Circadian Neurobiology and the Physiologic Regulation of Sleep and Wakefulness - PubMed (original) (raw)

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Circadian Neurobiology and the Physiologic Regulation of Sleep and Wakefulness

William J Schwartz et al. Neurol Clin. 2019 Aug.

Abstract

Endogenous central and peripheral circadian oscillators are key to organizing multiple aspects of mammalian physiology; this clock tracks the day-night cycle and governs behavioral and physiologic rhythmicity. Flexibility in the timing and duration of sleep and wakefulness, critical to the survival of species, is the result of a complex, dynamic interaction between 2 regulatory processes: the clock and a homeostatic drive that increases with wake duration and decreases during sleep. When circadian rhythmicity and sleep homeostasis are misaligned-as in shifted schedules, time zone transitions, aging, or disease-sleep, metabolic, and other disorders may ensue.

Keywords: Circadian rhythms; Homeostasis; Physiology; Sleep; Suprachiasmatic nucleus.

Copyright © 2019 Elsevier Inc. All rights reserved.

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Figures

Figure 1.

Figure 1.. Terminology and Displays

A. A schematic circadian rhythm, on the left entrained to a 12-h:12-h light:dark cycle and on the right free-running under constant darkness; white bar, light; black bar, dark. Under entrainment, zeitgeber time (zt) 0 represents the time of lights-on (dawn) and zt 12 the time of lights-off (dusk); during free-running, circadian time (ct) 0 represents “subjective” dawn and ct 12 “subjective” dusk. T = period length of the zeitgeber;τ = period length of the free-running rhythm; φ = phase, a defined, stable cycle-to-cycle reference point within the cycle of the rhythm (e.g.,φ1, the minimum value);Ψ = phase angle of entrainment, the time difference (phase relationship) between a defined phase of the rhythm and an external phase-reference (e.g., zt 0). B. A schematic circadian rhythm in “actogram” format. Data (e.g., locomotor activity) is plotted horizontally from left to right over the course of 24-h periods, with succeeding days stacked vertically from top to bottom. LD, light:dark cycle; DD, constant darkness. C. A schematic circular plot of the phases of individual rhythms from two populations, from 0 to 24 h (or sometimes graphed from 0 to 360°). The solid radial arrow represents the mean phase of each population and its length a measure of the scatter within each population. This format for plotting angular data illustrates that a mean of 15 h for the red population is not a larger value than a mean of 3 h for the blue population, but rather a difference in their relative phases.

Figure 2.

Figure 2.. A Network for Internal Time

Simplified cartoon of the mammalian circadian timekeeping system, with the suprachiasmatic nucleus (SCN) entrained by the light:dark cycle and “peripheral” body clocks entrained by SCN output signals, including rhythmic behaviors, body temperature, hormone levels, and nervous system activity; not shown are other clocks within the nervous system itself. These subsidiary clocks and rhythms also can respond to inputs downstream of the SCN (e.g., a shifted cycle of food availability on the hepatic clock). Of possible feedbacks, two exemplary ones are illustrated (i.e., locomotor activity itself affecting body temperature; adrenal output itself acting as a coupling signal).

Figure 3.

Figure 3.. The Regulation of Sleep and Wake

A. Inter-relationships of factors affecting multiple aspects of sleep and wake. B. Diagrams of changes in time of (left) Two-Process Model of sleep regulation. S=Process S, the sleep homeostatic factor; C=Process C, the circadian rhythm process; Grey areas=time of sleep. When an individual self-selects their sleep onset (one of the two vertical lines), the level of homeostatic drive is at the Process S level (upper line); homeostatic drive then declines in an exponential manner until it reaches the Process C level. Then the person awakens and the level of homeostatic drive begins to rise again (which is not shown). (Right) Sleep durations associated with different self-selected sleep onsets.

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