AJP - Regulatory, Integrative and Comparative Physiology, Vol 268, Issue 2 528-R535, Copyright © 1995 by American Physiological Society
Differential effects of bright light and social cues on reentrainment of human circadian rhythms
K. Honma, S. Honma, K. Nakamura, M. Sasaki, T. Endo and T. Takahashi
Department of Physiology, Hokkaido University School of Medicine, Sapparo, Japan.
Reentrainment of human circadian rhythm to an 8-h advanced schedule of
sleep and social contacts was assessed under two different conditions: with
and without bright light (4,000-6,000 lx). Subjects spent 15 days without
knowing the natural day-night alternation. On the fourth day, the social
schedule was phase-advanced by 8 h. In one experiment, a bright light pulse
of 3-h duration was given in every subjective morning, and in the other no
light pulse was applied. Plasma melatonin and rectal temperature rhythms
were measured. Seven of nine subjects showed an orthodromic phase shift,
the rate of which was significantly larger with bright light pulses than
without them. The maximum phase-advance shift by three consecutive light
pulses was observed when the first pulse was applied approximately 4 h
after the onset of melatonin rise. By contrast, the maximum phase shift of
a similar extent was detected at 1 h after the onset of melatonin rise,
when ordinary room light (300-500 lx) at the time corresponding to bright
light was regarded as a dim light pulse. It is concluded that bright light
accelerates the reentrainment of human circadian rhythm, and bright light
and social schedule have differential effects on the reentrainment.
