Bright light resets the human circadian pacemaker independent of the timing of the sleep-wake cycle

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Authors: Charles A. Czeisler, James S. Allan, Steven H. Strogatz, Jospeh M. Ronda, Ramiro Sanchez and C. David Rios
Date: Aug. 8, 1986
From: Science(Vol. 233)
Publisher: American Association for the Advancement of Science
Document Type: Article
Length: 4,741 words

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Bright Light Resets the Human Circadian Pacemaker Independent of the Timing of the Sleep-Wake Cycle

IN THE 25 YEARS SINCE DECOURSEY discovered the phase response curve to light in the flying squirrel (1), the resetting of biological clocks by light has been characterized in nearly all species studied except man. Synchronization of the human circadian system, which usually has an intrinsic period greater than 24 hours (2, 3), to a 24-hour day implies that our biological clocks are reset daily. Yet, a specific resetting stimulus that shifts the phase of the human circadian pacemaker has not been identified. In a controlled case study, we have demonstrated that critically timed exposure to bright indoor light can rapidly reset the human circadian pacemaker by about 6 hours, even when the timing of the sleep-wake cycle is constant.

Despite documentation of human neuroanatomic structures analogous to those subserving circadian rhythmicity and photic entrainment in other mammals (4), attempts to assess the specific role of light in the synchronization of the human circadian system have been methodologically difficult. In contrast to the results of animal studies, the light-dark cycle was reported to be too weak a synchronizing cue to entrain human circadian rhythms (5); however, these experiments were confounded by the subjects' access to auxiliary lighting. In 1981, we demonstrated that a true light-dark cycle could entrain human circadian rhythms (6). However, studies of light-dark cycle entrainment in humans cannot distinguish whether synchronization occurs (i) directly through an action of light on the endogenous circadian pacemaker or (ii) indirectly by an influence on the behavioral rest-activity cycle (7). Because subjects attempt to sleep when it is dark and are awakened by light, the light-dark cycle influences the timing of the subjects' sleep-wake cycle, which itself may be a synchronizing agent (6, 8).

Having demonstrated that bright light must exceed a minimum threshold (>2500 lux) to suppress melatonin secretion (9), Lewy has suggested that bright light may have a more powerful effect on circadian rhythms than ordinary indoor illumination (10). However, it is inherently difficult to demonstrate a direct physiologic synchronizing effect of light beyond its potential indirect influence via behavior because the maximally responsive phase of the circadian cycle normally occurs during the customary sleep time in diurnal animals (11). Thus, stimulation of the subject by light would intrude on the normal sleep-wake cycle. Hence, we decided to search for individuals with a normal sleep-wake cycle and a markedly advanced endogenous circadian phase (ECP) position (12), our reason being that the portion of the circadian cycle sensitive to phase-delay shifts by light might be accessible during their usual waking hours in the evening, a time ordinarily free from exposure to bright light.

Because there is an age-related shortening of the internally synchronized free-running period (13), we hypothesized that, on average, the circadian timing system in the elderly would be internally phase advanced with respect to sleep. On screening a group of healthy elderly subjects (14), we identified a woman whose sleep was normal, but...

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Source Citation
Czeisler, Charles A., et al. "Bright light resets the human circadian pacemaker independent of the timing of the sleep-wake cycle." Science, vol. 233, 8 Aug. 1986, pp. 667+. Accessed 8 Aug. 2022.

Gale Document Number: GALE|A4329767