Simulations of the effect of light on the human circadian
pacemaker: implications for assessment of intrinsic period.
Klerman, Elizabeth B., Derk-Jan Dijk, Richard E. Kronauer, Charles A.
Czeisler.
Section on Sleep Disorders/Circadian Medicine, Division of
Endocrinology-Hypertension, Department of Medicine, Harvard Medical
School & Brigham and Women's Hospital, Boston, MA USA; Institute
of Pharmacology, University of Z[umlaut]urich, Winterthurerstrasse
190, CH8057, Zurich, Switzerland; Division of Applied Sciences,
Harvard University, Cambridge, MA USA
APStracts 2:0248R, 1995.
The sensitivity of the human circadian system to light has been the
subject of considerable debate. Using computer simulations of a
recent quantitative model for the effects of light on the human
circadian system, we investigate these effects of light during
different experimental protocols. The results of the simulations
indicate that the non-uniform distribution over the circadian cycle
of exposure to ordinary room light seen in classical free-run
studies, in which subjects select their exposure to light and
darkness, can result in an observed period of approximately 25 hours,
even when the intrinsic period of the subject's endogenous circadian
pacemaker is much closer to 24 hours. Other simulation results
suggest that accurate assessment of the true intrinsic period of the
human circadian pacemaker requires low ambient light intensities
(approximately 10 -15 lux) during scheduled wake episodes,
desynchrony of the imposed light-dark cycle from the endogenous
circadian oscillator, and a study length of at least 20 days.
Although these simulations await further experimental substantiation,
they highlight the sensitivity to light of the human circadian system
and the potential confounding influence of light on the assessment of
the intrinsic period of the circadian pacemaker.
Received 31 March 1995; accepted in final form 14 August 1995.
APS Manuscript Number R212-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.