Temporal dynamics of late-night photic stimulation of the human circadian timing system

• Published in: American journal of physiology. Regulatory, integrative and comparative physiology Vol. 289; no. 3; pp. R839 - R844
• Main Authors: Zeitzer, Jamie M, Khalsa, Sat Bir S, Boivin, Diane B, Duffy, Jeanne F, Shanahan, Theresa L, Kronauer, Richard E, Czeisler, Charles A
• Format: Journal Article
• Language: English
• Published: United States 01.09.2005
• Subjects: Adult; Circadian Rhythm - physiology; Circadian Rhythm - radiation effects; Humans; Light; Logistic Models; Male; Melatonin - antagonists & inhibitors; Melatonin - blood; Melatonin - radiation effects; Models, Biological; Osmolar Concentration; Photic Stimulation - methods; Space life sciences; Time Factors; NASA Discipline Regulatory Physiology; Non-NASA Center
• ISSN: 0363-6119; 1522-1490
• DOI: 10.1152/ajpregu.00232.2005

2 Division of Sleep Medicine, Department of Medicine, Harvard Medical School, Brigham and Women’s Hospital, Boston; and 1 Division of Applied Sciences, Harvard University, Cambridge Massachusetts Submitted 4 April 2005 ; accepted in final form 6 May 2005 The light-dark cycle is the primary synchronizing factor that keeps the internal circadian pacemaker appropriately aligned with the environmental 24-h day. Although it is known that ocular light exposure can effectively shift the human circadian pacemaker and do so in an intensity-dependent manner, the curve that describes the relationship between light intensity and pacemaker response has not been fully characterized for light exposure in the late biological night. We exposed subjects to 3 consecutive days of 5 h of experimental light, centered 1.5 h after the timing of the fitted minimum of core body temperature, and show that such light can phase advance shift the human circadian pacemaker in an intensity-dependent manner, with a logistic model best describing the relationship between light intensity and phase shift. A similar sigmoidal relationship is also observed between light intensity and the suppression of plasma melatonin concentrations that occurs during the experimental light exposure. As with a simpler, 1-day light exposure during the early biological night, our data indicate that the human circadian pacemaker is highly sensitive even to typical room light intensities during the late biological night, with 100 lux evoking half of the effects observed with light 10 times as bright. light; phase shift; melatonin suppression Address for reprint requests and other correspondence: C. A. Czeisler, Div. of Sleep Medicine, Brigham and Women’s Hospital, 221 Longwood Av., Ste. 438, Boston MA 02115 (e-mail: caczeisler{at}rics.bwh.harvard.edu )