Circadian rhythms in bed rest: Monitoring core body temperature via heat-flux approach is superior to skin surface temperature

Continuous recordings of core body temperature (CBT) are a well-established approach in describing circadian rhythms. Given the discomfort of invasive CBT measurement techniques, the use of skin temperature recordings has been proposed as a surrogate. More recently, we proposed a heat-flux approach (the so-called Double Sensor) for monitoring CBT. Studies investigating the reliability of the heat-flux approach over a 24-hour period, as well as comparisons with skin temperature recordings, are however lacking. The first aim of the study was therefore to compare rectal, skin, and heat-flux temperature recordings for monitoring circadian rhythm. In addition, to assess the optimal placement of sensor probes, we also investigated the effect of different anatomical measurement sites, i.e. sensor probes positioned at the forehead vs. the sternum. Data were collected as part of the Berlin BedRest study (BBR2-2) under controlled, standardized, and thermoneutral conditions. 24-hours temperature data of seven healthy males were collected after 50 days of -6° head-down tilt bed-rest. Mean Pearson correlation coefficients indicated a high association between rectal and forehead temperature recordings (r > 0.80 for skin and Double Sensor). In contrast, only a poor to moderate relationship was observed for sensors positioned at the sternum (r = -0.02 and r = 0.52 for skin and Double Sensor, respectively). Cross-correlation analyses further confirmed the feasibility of the forehead as a preferred monitoring site. The phase difference between forehead Double Sensor and rectal recordings was not statistically different from zero (p = 0.313), and was significantly smaller than the phase difference between forehead skin and rectal temperatures (p = 0.016). These findings were substantiated by cosinor analyses, revealing significant differences for mesor, amplitude, and acrophase between rectal and forehead skin temperature recordings, but not between forehead Double Sensor and rectal temperature measurements. Finally, Bland-Altman analysis indicated narrower limits of agreement for rhythm parameters between rectal and Double Sensor measurements compared to between rectal and skin recordings, irrespective of the measurement site (i.e. forehead, sternum). Based on these data we conclude that (1) Double Sensor recordings are significantly superior to skin temperature measurements for non-invasively assessing the circadian rhythm of rectal temperature, and (2) temperature rhythms from the sternum are less reliable than from the forehead. We suggest that forehead Double Sensor recordings may provide a surrogate for rectal temperature in circadian rhythm research, where constant routine protocols are applied. Future studies will be needed to assess the sensor’s ecological validity outside the laboratory under changing environmental and physiological conditions.     

Assessment of circadian rhythms throughout the menstrual cycle of female rhesus monkeys

Reproductive cyclicity has a significant influence on the regulation of circadian rhythms in rodents. Studies have suggested that there are changes in body temperature rhythms between the follicular and luteal phases in human females. This study examined the effects of menstrual cyclicity on physiological and behavioral circadian rhythms in female rhesus monkeys (Macaca mulatta), an acknowledged biomedical model. Seven unrestrained subjects were implanted with a biotelemetry transmitter to measure body temperature and heart rate and an accelerometer was used to measure physical activity. Water was available ad libitum and drinking was measured via an electronic circuit attached to a water lixit. A video-based task system, the Psychomotor Test System, provided environmental enrichment and delivered a pelletized diet. Mean, phase, and amplitude of each rhythm were calculated. Estrogen and progesterone conjugates were assayed and quantified from daily urine samples to identify follicular and luteal phases of the menstrual cycle. Average circadian variables were then compared between these phases. Heart rate was significantly (P< or =0.05) delayed in the luteal phase. Albeit non-significant, analysis showed a trend toward decreased circadian amplitude of body temperature in the luteal phase.     

Intra- and inter-individual variability in the circadian rhythm of body temperature of rats,squirrels, dogs,and horses

In order to explore the often neglected issue of the relationship between intra- and inter-subject variability of circadian rhythms, we evaluated the variability in four parameters of the circadian rhythm of body temperature under controlled laboratory conditions. To avoid the bias of potential selection of an idiosyncratic species, we conducted the study on four different species: the laboratory rat, the thirteen-lined ground squirrel, the domestic dog, and the horse. In rats and squirrels, three of the four parameters of the body temperature rhythm (mean level, amplitude, and regularity of waveform) showed greater inter-subject variability than intra-subject variability. The two variabilities were not different from each other in dogs and horses. Intra- and inter-subject variabilities of acrophase (time of peak) were not significantly different in any of the species, and their magnitudes were similar in all species, which suggests that acrophase is a very dependable parameter in the analysis of circadian rhythms, even though its overall variability is not particularly low.     

Effects of suprachiasmatic nuclei lesions on circadian and ultradian rhythms in body temperature in ocular enucleated rats

Abstract

To test the hypothesis that an oscillator located outside the suprachiasmatic nuclei (SCN) controls the circadian rhythm of body temperature, we conducted a study with 14 blinded rats, 10 of which receiving a SCN lesion. Body temperature was automatically and continuously recorded for about one month by intraperitoneal radio transmitters. Food intake, drinking and locomotor activity were also recorded. Periodograms revealed that 3 rats with histologically verified total bilateral SCN lesions did not exhibit any circadian rhythmicity. The 7 other rats appeared to have partial lesions. They showed shortening of period and severe amplitude reduction in all functions. Thus, no support was found for the hypothesis of a separate circadian ‘temperature oscillator’ located outside the SCN. Nevertheless, after large partial lesions body temperature showed more persistency than some of the other behavioral rhythms.

Ultradian rhythms in temperature persisted after partial and total lesions. Other functions showed parallel ultradian rhythms. In intact rats the ultradian peaks were restricted predominantly to the subjective night. After total lesions these peaks became more or less homogeneously distributed in time but more heterogeneously after partial lesions. So the SCN plays a role in the temporal structure of ultradian rhythms but does not generate them. Non‐24‐hour actograms showed instabilities of period and phase of ultradian rhythms. Intact and lesioned rats were similar with respect to the mean (about 3.5 hrs) and standard deviation (about 1.5 hrs) of ultradian periods in temperature. These features indicate that a mechanism outside the SCN is underlying ultradian rhythmicity, capable of generating short‐term oscillations. Two approaches, homeostatic sleep‐wake relaxation oscillations and multiple circadian oscillators, are discussed.     

Sleep and Circadian Rhythms of Temperature and Urinary Excretion on a 22.8 hr “Day”

Two groups of subjects (total N = 6) were studied in an isolation chamber for a period of 3 weeks whilst living on a 22.8 hr “day”. Regular samples of urine were taken when the subjects were awake, deep body temperature was recorded continuously and polygraphic EEG recordings were made of alternate sleeps. The excretion in the urine of potassium, sodium, phosphate, calcium and a metabolite of melatonin were estimated.

Measurements of the quantity and quality of sleep were made together with assessments of the temperature profiles associated with sleep. In addition, cosinor analysis of circadian rhythmicity in urinary variables and temperature was performed.

The 22.8 hr “days” affected variables and subjects differently. These differences were interpreted as indicating that the endogenous component of half the subjects adjusted to the 22.8 hr “days” but that, for the other three, adjustment did not occur. When the behaviour of different variables was considered then some (including urinary potassium and melatonin, sleep length and REM sleep) appeared to possess a larger endogenous component than others (for example, urinary sodium, phosphate and calcium), with rectal temperature behaving in an intermediate manner. In addition, a comparison between different rhythms in any subject enabled inferences to be drawn regarding any links (or lack of them) that might exist between the rhythms. In this respect also, there was a considerable range in the results and no links between any of the rhythms appeared to exist in the group of subjects as a whole.

Two further groups (total N=8) were treated similarly except that the chamber clock ran at the correct rate. In these subjects, circadian rhythms of urinary excretion and deep body temperature (sleep stages and urinary melatonin were not measured) gave no evidence for deterioration. We conclude, therefore, that the results on the 22.8 hr “day” were directly due to the abnormal “day” length rather than to a prolonged stay in the isolation chamber.     

Locomotor activity and body temperature rhythms in the Mahali mole-rat (C. h. mahali): The effect of light and ambient temperature variations

African mole-rats (family: Bathyergidae) are strictly subterranean mammals that reside in extensive networks of underground tunnels. They are rarely, if ever, exposed to light and experience muted temperature ranges. Despite these constant conditions, the presence of a functional circadian clock capable of entraining to external light cues has been reported for a number of species. In this study, we examine a social mole-rat species, Cryptomys hottentotus mahali, to determine if it possesses a functional circadian clock that is capable of perceiving light and ambient temperature cycles, and can integrate these cues into circadian rhythms of locomotor activity and core body temperature. Eight male and eight female, non-reproductive individuals were subjected to six cycles of varying light and temperature regimes. The majority of the individuals displayed daily rhythms of locomotor activity and body temperature that are synchronised to the external light and temperature cycles. Furthermore, endogenous rhythms of both locomotor activity and core body temperature were displayed under constant conditions. Thus, we can conclude that C. h. mahali possesses a functional circadian clock that can integrate external light and temperature cues into circadian rhythms of locomotor activity and core-body temperature.     

The Utility of the Swine Model to Assess Biological Rhythms and Their Characteristics during Different Stages of Residence in a Simulated Intensive Care Unit: A Pilot Study

The purpose of this pilot study was to explore the utility of the mammalian swine model under simulated intensive care unit (sICU) conditions and mechanical ventilation (MV) for assessment of the trajectory of circadian rhythms of sedation requirement, core body temperature (CBT), pulmonary mechanics (PM) and gas exchange (GE). Data were collected prospectively with an observational time-series design to describe and compare circadian rhythms of selected study variables in four swine mechanically ventilated for up to seven consecutive days. We derived the circadian (total variance explained by rhythms of τ between 20 and 28?h)/ultradian (total variance explained by rhythms of τ between 1 and <20?h) bandpower ratio to assess the robustness of circadian rhythms, and compare findings between the early (first 3 days) and late (subsequent days) sICU stay. All pigs exhibited statistically significant circadian rhythms (τ between 20 and 28?h) in CBT, respiratory rate and peripheral oxygen saturation, but circadian rhythms were detected less frequently for sedation requirement, spontaneous minute volume, arterial oxygen tension, arterial carbon dioxide tension and arterial pH. Sedation did not appear to mask the circadian rhythms of CBT, PM and GE. Individual subject observations were more informative than group data, and provided preliminary evidence that (a) circadian rhythms of multiple variables are lost or desynchronized in mechanically ventilated subjects, (b) robustness of circadian rhythm varies with subject morbidity and (c) healthier pigs develop more robust circadian rhythm profiles over time in the sICU. Comparison of biological rhythm profiles among sICU subjects with similar severity of illness is needed to determine if the results of this pilot study are reproducible. Identification of consistent patterns may provide insight into subject morbidity and timing of such therapeutic interventions as weaning from MV.     

Social stimuli fail to act as entraining agents of circadian rhythms in the golden hamster

Summary The ability of social stimuli to act as entraining agents of circadian rhythms was investigated in golden hamsters (Mesocricetus auratus). In a first experiment, pairs of male hamsters (one of them enucleated and the other intact) were maintained under a light-dark (LD) cycle with a period of 23.3 h. Running-wheel activity was recorded to determine the effect of social interaction on the free-running circadian rhythm of activity. In several pairs, general activity and body temperature were also recorded. In all pairs the intact animals entrained to the LD cycle, whereas the activity rhythms of the enucleated animals free-ran with periods of approximately 24 h and showed no apparent sign of synchronization or relative coordination with the other member of the pair. In a second experiment, male hamsters maintained in constant darkness received pulses of social interaction, which have been reported to induce phase shifts of the activity rhythm. Consistent phase shifts in the running-wheel activity rhythm were not induced by the social pulses in our experiment. These results suggest strongly that social stimuli are not effective entraining agents of circadian rhythms in the golden hamster.Abbreviations CT circadian time - LD light-dark     

Uncovering Different Masking Factors on Wrist Skin Temperature Rhythm in Free-Living Subjects

Most circadian rhythms are controlled by a major pacemaker located in the hypothalamic suprachiasmatic nucleus. Some of these rhythms, called marker rhythms, serve to characterize the timing of the internal temporal order. However, these variables are susceptible to masking effects as the result of activity, body position, light exposure, environmental temperature and sleep. Recently, wrist skin temperature (WT) has been proposed as a new index for evaluating circadian system status. In light of previous evidence suggesting the important relationship between WT and core body temperature regulation, the aim of this work was to purify the WT pattern in order to obtain its endogenous rhythm with the application of multiple demasking procedures. To this end, 103 subjects (18–24 years old) were recruited and their WT, activity, body position, light exposure, environmental temperature and sleep were recorded under free-living conditions for 1 week. WT demasking by categories or intercepts was applied to simulate a “constant routine” protocol (awakening, dim light, recumbent position, low activity and warm environmental temperature). Although the overall circadian pattern of WT was similar regardless of the masking effects, its amplitude was the rhythmic parameter most affected by environmental conditions. The acrophase and mesor were determined to be the most robust parameters for characterizing this rhythm. In addition, a circadian modulation of the masking effect was found for each masking variable. WT rhythm exhibits a strong endogenous component, despite the existence of multiple external influences. This was evidenced by simultaneously eliminating the influence of activity, body position, light exposure, environmental temperature and sleep. We therefore propose that it could be considered a valuable and minimally-invasive means of recording circadian physiology in ambulatory conditions.     

RETINAL CIRCADIAN RHYTHMS IN HUMANS *

Circadian rhythms in the retina may reflect intrinsic rhythms in the eye. Previous reports on circadian variability in electrophysiological human retinal measures have been scanty, and the results have been somewhat inconsistent. We studied the circadian variation of the electrooculography (EOG), electroretinography (ERG), and visual threshold (VTH) in subjects undergoing a 36h testing period. We used an ultrashort sleep-wake cycle to balance effects of sleep and light-dark across circadian cycles. Twelve healthy volunteers (10 males, 2 females; mean age 26.3 years, standard deviation [SD] 8.0 years, range 19-40 years) participated in the study. The retinal functions and oral temperature were measured every 90 min. The EOG was measured in the light, whereas the ERG and the VTH were measured in the dark. Sleep was inferred from activity detected by an Actillume monitor. The EOG peak-to-peak responses followed a circadian rhythm, with the peak occurring late in the morning (acrophase 12:22). The ERG b-wave implicit time peaked in the early morning (acrophase 06:46). No statistically significant circadian rhythms could be demonstrated in the ERG a-wave implicit time or peak-to-peak amplitude. The VTH rhythm peaked in the early morning (acrophases 07:59 for blue and 07:32 for red stimuli). All retinal rhythms showed less-consistent acrophases than the temperature and sleep rhythms. This study demonstrated several different circadian rhythms in retinal electrophysiological and psychophysical measures of healthy subjects. As the retinal rhythms had much poorer signal-to-noise ratios than the temperature rhythm, these measures cannot be recommended as circadian markers. (Chronobiology International, 18(6), 957-971, 2001)     

Pineal and retinal melatonin is involved in the control of circadian locomotor activity and body temperature rhythms in the pigeon

Summary Although pinealectomy or blinding resulted in loss of the clarity of the free-running rhythm of locomotor activity and body temperature and reduced the peak level of circulating melatonin rhythms to approximately a half in intact pigeons, neither pinealectomy nor blinding abolished any of these rhythms. However, when pinealectomy and blinding were combined, the rhythms of locomotor activity and body temperature disappeared in prolonged constant dim light, and melatonin concentration was reduced to the minimum level of detection. In order to examine the role of melatonin in the pigeon's circadian system, it was administered either daily or continuously to PX + EX-pigeons in LLdim. Daily administration of melatonin restored circadian rhythms of locomotor activity which entrained to melatonin injections, but continuous administration did not induce any remarkable change of locomotor activity. These results suggest that melatonin synthesized in the pineal body and the eye contributes to circulating melatonin and its rhythmicity is important for the control of circadian rhythms of locomotor activity and body temperature in the pigeon.Abbreviations LD Light-dark - LLdim constant dim light - LLbright constant bright light - PX pinealectomy - EX blinding - SCN suprachiasmatic nucleus     

Failure of extraocular light to facilitate circadian rhythm reentrainment in humans

Although extraocular light can entrain the circadian rhythms of invertebrates and nonmammalian vertebrates, almost all studies show that the mammalian circadian system can only be affected by light to the eyes. The exception is a recent study by Campbell and Murphy that reported phase shifts in humans to bright light applied with fiber-optic pads behind the knees (popliteal region). We tested whether this extraocular light stimulus could accelerate the entrainment of circadian rhythms to a shift of the sleep schedule, as occurs in shift work or jet lag. In experiment 1, the sleep/dark episodes were delayed 8h from baseline for 2 days, and 3h light exposures were timed to occur before the temperature minimum to help delay circadian rhythms. There were three groups: (1) bright (about 13,000 lux) extraocular light from fiber-optic pads, (2) control (dim light, 10-20 lux), and (3) medium-intensity (about 1000 lux) ocular light from light boxes. In experiment 2, the sleep/dark episodes were inverted, and extraocular light was applied either before the temperature minimum to help delay circadian rhythms or after the temperature minimum to help advance rhythms. Circadian phase markers were the salivary dim light melatonin onset (DLMO) and the rectal temperature minimum. There was no evidence that the popliteal extraocular light had a phase-shifting effect in either experiment. Possible reasons for phase shifts in the Campbell and Murphy study and not the current study include the many differences between the protocols. In the current study, there was substantial sleep deprivation before the extraocular light was applied. There was a large shift in the sleep/dark schedule, rather than allowing subjects to sleep each day from midnight to noon, as in the Campbell and Murphy study. Also, when extraocular light was applied in the current protocol, subjects did not experience a change from sleeping to awake, a change in posture (from lying in bed to sitting in a chair), or a change in ocular light (from dark to dim light). Further research is necessary to determine the conditions under which extraocular light might produce phase shifts in human circadian rhythms. (Chronobiology International, 17(6), 807-826, 2000).     

Chronobiological Aspects of Weight Loss in Obesity: Effects of Different Meal Timing Regimens

A series of short- and long-lasting experimental protocols of different meal timing regimes were performed in obese subjects to assess the possible occurrence of (1) a different metabolic fate of nutrients; (2) a phase shift of circadian rhythms of metabolic and hormonal parameters strictly related to nutrition; (3) a different weight loss. (A) In a short-lasting protocol (3 days) IS obese subjects were fed a hypocaloric diet (684 kcal/day) (a) at 10 hr only, (b) at 1800 hr only; (c) at 1000 hr, 1400 hr and 1800 hr, or (d) studied during a 36-hr fasting. Measures of calorimetry (R.Q., CHO and lipid oxidations, energy expenditure), hormones (plasma Cortisol, insulin, HGH, urinary catecholamines), urinary electrolytes (Na, K) and vital parameters (body temperature, heart rate, blood pressure) were carried out at 4-hr intervals for three days. A significantly higher lipid oxidation and a lower CHO oxidation were documented with the meal at 1800 hr, in comparison with the meal at 1000 hr. CHO and lipid oxidation circadian rhythms appeared the most affected by meal timing. (B) In a long-lasting protocol (18 days) 10 obese subjects were fed the same hypocaloric diet (a) at 1000 hr only and (b) at 1800 hr only. Calorimetric measures were performed every other day for 2 hr preceding each meal. Before and after the 18-days single meal period, body temperature, plasma Cortisol, PRL and TSH were recorded (δt = 4 hr). A higher lipid oxidation and a lower CHO oxidation were again demonstrated with the meal at 18 hr. Minimal changes of hormonal circadian rhythms were documented suggesting that the hypothalamus-hypophysis network is scarcely affected by meal timing. Weight loss did not vary in both short- and long-term protocol.     

Acute dim light at night increases body mass,alters metabolism,and shifts core body temperature circadian rhythms

The circadian system is primarily entrained by the ambient light environment and is fundamentally linked to metabolism. Mounting evidence suggests a causal relationship among aberrant light exposure, shift work, and metabolic disease. Previous research has demonstrated deleterious metabolic phenotypes elicited by chronic (>4 weeks) exposure to dim light at night (DLAN) (～5?lux). However, the metabolic effects of short-term (<2 weeks) exposure to DLAN are unspecified. We hypothesized that metabolic alterations would arise in response to just 2 weeks of DLAN. Specifically, we predicted that mice exposed to dim light would gain more body mass, alter whole body metabolism, and display altered body temperature (T_b) and activity rhythms compared to mice maintained in dark nights. Our data largely support these predictions; DLAN mice gained significantly more mass, reduced whole body energy expenditure, increased carbohydrate over fat oxidation, and altered temperature circadian rhythms. Importantly, these alterations occurred despite similar activity locomotor levels (and rhythms) and total food intake between groups. Peripheral clocks are potently entrained by body temperature rhythms, and the deregulation of body temperature we observed may contribute to metabolic problems due to “internal desynchrony” between the central circadian oscillator and temperature sensitive peripheral clocks. We conclude that even relatively short-term exposure to low levels of nighttime light can influence metabolism to increase mass gain.     

Circadian regulation of cardiovascular function: a role for vasoactive intestinal peptide

The circadian system, driven by the suprachiasmatic nucleus (SCN), regulates properties of cardiovascular function. The dysfunction of this timing system can result in cardiac pathology. The neuropeptide vasoactive intestinal peptide (VIP) is crucial for circadian rhythms in a number of biological processes including SCN electrical activity and wheel running behavior. Anatomic evidence indicates that SCN neurons expressing VIP are well positioned to drive circadian regulation of cardiac function through interactions with the autonomic centers. In this study, we tested the hypothesis that loss of VIP would result in circadian deficits in heart rate (HR) and clock gene expression in cardiac tissue. We implanted radiotelemetry devices into VIP-deficient mice and wild-type (WT) controls and continuously recorded HR, body temperature, and cage activity in freely moving mice. Under light-dark conditions, VIP-deficient mice displayed weak rhythms in HR, body temperature, and cage activity, with onsets that were advanced in phase compared with WT mice. Similarly, clock gene expression in cardiac tissue was rhythmic but phase advanced in mutant mice. In constant darkness, the normal circadian rhythms in HR were lost in VIP-deficient mice; however, most mutant mice continued to exhibit circadian rhythms of body temperature with shortened free-running period. The loss of VIP altered, but did not abolish, autonomic regulation of HR. Analysis of the echocardiograms did not find any evidence for a loss of cardiac function in VIP-deficient mice, and the size of the hearts did not differ between genotypes. These results demonstrate that VIP is an important regulator of physiological circadian rhythmicity in the heart.     

RETINAL CIRCADIAN RHYTHMS IN HUMANS*

Circadian rhythms in the retina may reflect intrinsic rhythms in the eye. Previous reports on circadian variability in electrophysiological human retinal measures have been scanty, and the results have been somewhat inconsistent. We studied the circadian variation of the electrooculography (EOG), electroretinography (ERG), and visual threshold (VTH) in subjects undergoing a 36h testing period. We used an ultrashort sleep-wake cycle to balance effects of sleep and light-dark across circadian cycles. Twelve healthy volunteers (10 males, 2 females; mean age 26.3 years, standard deviation [SD] 8.0 years, range 19–40 years) participated in the study. The retinal functions and oral temperature were measured every 90 min. The EOG was measured in the light, whereas the ERG and the VTH were measured in the dark. Sleep was inferred from activity detected by an Actillume monitor. The EOG peak-to-peak responses followed a circadian rhythm, with the peak occurring late in the morning (acrophase 12:22). The ERG b-wave implicit time peaked in the early morning (acrophase 06:46). No statistically significant circadian rhythms could be demonstrated in the ERG a-wave implicit time or peak-to-peak amplitude. The VTH rhythm peaked in the early morning (acrophases 07:59 for blue and 07:32 for red stimuli). All retinal rhythms showed less-consistent acrophases than the temperature and sleep rhythms. This study demonstrated several different circadian rhythms in retinal electrophysiological and psychophysical measures of healthy subjects. As the retinal rhythms had much poorer signal-to-noise ratios than the temperature rhythm, these measures cannot be recommended as circadian markers. (Chronobiology International, 18(6), 957–971, 2001)     

Masking effects of posture and sleep onset on core body temperature have distinct circadian rhythms: results from a 90-min/day protocol

Both recumbency and sleep affect core body temperature (CBT). To characterize their circadian effects and interactions, the authors examined the bedtime temperature drops (TDs) of nine men and eight women (aged 20 to 30) who repeated 90-min sleep-wake cycles over 2.5 days. While awake, subjects were exposed to 50 to 250 lux; while asleep, lights were off. Electroencephalogram-monitored time inbed lasted 30 min during each cycle. Cosinor nonlinear mixed-effects regressions modeled the circadian rhythm of TDs. The circadian maximum of TDs occurred approximately 4 h before the time of circadian CBT minimum, in a model that included the effects of baseline expected CBT, deviations from baseline CBT, time in study, and gender-dependent 24- and 12-h adjustments. Rates of temperature drops were faster during initial periods of lying awake than during periods of initially sleeping. Both rates followed separate circadian rhythms. The circadian maximum of TDs was located near customary nocturnal bedtimes, suggesting its role in fostering sleep during a normal bedtime routine. The apparent deceleration of temperature dropping at sleep onset supports the notion that the sleep onset period has complicated circadian neuroregulatory dynamics. These findings confirm the need for nonlinear models of temperature responses to postural changes and sleep that incorporate circadian variability in these masking effects.     

Physical activity,and not fat mass is a primary predictor of circadian parameters in young men

Circadian rhythms are ≈24?h oscillations in physiology and behavior, and disruptions have been shown to have negative effects on health. Wrist skin temperature has been used by several groups as a valid method of assessing circadian rhythms in humans. We tested the hypothesis that circadian temperature amplitude (TempAmp) and stability (TempStab) would significantly differ among groups of healthy young men of varying adiposities, and that we could identify physiological and behavioral measures that were significantly associated with these temperature parameters. Wrist skin temperatures taken at 10?min intervals for 7 consecutive days were determined in 18 optimal (OGroup), 20 fair (FGroup) and 21 poor (PGroup) %Fat grouped young men and subsequently analyzed using available validated software. Body composition, cardiorespiratory fitness, actigraphy, daily nutritional and sleep data, and fasting lipid, insulin and glucose concentration measures were also determined. Significant changes in TempAmp and TempStab parameters in subjects with a single metabolic syndrome (MetS) risk factor compared to those with no MetS factors was observed. In addition, stepwise multivariate regression analyses showed that 50% of the variance in TempAmp was explained by actigraphy (mean steps taken per day; MSTPD), cardiorespiratory fitness, and late night eating per week (#LNE); and 57% in TempStab by MSTPD, time spent in moderate-to-vigorous activity per day, fat mass, and #LNE. Overwhelmingly, physical activity was the most important measure associated with the differences in circadian rhythm parameters. Further research is warranted to determine the effects of increasing the amount and timing of physical activity on the status of the circadian system in a variety of populations.     

Different Behavior of the Circadian Rhythms of Activity and Body Temperature During Resynchronization Following an Advance of the LD Cycle

Spontaneous activity and the body temperature of laboratory mice were recorded telemetrically using implantable transmitters. Following ten control days (L : D = 12 : 12; light from 07:00 to 19:00), the LD cycle was phase-advanced by shortening the light time by 8 h. Recordings were continued for a further 3 weeks. The raw temperature data were unmasked or ‘purified’ — that is, the temperature changes due to locomotor activity were removed, so revealing the endogenous component of the rhythm — using a regression method previously developed by us. The circadian rhythms of activity and measured body temperature resynchronized on average after 8 days. During resynchronization, both rhythms tended to show two components, one adjusting by a phase advance and the other by a phase delay. However, after purification of the body temperature rhythm, only the advancing component remained. These results indicate that the delaying component of the measured temperature rhythm was caused by masking due to activity, and that the endogenous component of this rhythm did not divide into two components during the resynchronization process. Also, the endogenous component of the circadian rhythm of body temperature and one component of the activity rhythm seemed to be controlled by the same oscillator. It remains uncertain how the other component of the activity rhythm is regulated.     

The circadian rhythm of selected parameters of heart rate variability

At present, two main circadian oscillators are known, responsible for the rhythm of body temperature (BT) and body activity. Their independence has been demonstrated by the dissociation of these two rhythms in people during long-term isolation. In order to ascertain the circadian rhythm (CR) of heart rate variability (HRV), the ECG was recorded in 24 healthy awake men every two hours in the sitting position, from Friday 5 p.m. to Monday 6 a.m., who were maintained on a standard regime. One hundred consecutive RR intervals in every ECG were measured and from these 11 selected indicators of HRV were computed. Chronograms from the means of BT, respiratory rate, and electrical skin resistance showed pronounced CR with acrophases at 6 to 8 p.m. "Frequency" parameters of HRV, especially the frequency of reversal points, behaved similarly. CR in the remaining 7 "amplitude" parameters was also detected in individual persons, but their acrophases were different, and averaged chronograms mostly exhibited a flat course. The study has shown that there are at least two circadian components of HRV: the first phase has the CR synchronized with BT and is interindividually more homogeneous; the second phase is synchronized with body activity rhythm and is interindividually heterogeneous. On this basis, three equal subgroups of subjects arose, tentatively called afternoon, night, and forenoon types, respectively, in accordance with information about their preference for working and sleeping.