Janet C. Zimmerman

Chronobiology of aging: Temperature, sleep-wake rhythms and entrainment

Studies were carried out on a group of six young (ages 23-30) and six older (ages 53-70) normal men who lived under conditions of temporal, but not social isolation, from three to eight weeks. During entrained and non-entrained (free-running) conditions, comparative measurements were made of sleep-wake cycles, sleep stages and rectal temperature rhythms for these two age groups. Results demonstrated a reduction in the period and amplitude of the body temperature rhythms during free-running in the older group. Sleep efficiency, total sleep time, REM sleep latency, REM episode length, percent REM in the last 2 hours of sleep, the length and frequency of arousals during sleep, and the terminal wake latency were all age related and dependent on entrainment. The period of the sleep-wake cycle, terminal awakenings from REM and percent REM in the first 3 hours of sleep were not age related but were dependent on entrainment. Sleep stages as percents of total sleep time were found to be age related but independent of entrainment, while sleep latency, mid-REM to mid-REM cycle length and the ratio of sleep to total time were neither age related nor dependent on entrainment. In addition, individual chronobiological differences were prominent in the older group. Changes of period and of the phase relationship of sleep-wake and temperature rhythms occurred in several subjects during the non-entrained condition.

ENTRAINMENT OF HUMAN ORCADIAN RHYTHMS BY LIGHT‐DARK CYCLES: A REASSESSMENT

Abstract— Man is the only eukaryotic organism in which it has been reported that the circadian system cannot be entrained to a 24‐h period by a simple light‐dark (LD) cycle. In this paper, we reexamine the evidence for that claim and demonstrate that there were some fundamental flaws in the experimental design of the previous studies on which this conclusion was based. We report new studies in which we tested the efficacy of LD cycles in entraining the circadian rhythms of human subjects living in isolation from environmental time cues. We found that the cyclic alternation of light and dark, when applied to human subjects in a comparable way to experiments in other species, was an effective entraining agent. Our results and a critical review of the literature indicate that a LD cycle alone can be an effective environmental synchronizer of the human circadian timing system. Other factors, such as the knowledge of time of day, social contacts, the feeding schedule, and the imposed rest‐activity schedule may contribute to stable entrainment, although their relative strengths as synchronizers have yet to be determined.

Meal patterns in “free-running” humans☆

Abstract The meal patterns of men who were initiating meals for extended periods in the absence of time cues were examined. A significant, positive correlation was found between the sizes (in kcals) of mixed and varied meals and the lengths of postprandial intervals. This quantitative relationship between meal size and meal timing is similar to patterns reported for freely feeding rats. If postprandial correlations reflect a short term mechanism for energy regulation then when humans schedule their meals due to social or time considerations they may negate this contribution to the regulation of their energy intake.

The Sleep-Wake Pattern of Cortisol and Growth Hormone Secretion during Non-Entrained (Free-Running) Conditions in Man

It is now well established that all the peptide hormones secreted from the pituitary in man are related to the daily sleep-wake cycle. In addition to a temporal pattern of episodic secretion, the timing of the daily rhythm has been shown to be closely associated with sleep stages (1–6). The field of biological rhythm research has demonstrated that biological organisms have endogenous oscillations and that when these rhythms are no longer entrained to specific daily time cues (zeitgebers) they will develop free-running rhythms with period lengths greater than or less than 24 hours (circadian) (7–15).