CIRCADIAN RHYTHMS DURING AND AFTER THREE MONTHS IN SOLITUDE UNDERGROUND

Sleep-independent circadian rhythm of aldosterone secretion in healthy young adults

OBJECTIVE

A diurnal variation in urine output has been described in humans, whereby it is lowest at night. Fluid balance hormones such as vasopressin and aldosterone as well as urine output have a diurnal variation. Although the diurnal variation of vasopressin results in part from a circadian rhythm, the variation in aldosterone has until recently been reported to be due to the sleep/wake cycle. The present study used a specialized protocol to explore whether aldosterone has an underlying circadian rhythm.

METHODS

Ten healthy participants (average age 23.1) were enrolled in the 57.3-hour protocol that included an 8-hour baseline sleep episode, 40 hours in constant routine conditions (wakefulness, food and fluid intake, posture, and dim light), and a 9.3-hour recovery sleep. Blood samples for aldosterone were taken every 4 hours. Cosinor analysis was performed on the constant routine data to test the effect of the sleep/wake cycle on overall aldosterone secretion.

RESULTS

There was a significant circadian rhythm during the 40-hour constant routine, independent of sleep, with aldosterone higher at the end of the biological night and lower at the end of the biological day. When analyzing data from the entire 57.3-hour protocol and controlling for this circadian rhythm, aldosterone concentration was significantly higher during the recovery night following the 40-hour sleep deprivation compared to the night spent awake.

CONCLUSION

We found a significant endogenous circadian rhythm in the secretion of aldosterone, independent of sleep. In addition, as shown previously, there was a significant effect of the sleep/wake cycle on aldosterone secretion.

Timeless spaces: Field experiments in the physiological study of circadian rhythms, 1938-1963

In the middle of the twentieth century, physiologists interested in human biological rhythms undertook a series of field experiments in natural spaces that they believed could closely approximate conditions of biological timelessness. With the field of rhythms research was still largely on the fringes of the life sciences, natural spaces seemed to offer unique research opportunities beyond what was available to physiologists in laboratory spaces. In particular, subterranean caves and the High Arctic became archetypal 'natural laboratories' for the study of human circadian (daily) rhythms. This paper is explores the field experiments which occurred in these 'timeless spaces'. It considers how scientists understood these natural spaces as suitably 'timeless' for studying circadian rhythms and what their experimental practices can tell us about contemporary physiological notions of biological time, especially its relationship to 'environmentality' (Formosinho et al. in Stud History Philos Sci 91:148-158, 2022). In so doing, this paper adds to a growing literature on the interrelationship of field sites by demonstrating the ways that caves and the Arctic were connected by rhythms scientists. Finally, it will explore how the use of these particular spaces were not just scientific but also political - leveraging growing Cold War anxieties about nuclear fallout and the space race to bring greater prestige and funding to the study of circadian rhythms in its early years.

Circadian clocks of the kidney: function, mechanism, and regulation

An intrinsic cellular circadian clock is located in nearly every cell of the body. The peripheral circadian clocks within the cells of the kidney contribute to the regulation of a variety of renal processes. In this review, we summarize what is currently known regarding the function, mechanism, and regulation of kidney clocks. Additionally, the effect of extrarenal physiological processes, such as endocrine and neuronal signals, on kidney function is also reviewed. Circadian rhythms in renal function are an integral part of kidney physiology, underscoring the importance of considering time of day as a key biological variable. The field of circadian renal physiology is of tremendous relevance, but with limited physiological and mechanistic information on the kidney clocks this is an area in need of extensive investigation.

Health Effects of Underground Workspaces cohort: study design and baseline characteristics

The development of underground workspaces is a strategic effort towards healthy urban growth in cities with ever-increasing land scarcity. Despite the growth in underground workspaces, there is limited information regarding the impact of this environment on workers' health. The Health Effects of Underground Workspaces (HEUW) study is a cohort study that was set up to examine the health effects of working in underground workspaces. In this paper, we describe the rationale for the study, study design, data collection, and baseline characteristics of participants. The HEUW study recruited 464 participants at baseline, of whom 424 (91.4%) were followed-up at 3 months and 334 (72.0%) at 12 months from baseline. We used standardized and validated questionnaires to collect information on socio-demographic and lifestyle characteristics, medical history, family history of chronic diseases, sleep quality, health-related quality of life, chronotype, psychological distress, occupational factors, and comfort levels with indoor environmental quality parameters. Clinical and anthropometric parameters including blood pressure, spirometry, height, weight, and waist and hip circumference were also measured. Biochemical tests of participants' blood and urine samples were conducted to measure levels of glucose, lipids, and melatonin. We also conducted objective measurements of individuals' workplace environment, assessing air quality, light intensity, temperature, thermal comfort, and bacterial and fungal counts. The findings this study will help to identify modifiable lifestyle and environmental parameters that are negatively affecting workers' health. The findings may be used to guide the development of more health-promoting workspaces that attempt to negate any potential deleterious health effects from working in underground workspaces.

Human Physiology During Exposure to the Cave Environment: A Systematic Review With Implications for Aerospace Medicine

Background: Successful long-duration missions outside low-Earth orbit will depend on technical and physiological challenges under abnormal environmental conditions. Caves, characterized by absence of light, confinement, three-dimensional human movement and long-duration isolation, are identifiably one of the earliest examples of scientific enquiry into space analogs. However, little is known about the holistic human physiological response during cave exploration or prolonged habitation.

Objectives: The aim of our review was to conduct a systematic bibliographic research review of the effects of short and prolonged exposure to a cave environment on human physiology, with a view to extend the results to implications for human planetary exploration missions.

Methods: A systematic search was conducted following the structured PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines for electronic databases.

Results: The search retrieved 1,519 studies. There were 50 articles selected for further consideration, of which 31 met our inclusion criteria. Short-term cave exposure studies have investigated visual dysfunction, cardiovascular, endocrine-metabolic, immunologic-hematological and muscular responses in humans. Augmentations of heart rate, muscular damage, initial anticipatory stress reaction and inflammatory responses were reported during caving activity. Prolonged exposure studies mainly investigated whether biological rhythms persist or desist in the absence of standard environmental conditions. Changes were evident in estimated vs. actual rest-activity cycle periods and external desynchronization, body temperature, performance reaction time and heart rate cycles. All studies have shown a marked methodological heterogeneity and lack reproduction under controlled conditions.

Conclusions: This review facilitates a further comparison of the proposed physiological impact of a subterranean space analog environment, with existing knowledge in related disciplines pertaining to human operative preparation under challenging environmental conditions. This comprehensive overview should stimulate more reproducible research on this topic and offer the opportunity to advance study design and focus future human research in the cave environment on noteworthy, reproducible projects.

Expedition Cognition: A Review and Prospective of Subterranean Neuroscience With Spaceflight Applications

Renewed interest in human space exploration has highlighted the gaps in knowledge needed for successful long-duration missions outside low-Earth orbit. Although the technical challenges of such missions are being systematically overcome, many of the unknowns in predicting mission success depend on human behavior and performance, knowledge of which must be either obtained through space research or extrapolated from human experience on Earth. Particularly in human neuroscience, laboratory-based research efforts are not closely connected to real environments such as human space exploration. As caves share several of the physical and psychological challenges of spaceflight, underground expeditions have recently been developed as a spaceflight analog for astronaut training purposes, suggesting that they might also be suitable for studying aspects of behavior and cognition that cannot be fully examined under laboratory conditions. Our objective is to foster a bi-directional exchange between cognitive neuroscientists and expedition experts by (1) describing the cave environment as a worthy space analog for human research, (2) reviewing work conducted on human neuroscience and cognition within caves, (3) exploring the range of topics for which the unique environment may prove valuable as well as obstacles and limitations, (4) outlining technologies and methods appropriate for cave use, and (5) suggesting how researchers might establish contact with potential expedition collaborators. We believe that cave expeditions, as well as other sorts of expeditions, offer unique possibilities for cognitive neuroscience that will complement laboratory work and help to improve human performance and safety in operational environments, both on Earth and in space.

Resynchronization of circadian oscillators and the east-west asymmetry of jet-lag

Cells in the brain's Suprachiasmatic Nucleus (SCN) are known to regulate circadian rhythms in mammals. We model synchronization of SCN cells using the forced Kuramoto model, which consists of a large population of coupled phase oscillators (modeling individual SCN cells) with heterogeneous intrinsic frequencies and external periodic forcing. Here, the periodic forcing models diurnally varying external inputs such as sunrise, sunset, and alarm clocks. We reduce the dimensionality of the system using the ansatz of Ott and Antonsen and then study the effect of a sudden change of clock phase to simulate cross-time-zone travel. We estimate model parameters from previous biological experiments. By examining the phase space dynamics of the model, we study the mechanism leading to the difference typically experienced in the severity of jet-lag resulting from eastward and westward travel.

Possible contribution of chronobiology to cardiovascular health

The daily variations found in many aspects of physiology are collectively known as circadian rhythm (from "circa" meaning "about" and "dien" meaning "day"). Circadian oscillation in clock gene expression can generate quantitative or functional variations of the molecules directly involved in many physiological functions. This paper reviews the molecular mechanisms of the circadian clock, the transmission of circadian effects to cardiovascular functions, and the effects of circadian dysfunction on cardiovascular diseases. An evaluation of the operation of the internal clock is needed in clinical settings and will be an effective tool in the diagnosis of circadian rhythm disorders. Toward this end, we introduce a novel non-invasive method for assessing circadian time-regulation in human beings through the utilization of hair follicle cells.

Blacks (African Americans) have shorter free-running circadian periods than whites (Caucasian Americans)

The length of the free-running period (τ) affects how an animal re-entrains after phase shifts of the light-dark (LD) cycle. Those with shorter periods adapt faster to phase advances than those with longer periods, whereas those with longer periods adapt faster to phase delays than those with shorter periods. The free-running period of humans, measured in temporal isolation units and in forced desychrony protocols in which the day length is set beyond the range of entrainment, varies from about 23.5 to 26 h, depending on the individual and the experimental conditions (e.g., temporal isolation vs. forced desychrony). We studied 94 subjects free-running through an ultradian LD cycle, which was a forced desychrony with a day length of 4 h (2.5 h awake in dim light, ~35 lux, alternating with 1.5 h for sleep in darkness). Circadian phase assessments were conducted before (baseline) and after (final) three 24-h days of the ultradian LD cycle. During these assessments, saliva samples were collected every 30 min and subsequently analyzed for melatonin. The phase shift of the dim light melatonin onset (DLMO) from baseline to final phase assessment gave the free-running period. The mean ± SD period was 24.31 ± .23 h and ranged from 23.7 to 24.9 h. Black subjects had a significantly shorter free-running period than Whites (24.18 ± .23 h, N =20 vs. 24.37 ± .22 h, N = 55). We had a greater proportion of women than men in our Black sample, so to check the τ difference we compared the Black women to White women. Again, Black subjects had a significantly shorter free-running period (24.18 ± .23, N = 17 vs. 24.41 ± .23, N = 23). We did not find any sex differences in the free-running period. These findings give rise to several testable predictions: on average, Blacks should adapt quicker to eastward flights across time zones than Whites, whereas Whites should adjust quicker to westward flights than Blacks. Also, Blacks should have more difficulty adjusting to night-shift work and day sleep, which requires a phase delay. On the other hand, Whites should be more likely to have trouble adapting to the early work and school schedules imposed by society. More research is needed to confirm these results and predictions.

Season of birth and mood seasonality in late childhood and adolescence

A significant season of birth effect on mood seasonality has been detected in young adults, with higher sensitivity to seasonal changes for people born during spring or summer months (long photoperiod) than those born during autumn or winter months (short photoperiod). The aim of this study was to verify whether the birth season effect on mood seasonality is already present in late childhood and adolescence. To this end, the Seasonal Pattern Assessment Questionnaire for Children and Adolescents (SPAQ-CA) was administered to 1523 Italian participants (870 females, 653 males), ranging from 10 to 17 years of age. The Global Seasonality Score (GSS) was computed as a measure of mood seasonality. Analysis of covariance showed a significant season of birth effect on GSS; although no comparisons were significant when Tukey's posthoc test for unequal samples was performed, it was observed that adolescents born in summer achieved the highest scores while those born in winter obtained the lowest. The present data point out that a significant season of birth effect on mood seasonality is already present in late childhood and adolescence.

Revisiting spontaneous internal desynchrony using a quantitative model of sleep physiology

Early attempts to characterize free-running human circadian rhythms generated three notable results: 1) observed circadian periods of 25 hours (considerably longer than the now established 24.1- to 24.2-hour average intrinsic circadian period) with sleep delayed to later circadian phases than during entrainment; 2) spontaneous internal desynchrony of circadian rhythms and sleep/wake cycles--the former with an approximately 24.9-hour period, and the latter with a longer (28-68 hour) or shorter (12-20 hour) period; and 3) bicircadian (48-50 hour) sleep/wake cycles. All three results are reproduced by Kronauer et al.'s (1982) coupled oscillator model, but the physiological basis for that phenomenological model is unclear. We use a physiologically based model of hypothalamic and brain stem nuclei to investigate alternative physiological mechanisms that could underlie internal desynchrony. We demonstrate that experimental observations can be reproduced by changes in two pathways: promotion of orexinergic (Orx) wake signals, and attenuation of the circadian signal reaching hypothalamic nuclei. We reason that delayed sleep is indicative of an additional wake-promoting drive, which may be of behavioral origin, associated with removal of daily schedules and instructions given to participants. We model this by increasing Orx tone during wake, which reproduces the observed period lengthening and delayed sleep. Weakening circadian input to the ventrolateral preoptic nucleus (possibly mediated by the dorsomedial hypothalamus) causes desynchrony, with observed sleep/wake cycle period determined by degree of Orx up-regulation. During desynchrony, sleep/wake cycles are driven by sleep homeostasis, yet sleep bout length maintains circadian phase dependence. The model predicts sleep episodes are shortest when started near the temperature minimum, consistent with experimental findings. The model also correctly predicts that it is possible to transition to bicircadian rhythms from either a synchronized or desynchronized state. Our findings suggest that feedback from behavioral choices to physiology could play an important role in spontaneous internal desynchrony.

Neurobehavioral performance in young adults living on a 28-h day for 6 weeks

OBJECTIVES

Performance on many cognitive tasks varies with time awake and with circadian phase, and the forced desynchrony (FD) protocol can be used to separate these influences on performance. Some performance tasks show practice effects, whereas the Psychomotor Vigilance Task (PVT) has been reported not to show such effects. We aimed to compare performance on the PVT and on an addition test (ADD) across a 6-week FD study, to determine whether practice effects were present and to analyze the circadian and wake-dependent modulation of the 2 measures.

DESIGN AND SETTING

A 47-day FD study conducted at the Brigham and Women's Hospital General Clinical Research Center.

PARTICIPANTS

Eleven healthy adults (mean age: 24.4 years, 2 women).

MEASUREMENTS AND RESULTS

For 2 baseline days and across 6 weeks of FD, we gave a test battery (ADD, PVT, self-rating of effort and performance) every 2 hours. During FD, there was a significant (P < 0.0001) improvement in ADD performance (more correct calculations completed), whereas PVT performance (mean reaction time, fastest 10% reaction times, lapses) significantly (P < 0.0001) declined week by week. Subjective ratings of PVT performance indicated that subjects felt their performance improved across the study (P < 0.0001), but their rating of whether they could have performed better with greater effort did not change across the study (P > 0.05).

CONCLUSIONS

The decline in PVT performance suggests a cumulative effect of sleep loss across the 6-week study. Subjects did not accurately detect their declining PVT performance, and a motivational factor could not explain this decline.

Circadian rhythm disturbances in depression

OBJECTIVE

The aim of this article is to review progress in understanding the mechanisms that underlie circadian and sleep rhythms, and their role in the pathogenesis and treatment of depression.

METHODS

Literature was selected principally by Medline searches, and additional reports were identified based on ongoing research activities in the authors' laboratory.

RESULTS

Many physiological processes show circadian rhythms of activity. Sleep and waking are the most obvious circadian rhythms in mammals. There is considerable evidence that circadian and sleep disturbances are important in the pathophysiology of mood disorders. Depressed patients often show altered circadian rhythms, sleep disturbances, and diurnal mood variation. Chronotherapies, including bright light exposure, sleep deprivation, and social rhythm therapies, may be useful adjuncts in non-seasonal and seasonal depression. Antidepressant drugs have marked effects on circadian processes and sleep.

CONCLUSIONS

Recent progress in understanding chronobiological and sleep regulation mechanisms may provide novel insights and avenues into the development of new pharmacological and behavioral treatment strategies for mood disorders.

Sleep and circadian rhythms in humans

During the past 50 years, converging evidence reveals that the fundamental properties of the human circadian system are shared in common with those of other organisms. Concurrent data from multiple physiological rhythms in humans revealed that under some conditions, rhythms oscillated at different periods within the same individuals and led to the conclusion 30 years ago that the human circadian system was composed of multiple oscillators organized hierarchically; this inference has recently been confirmed using molecular techniques in species ranging from unicellular marine organisms to mammals. Although humans were once thought to be insensitive to the resetting effects of light, light is now recognized as the principal circadian synchronizer in humans, capable of eliciting weak (Type 1) or strong (Type 0) resetting, depending on stimulus strength and timing. Realization that circadian photoreception could be maintained in the absence of sight was first recognized in blind humans, as was the property of adaptation of the sensitivity of circadian photoreception to prior light history. In sighted humans, the intrinsic circadian period is very tightly distributed around approximately 24.2 hours and exhibits aftereffects of prior entrainment. Phase angle of entrainment is dependent on circadian period, at least in young adults. Circadian pacemakers in humans drive daily variations in many physiologic and behavioral variables, including circadian rhythms in alertness and sleep propensity. Under entrained conditions, these rhythms interact with homeostatic regulation of the sleep/wake cycle to determine the ability to sustain vigilance during the day and to sleep at night. Quantitative understanding of the fundamental properties of the multioscillator circadian system in humans and their interaction with sleep/wake homeostasis has many applications to health and disease, including the development of treatments for circadian rhythm and sleep disorders.

Sleep-wake as a biological rhythm

Evidence that the sleep-wake rhythm is generated endogenously has been provided by studies employing a variety of experimental paradigms such as sleep deprivation, sleep displacement, isolating subjects in environments free of time cues, or imposing on subjects sleep-wake schedules widely deviating from 24 hours. The initial observations obtained in isolated subjects revealed that the period of the endogenous circadian pacemaker regulating sleep is of approximately 25 hours. More recent studies, however, in which a more rigorous control of subjects' behavior was exerted, particularly over lighting conditions, have shown that the true periodicity of the endogenous pacemaker deviates from 24 hours by a few minutes only. Besides sleep propensity, the circadian pacemaker has been shown to regulate sleep consolidation, sleep stage structure, and electroencephalographic activities. The pattern of light exposure throughout the 24 hours appears to participate in the entrainment of the circadian pacemaker to the geophysical day-night cycle. Melatonin, the pineal hormone produced during the dark hours, participates in communicating both between the environmental light-dark cycle and the circadian pacemaker, and between the circadian pacemaker and the sleep-wake-generating mechanism. In contrast to prevailing views that have placed great emphasis on homeostatic sleep drive, recent data have revealed a potent circadian cycle in the drive for wakefulness, which is generated by the suprachiasmatic nucleus. This drive reaches a peak during the evening hours just before habitual bedtime.

Stability, precision, and near-24-hour period of the human circadian pacemaker

Regulation of circadian period in humans was thought to differ from that of other species, with the period of the activity rhythm reported to range from 13 to 65 hours (median 25.2 hours) and the period of the body temperature rhythm reported to average 25 hours in adulthood, and to shorten with age. However, those observations were based on studies of humans exposed to light levels sufficient to confound circadian period estimation. Precise estimation of the periods of the endogenous circadian rhythms of melatonin, core body temperature, and cortisol in healthy young and older individuals living in carefully controlled lighting conditions has now revealed that the intrinsic period of the human circadian pacemaker averages 24.18 hours in both age groups, with a tight distribution consistent with other species. These findings have important implications for understanding the pathophysiology of disrupted sleep in older people.

Nonphotic entrainment of the human circadian pacemaker

In organisms as diverse as single-celled algae and humans, light is the primary stimulus mediating entrainment of the circadian biological clock. Reports that some totally blind individuals appear entrained to the 24-h day have suggested that nonphotic stimuli may also be effective circadian synchronizers in humans, although the nonphotic stimuli are probably comparatively weak synchronizers, because the circadian rhythms of many totally blind individuals "free run" even when they maintain a 24-h activity-rest schedule. To investigate entrainment by nonphotic synchronizers, we studied the endogenous circadian melatonin and core body temperature rhythms of 15 totally blind subjects who lacked conscious light perception and exhibited no suppression of plasma melatonin in response to ocular bright-light exposure. Nine of these fifteen blind individuals were able to maintain synchronization to the 24-h day, albeit often at an atypical phase angle of entrainment. Nonphotic stimuli also synchronized the endogenous circadian rhythms of a totally blind individual to a non-24-h schedule while living in constant near darkness. We conclude that nonphotic stimuli can entrain the human circadian pacemaker in some individuals lacking ocular circadian photoreception.

The effect of the timing, quality and quantity of sleep upon the depression (masking) of body temperature on an irregular sleep/wake schedule

Eight subjects were studied on an irregular sleep/wake schedule which was designed so that prior wake time and the time of day when 4-h sleep periods were taken were both balanced. Rectal temperature and the sleep EEG were measured throughout the 8-d protocol. A purification method was used to estimate the depression of rectal temperature (masking) caused by the first and second halves of each 4-h sleep period. On average this was 0.187 +/- 0.015 degrees C during the first 2 h and 0.262 +/- 0.017 degrees C during the second 2 h. Masking increases beyond the first 2 h of sleep, but any effects due to the phase of the rectal temperature when sleep is taken are very weak. However, masking did increase (that is, the temperature was depressed more) when the amount of prior wake time was greater than 4 h. When the effects of sleep content variables were considered also, masking was still greater in the second half of sleep and tended to increase with the amount of slow-wave sleep, but it decreased with increasing amounts of time awake during a sleep period (sleep latency or sleep discontinuity). Some implications of these results for the mechanism of sleep-induced masking are considered.

Orientation in time: implications for psychopathology and psychotherapy

Considerations derived from the fields of physics, philosophy, anthropology, and psychology suggest that our popular conception of time as a unidirectional and uniform flow may not be an adequate description of the human experience of time. Indeed, other dimensions of temporality may constitute important aspects of human phenomenology with respect to both adaptive and maladaptive or psychopathological mental states. Exploration of the temporal aspect of experience and temporal reorientation may be helpful psychotherapeutic maneuvers that are greatly facilitated by hypnosis.

Research on sleep, circadian rhythms and aging: applications to manned spaceflight

Disorders of sleep and circadian rhythmicity are characteristic of both advancing age and manned spaceflight. Sleep fragmentation, reduced nocturnal sleep tendency and sleep efficiency, reduced daytime alertness, and increased daytime napping are common to both of these conditions. Recent research on the pathophysiology and treatment of disrupted sleep in older people has led to a better understanding of how the human circadian pacemaker regulates the timing of the daily sleep-wake cycle and how it responds to the periodic changes in the light-dark cycle to which we are ordinarily exposed. These findings have led to new treatments for some of the sleep disorders common to older individuals, using carefully timed exposure to bright light and darkness to manipulate the phase and/or amplitude of the circadian timing system. These insights and treatment approaches have direct applications in the design of countermeasures allowing astronauts to overcome some of the challenges which manned spaceflight poses for the human circadian timing system. We have conducted an operational feasibility study on the use of scheduled exposure to bright light and darkness prior to launch in order to facilitate adaptation of the circadian system of a NASA space shuttle crew to the altered sleep-wake schedule required for their mission. The results of this study illustrate how an understanding of the properties of the human circadian timing system and the consequences of circadian disruption can be applied to manned spaceflight.

Entrainment of a free-running human with bright light?

The case of a 40-year-old sighted woman with free-running sleep-wake and melatonin rhythms is presented. The subject was studied for 102 days. During the pre-treatment period, both the sleep-wake and melatonin rhythms had a period of 25.1 hr, similar to the average period of humans living in temporal isolation. Treatment consisted of bright artificial light exposure (2500 lx Vita-Lite) for 2 hr each day upon awakening. Clock time of light exposure was held constant for 6 days and then slowly advanced until the subject was arising at her desired time of day. The subject continued the light treatment at home and was able to live on a 24-hr day for the 30-day follow-up study. While other factors may be operating in this situation, it is possible that the light treatment caused the stabilization of the free-running rhythms, advancement to a normal phase and entrainment to the 24-hr day. We suspect that the tendency to free-run was related to sleep onsets that were abnormally delayed relative to the circadian phase response curve for light. By scheduling a 2-hr pulse of bright light each morning, this tendency to delay would be counteracted by light-induced advances, resulting in normal entrainment.

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.

The effect of real and simulated time-zone shifts upon the circadian rhythms of body temperature, plasma 11-hydroxycorticosteroids, and renal excretion in human subjects

1. Observations were made upon five subjects who flew through 4(1/2)-6 time zones, four of them returning later to their starting point, and upon twenty-three subjects experiencing simulated 6 or 8 hr time zones shifts in either direction in an isolation unit.2. Measurements were made of plasma concentration of 11-hydroxycorticosteroids, of body temperature, and of urinary excretion of sodium, potassium and chloride. Their rhythm was defined, where possible, by fitting a sine curve of period 24 hr to each separate 24-hr stretch of data and computing the acrophase, or maximum predicted by the sine curve.3. The adaptation of the plasma steroid rhythm was assessed by the presence of a sharp fall in concentration after the sample collected around 08.00 hr. The time course of adaptation varied widely between individuals; it was usually largely complete by the fourth day after westward, and rather later after eastward, flights. After time shift the pattern often corresponded neither to an adapted nor to an unadapted one, and in a subject followed for many months after a real flight a normal amplitude only appeared 2-3 months after flight.4. Temperature rhythm adapted by a movement of the acrophase, without change in amplitude, although on some days no rhythm could be observed. This movement was always substantial even on the first day, and was usually nearly complete by the fifth.5. High nocturnal excretion of electrolyte was often seen in the early days after time shift, more notably after simulated westward flights. Adaptation of urinary electrolyte rhythms usually proceeded as with temperature, but the movement of the acrophase was slower, more variable between individuals, more erratic, and sometimes reversed after partial adaptation. On a few days there were two maxima corresponding to those expected on real and on experimental time.6. Sodium excretion was much less regular than that of potassium, but adapted more rapidly to time shift, so that the two often became completely dissociated. Chloride behaved much as sodium.7. The time course of adaptation of the plasma steroid and urinary potassium rhythms were sufficiently similar to suggest a causal connexion. The time course of adaptation of the temperature rhythm did not coincide with that of any other component considered here.

Bibliography of sensory and perceptual deprivation, isolation, and related areas

This 1199-item bibliography is concerned with the general topics of sensory and perceptual deprivation, isolation, relative restriction of sensory input, brain-washing, life in isolated outposts, submarines, etc.