Repeated exposures to daytime bright light increase nocturnal melatonin rise and maintain circadian phase in young subjects under fixed sleep schedule

• Published in: American journal of physiology. Regulatory, integrative and comparative physiology Vol. 291; no. 6; pp. R1799 - R1807
• Main Authors: Takasu, Nana N, Hashimoto, Satoko, Yamanaka, Yujiro, Tanahashi, Yusuke, Yamazaki, Ayano, Honma, Sato, Honma, Ken-ichi
• Format: Journal Article
• Language: English
• Published: United States American Physiological Society 01.12.2006
• Subjects: Adult; Biological Clocks - physiology; Biological Clocks - radiation effects; Body temperature; Body Temperature - physiology; Body Temperature - radiation effects; Circadian rhythm; Circadian Rhythm - physiology; Circadian Rhythm - radiation effects; Circadian rhythms; Environmental Exposure; Hormones; Humans; Light; Male; Metabolic Clearance Rate - radiation effects; Plasma; Sleep; Sleep - physiology; Sleep - radiation effects
• ISSN: 0363-6119; 1522-1490
• DOI: 10.1152/ajpregu.00211.2006

Department of Physiology, Hokkaido University Graduate School of Medicine, Sapporo, Japan Submitted 24 March 2006 ; accepted in final form 6 July 2006 Effects of two different light intensities during daytime were examined on human circadian rhythms in plasma melatonin, core body temperature, and wrist activity under a fixed sleep schedule. Sleep qualities as indicated by polysomnography and subjective sleepiness were also measured. In the first week, under dim light conditions ( 10 lx), the onset and peak of nocturnal melatonin rise were significantly delayed, whereas the end of melatonin rise was not changed. The peak level of melatonin rise was not affected. As a result, the width of nocturnal melatonin rise was significantly shortened. In the second week, under bright light conditions ( 5,000 lx), the phases of nocturnal melatonin rise were not changed further, but the peak level was significantly increased. Core body temperature at the initial sleep phase was progressively elevated during the course of dim light exposure and reached the maximum level at the first night of bright light conditions. Subjective sleepiness gradually declined in the course of dim light exposure and reached the minimum level at the first day of bright light. These findings indicate that repeated exposures to daytime bright light are effective in controlling the circadian phase and increasing the peak level of nocturnal melatonin rise in plasma and suggest a close correlation between phase-delay shifts of the onset of nocturnal melatonin rise or body temperature rhythm and daytime sleepiness. light intensity; core temperature; sleepiness; polysomnography; entrainment Address for reprint requests and other correspondence: K. Honma, Dept. of Physiology, Hokkaido Univ. Graduate School of Medicine, Sapporo 060-8638, Japan (e-mail: kenhonma{at}med.hokudai.ac.jp )