accumulated during the race than those engaged for a shorter time (2-3 days). Poor sleep combined with prolonged and demanding mental focus for 2 days or more has a negative influence upon the rest-activity cycle, though complete restoration of the cycle occurs over 1 week or more. Being in such a condition for ≥ 5 days leads to disruptions of the circadian component of the sleep-wake cycle that hampers the sleep recovery process. Disrupted sleep and demanding mental requirements are common in long-distance sporting events as well as among shift workers. Follow-up intervention should be made in such cases to ensure the return of a healthy rest-activity rhythm and sleep quality.

P.2.c.025 Preliminary findings of the rest-activity cycles in patients with major depression following agomelatine and sertraline treatment

Positron emission tomography (PET) studies have shown that the antidepressant effect of chronic deep brain stimulation (DBS) of the subcallosal cingulate gyrus (SCG) may be consequence of modifications of brain metabolism at key structures involved in depression. Like clinical benefits, these metabolic changes may reverse when the stimulation is discontinued, even preceding clinical worsening. However no data on immediate effects of DBS discontinuation are available. The aim of this study was to determine immediate cerebral metabolism changes during a short switch-off of electrical stimulation in implanted patients with treatment-resistant depression (TRD) who had achieved clinical improvement after a period of chronic DBS.Seven patients with TRD who had been previously implanted for DBS in SCG were included. After a period of clinical stabilization two consecutive FDG-PET were acquired, the first with active stimulation and the second after 48 h of inactive stimulation. A HAMD-17 to assess depressive symptoms was performed before both scans. Analyses were performed with SnPM8.Inactive stimulation was characterized by metabolism decreases in dorsal anterior cingulate (Broadmann Area, BA24), premotor region (BA6) and putamen with respect to active stimulation. No clinical changes according to HAMD-17 were detected.The main limitation of this study is the small sample size.Our results point to immediate effects of DBS discontinuation on metabolism of brain depressive network which precede clinical changes, helping to disentangle the rationale behind DBS efficacy in TRD.

Day or Night Administration of Ketamine and Pentobarbital Differentially Affect Circadian Rhythms of Pineal Melatonin Secretion and Locomotor Activity in Rats

Surgery with general anesthesia disturbs circadian rhythms, which may lead to postoperative sleep disorders and delirium in patients. However, it is unclear how circadian rhythms are affected by different anesthetics administered at different times during the rest-activity cycle. We hypothesized that pentobarbital (an agonist at the γ-aminobutyric acid A receptors) and ketamine (an antagonist at the N-methyl-d-aspartate receptors) would have differential effects on circadian rhythms, and these effects would also be influenced by the time of their administration (the active versus resting phase). Rats were divided into 4 groups according to the anesthetic administered (pentobarbital or ketamine) and the timing of intraperitoneal administration (active/night phase or resting/day phase). Using online pineal microdialysis, we analyzed pineal melatonin secretion and locomotor activity rhythms in rats under a light/dark (12/12-hour) cycle for 5 days after anesthesia and microdialysis catheter implantation. The data were analyzed for rhythmicity by cosinor analysis. Ketamine administered during the resting phase produced 65- and 153-minute phase advances, respectively, in melatonin secretion and locomotor activity rhythms on the first day after anesthesia. In contrast, ketamine administered during the active phase produced 43- and 235-minute phase delays. Pentobarbital had no effect on the phase of either melatonin secretion or locomotor activity, irrespective of the timing of administration. When administered during the active phase, both anesthetics decreased the amplitude of melatonin secretion on the day after anesthesia; when administered during the resting phase, however, neither anesthetic affected the amplitude. The amplitude of locomotor activity decreased in all animals for 3 days after anesthesia. Ketamine has opposite phase-shifting effects on circadian rhythms according to the time of administration, whereas pentobarbital has no effect. Furthermore, both anesthetics decrease the postoperative amplitude of pineal melatonin secretion if administered during the active, but not the resting, phase of the 24-hour rest-activity cycle.

Individual Variation in Sleep-Wake Rhythms in Free-Living Birds

Ultradian rhythms, such as sleep-wake periodicities, during the night might represent basic rest-activity cycles of organisms that are fundamental to the temporal organization and synchronization of behavior throughout the day. However, in contrast to circadian rhythms, little is known about the underlying oscillators and molecular mechanisms of higher-frequency rhythms. A fundamental step for the understanding of the mechanisms of these latter periodicities is the analysis of variation in sleep-wake cycles in free-living animals, which can help in estimating the relative importance of genetic and environmental influence on the rhythmicity. We analyzed variation in the level of rhythmicity and period length (τ) of behaviorally defined sleep-wake cycles in a natural population of blue tits Cyanistes caeruleus. Our results indicate that the expression of periodicity in sleep-wake patterns, but not τ, has a strong individual-specific basis. The within-individual repeatability estimate of the expression of periodicity was .45 (95% confidence interval: .35–.55) when data from males and females were combined. In addition, periodicity was influenced by specific environmental factors, such as night temperature, seasonal date, and age of the individual. Most strikingly, low nighttime temperature negatively affected periodicity of sleep-wake patterns, potentially via a hypothermic response of the birds. Our results further suggest that τ is influenced by photoperiod. Blue tits showed longer sleep-wake rhythms when the nights were longer. These observations suggest a genetic basis for the incidence of rhythmic sleep-wake behavior in addition to environmental modifications of their specific expression. (Author correspondence: mueller@orn.mpg.de)

Restless Flies Show Their Legs

Physiology Your leg may not seem to have much in common with that of a fly; however, it may be more than you think. Freeman et al. show that a relatively common human neurological disorder called restless legs syndrome, characterized by a strong urge to move one's legs and “fragmented” sleep, can be modeled in the fruit fly Drosophila melanogaster . A polymorphism in the gene BTBD9 confers approximately 50% of the population-attributable risk in the human disorder. Deletion of the homolog in Drosophila, dBTBD9 , recapitulated features of this condition, such as altered rest activity cycles, disrupted sleep, and hyperlocomotion. Knockdown of dBTBD9 in a specific set of dopaminergic neurons also resulted in fragmented sleep. Also similar to the human disorder, aberrant dopamine levels were observed in the dBTBD9 mutant fly brain. Treatment of mutant flies with a dopamine agonist improved sleep patterns. In human cells, iron homeostasis was controlled by BTBD9. These findings support prior studies suggesting a role for dopamine regulation and iron metabolism in restless legs syndrome. Furthermore, the fly may serve as a useful model for understanding this disorder. Curr. Biol. 22 , 1142 (2012).

Twelve‐hour days in the brain and behavior of split hamsters

Hamsters will spontaneously 'split' and exhibit two rest-activity cycles each day when housed in constant light (LL). The suprachiasmatic nucleus (SCN) is the locus of a brain clock organizing circadian rhythmicity. In split hamsters, the right and left SCN oscillate 12 h out of phase with each other, and the twice-daily locomotor bouts alternately correspond to one or the other. This unique configuration of the circadian system is useful for investigation of SCN communication to efferent targets. To track phase and period in the SCN and its targets, we measured wheel-running and FOS expression in the brains of split and unsplit hamsters housed in LL or light-dark cycles. The amount and duration of activity before splitting were correlated with latency to split, suggesting behavioral feedback to circadian organization. LL induced a robust rhythm in the SCN core, regardless of splitting. The split hamsters' SCN exhibited 24-h rhythms of FOS that cycled in antiphase between left and right sides and between core and shell subregions. In contrast, the medial preoptic area, paraventricular nucleus of the hypothalamus, dorsomedial hypothalamus and orexin-A neurons all exhibited 12-h rhythms of FOS expression, in-phase between hemispheres, with some detectable right-left differences in amplitude. Importantly, in all conditions studied, the onset of FOS expression in targets occurred at a common phase reference point of the SCN oscillation, suggesting that each SCN may signal these targets once daily. Finally, the transduction of 24-h SCN rhythms to 12-h extra-SCN rhythms indicates that each SCN signals both ipsilateral and contralateral targets.

Optimized light therapy for non-seasonal major depressive disorder: Effects of timing and season

Light Therapy (LT) when combined with standard antidepressant treatment for unipolar depression hastens recovery. We studied the influence of LT timing on the antidepressant efficacy of LT and the influence of the season of treatment and recurrence on the response to treatment. We studied 70 inpatients affected by Unipolar Depression, treated for three weeks with combined LT and venlafaxine. Two-third of the patients received LT following a predictive algorithm based on MEQ scores; the others received LT at 11:00 a.m. Severity of depression was rated on the Hamilton Depression Rating Scale (HDRS). A subgroup of patients wore activity monitors. HDRS scores significantly decreased during treatment (Friedman's ANOVA: χ2=186.82, p<0.00001). LT administered in the early morning showed a better relative efficacy than late morning (F=4.576; p=0.012) with the clinical improvement correlating with an advance in rest-activity rhythm acrophase (r=-0.336; p=0.017). Season of hospitalization interacted with LT timing and time in influencing response to treatment (F=3.101; p=0.049) and season of episode recurrence significantly interacted with LT timing, season of hospitalization and time (F=5.925; p=0.0035). The major limitation of the study is the small sample size when considering simultaneously LT schedules, season of treatment and recurrence. Moreover, even if none of the patients fulfilled DSM-IV criteria for seasonal pattern of recurrence, they were not administered any questionnaire about seasonality. We confirmed the usefulness of LT as a non-pharmacological antidepressant therapy for non-seasonal depression. Season and timing of administration and timing of the rest-activity cycle affected response to treatment.

Actigraphic Measurements in Opioid Detoxification With Methadone or Buprenorphine

The objective of the present naturalistic study was to assess the differential effects of opioid detoxification with methadone or buprenorphine on activity, circadian rhythm, and sleep. Forty-two consecutive inpatients with opiate addiction were switched to either methadone or buprenorphine and gradually tapered down over the course of 2 to 3 weeks. There were no significant differences in comedication (lofexidine, quetiapine, and valproic acid) between the methadone and buprenorphine groups. Patients in the methadone group showed 11% lower activity and were 24 minutes phase delayed as compared with buprenorphine-treated patients, whereas the latter had 2.5% lower sleep efficiency and 9% shorter actual sleep time. These significant group differences were most pronounced for the lowest doses (≤20% of maximum individual daily dose, ie, at the end of withdrawal representing late withdrawal effects). Furthermore, for the total sample, we found a significant decrease in the relative amplitude of the sleep-wake cycle and worsening of all actigraphic sleep parameters from the higher (100% to 20%) to the lowest doses (20% to 0%). The acrophase of the circadian rhythm displayed a phase advance (-88 minutes) from the highest (100% to 80%) to the lower doses (80% to 0%) in methadone-treated patients. Opioid tapering with methadone or buprenorphine leads to characteristic changes of the rest-activity cycle, but further study is required to validate these results.

The human circadian metabolome

The circadian clock orchestrates many aspects of human physiology, and disruption of this clock has been implicated in various pathologies, ranging from cancer to metabolic syndrome and diabetes. Although there is evidence that metabolism and the circadian clockwork are intimately linked on a transcriptional level, whether these effects are directly under clock control or are mediated by the rest-activity cycle and the timing of food intake is unclear. To answer this question, we conducted an unbiased screen in human subjects of the metabolome of blood plasma and saliva at different times of day. To minimize indirect effects, subjects were kept in a 40-h constant routine of enforced posture, constant dim light, hourly isocaloric meals, and sleep deprivation. Under these conditions, we found that ~15% of all identified metabolites in plasma and saliva were under circadian control, most notably fatty acids in plasma and amino acids in saliva. Our data suggest that there is a strong direct effect of the endogenous circadian clock on multiple human metabolic pathways that is independent of sleep or feeding. In addition, they identify multiple potential small-molecule biomarkers of human circadian phase and sleep pressure.

Experimental Jetlag Disrupts Circadian Clock Genes But Improves Performance in Racehorses After Light-Dependent Rapid Resetting of Neuroendocrine Systems and the Rest-Activity Cycle

Abrupt alterations in the 24-h light : dark cycle, such as those resulting from transmeridian air travel, disrupt circadian biological rhythms in humans with detrimental consequences on cognitive and physical performance. In the present study, a jetlag-simulated phase shift in photoperiod temporally impaired circadian peaks of peripheral clock gene expression in racehorses but acutely enhanced athletic performance without causing stress. Indices of aerobic and anaerobic capacities were significantly increased by a phase-advance, enabling prolonged physical activity before fatigue occurred. This was accompanied by rapid re-entrainment of the molecular clockwork and the circadian pattern of melatonin, with no disturbance of the adrenal cortical axis, but a timely rise in prolactin, which is a hormone known to target organs critical for physical performance. Subsequent studies showed that, unlike the circadian pattern of melatonin, and in contrast to other species, the daily rhythm of locomotor activity was completely eliminated under constant darkness, but it was restored immediately upon the reintroduction of a light : dark cycle. Resetting of the rhythm of locomotion was remarkably fast, revealing a rapid mechanism of adaptation and a species dependency on light exposure for the expression of daily diurnal activity. These results show that horses are exquisitely sensitive to sudden changes in photoperiod and that, unlike humans, can benefit from them; this appears to arise from powerful effects of light underlying a fast and advantageous process of adjustment to the phase shift.

Heating and eating: Brown adipose tissue thermogenesis precedes food ingestion as part of the ultradian basic rest–activity cycle in rats

Laboratory rats, throughout the 24hour day, alternate between behaviorally active and non active episodes that Kleitman called the basic rest–activity cycle (BRAC). We previously demonstrated that brown adipose tissue (BAT), body and brain temperatures and arterial pressure and heart rate increase in an integrated manner during behaviorally active phases. Studies show that eating is preceded by increases in body and brain temperature, but whether eating is integrated into the BRAC has not been investigated. In the present study of chronically instrumented, unrestrained Sprague–Dawley rats, peaks in BAT temperature occurred every 96±7 and 162±16min (mean±SE, n=14 rats) in dark and light periods respectively, with no apparent underlying regularity. With food available ad libitum, eating was integrated into the BRAC in a temporally precise manner. Eating occurred only after an increase in BAT temperature, commencing 15±1min (mean±SE) after the onset of an increase, with no difference between dark and light phases. There were either no or weak preprandial and postprandial relations between intermeal interval and amount eaten during a given meal. Remarkably, with no food available the rat still disturbed the empty food container 16±1 min (p>0.05 versus ad libitum food) after the onset of increases in BAT temperature, and not at other times. Rather than being triggered by changes in levels of body fuels or other meal-associated factors, in sedentary laboratory rats with ad libitum access to food eating commences as part of the ultradian BRAC, a manifestation of intrinsic brain activity.