Time-of-day-dependent effects of bright light exposure on human psychophysiology: comparison of daytime and nighttime exposure Melanie Rüger, Marijke C. M. Gordijn, Domien G. M. Beersma, Bonnie de Vries and Serge Daan Am J Physiol Regulatory Integrative Comp Physiol 290:1413-1420, 2006. First published Dec 22, 2005; doi:10.1152/ajpregu.00121.2005 You might find this additional information useful... This article cites 48 articles, 15 of which you can access free at: http://ajpregu.physiology.org/cgi/content/full/290/5/R1413#BIBL Updated information and services including high-resolution figures, can be found at: http://ajpregu.physiology.org/cgi/content/full/290/5/R1413 Additional material and information about American Journal of Physiology - Regulatory, Integrative and Comparative Physiology can be found at: Downloaded from http://www.the-aps.org/publications/ajpregu This information is current as of November 30, 2006 . ajpregu.physiology.org on November 30, 2006 The American Journal of Physiology - Regulatory, Integrative and Comparative Physiology publishes original investigations that illuminate normal or abnormal regulation and integration of physiological mechanisms at all levels of biological organization, ranging from molecules to humans, including clinical investigations. It is published 12 times a year (monthly) by the American Physiological Society, 9650 Rockville Pike, Bethesda MD 20814-3991. Copyright © 2005 by the American Physiological Society. ISSN: 0363-6119, ESSN: 1522-1490. Visit our website at http://www.the-aps.org/. Am J Physiol Regul Integr Comp Physiol 290: R1413–R1420, 2006. First published December 22, 2005; doi:10.1152/ajpregu.00121.2005. Time-of-day-dependent effects of bright light exposure on human psychophysiology: comparison of daytime and nighttime exposure Melanie Ru¨ger,1,2 Marijke C. M. Gordijn,1 Domien G. M. Beersma,1 Bonnie de Vries,1 and Serge Daan1 1Department of Chronobiology, University of Groningen, Groningen, The Netherlands; and 2Center for Brain Health, New York University Medical School, New York, New York Submitted 17 February 2005; accepted in final form 15 December 2005 Ru¨ger, Melanie, Marijke C. M. Gordijn, Domien G. M. in CBT, it is sometimes assumed that melatonin is the causal Beersma, Bonnie de Vries, and Serge Daan. Time-of-day-depen- factor in this process (4, 9, 22). However, data from daytime dent effects of bright light exposure on human psychophysiology: bright light exposure studies show that bright light can reduce comparison of daytime and nighttime exposure. Am J Physiol Regul sleepiness even though melatonin is virtually absent and CBT Integr Comp Physiol 290: R1413–R1420, 2006. First published De- is nearly constant (42, 43). We have shown that the relation cember 22, 2005; doi:10.1152/ajpregu.00121.2005.—Bright light can influence human psychophysiology instantaneously by inducing en- between melatonin suppression and reduction of sleepiness/ docrine (suppression of melatonin, increasing cortisol levels), other fatigue is weak, and therefore, melatonin suppression cannot Downloaded from physiological changes (enhancement of core body temperature), and be the sole explanation for the activating properties of bright psychological changes (reduction of sleepiness, increase of alertness). light (45). Its broad range of action is reflected in the wide field of applications, Few studies have focused on the effects of light on the ranging from optimizing a work environment to treating depressed autonomic nervous system. The results of these studies are patients. For optimally applying bright light and understanding its moreover difficult to compare because they vary greatly in the mechanism, it is crucial to know whether its effects depend on the amount of light used and in output variables measured. Saito et time of day. In this paper, we report the effects of bright light given al. (46) and Scheer et al. (48) showed an increase in muscle ajpregu.physiology.org at two different times of day on psychological and physiological sympathetic nerve activity and heart rate in response to bright parameters. Twenty-four subjects participated in two experiments (n ϭ 12 each). All subjects were nonsmoking, healthy young males light, and Gilbert and coworkers (22) found a reduction of the (18–30 yr). In both experiments, subjects were exposed to either heart rate of healthy young males after the administration of bright light (5,000 lux) or dim light Ͻ10 lux (control condition) either exogenous melatonin (5 mg) during the afternoon. Burgess et between 12:00 P.M. and 4:00 P.M. (experiment A) or between mid- al. (8) failed to find a clear effect of bright light on cardiac night and 4:00 A.M. (experiment B). Hourly measurements included output measures such as respiratory sine arrhythmia, preejec- salivary cortisol concentrations, electrocardiogram, sleepiness (Karo- tion period, and diastolic blood pressure. Tsunoda et al. (50) linska Sleepiness Scale), fatigue, and energy ratings (Visual Analog observed an increase in the low frequency-to-high frequency on November 30, 2006 Scale). Core body temperature was measured continuously throughout ratio of the heart rate variability after bright light exposure and the experiments. Bright light had a time-dependent effect on heart rate after exposure to complete darkness. Besides heart rate vari- and core body temperature; i.e., bright light exposure at night, but not ability, cortisol also shows a clear circadian rhythm with a peak in daytime, increased heart rate and enhanced core body temperature. around awakening (31). The circadian rhythm in cortisol is It had no significant effect at all on cortisol. The effect of bright light on the psychological variables was time independent, since nighttime largely under the control of the circadian pacemaker in the and daytime bright light reduced sleepiness and fatigue significantly suprachiasmatic nucleus (SCN; see Ref. 7). Therefore, it is to and similarly. be expected that the rhythm and the concentration of cortisol will be influenced by light. Indeed Leproult et al. (33) showed sleepiness; core body temperature; cortisol; heart rate that in sleep-deprived subjects,3hofbright light exposure (4,500 lux) in the early morning (5:00–8:00 A.M.) induced an BRIGHT LIGHT IS A prominent agent influencing human psycho- increase in cortisol levels, whereas afternoon (1:00–4:00 P.M.) physiology. Besides the ability to reset or shift the biological bright light exposure had no effect on cortisol. The cortisol clock (6, 25, 27, 38, 44, 50), bright light is thought to have an peak after awakening is present in total darkness and can be immediate activating effect on the central nervous system. This enhanced by1hof800luxapplied at habitual time of waking immediate effect has been studied mostly in the context of (47). Thorn et al. (49) showed that gradually increasing lumi- prolonged wakefulness to explore beneficial effects of bright nance levels (250 lux over 30 min) during awakening (dawn light on alertness and performance, for instance in shift work- simulation) increased cortisol levels compared with the control ers. Nighttime bright light exposure is known to reduce sleep- condition, in which subjects used their regular alarm clock to iness (11, 13, 44), enhance alertness (3, 5, 12, 17, 39), and to wake up, without additional increasing luminance. This in- improve mood and performance in healthy subjects (16, 21, crease in cortisol was accompanied by a higher level of 40). At the same time, it suppresses melatonin, enhances core reported arousal but not of reported stress. body temperature (CBT), and increases heart rate (35, 44, 46, Although the literature suggests that some of the variance in 48). Because the reduction of sleepiness is often accompanied the responses to bright light may be associated with time of by the suppression of nocturnal melatonin and/or the increase day, there is no straightforward analysis of such variation in Address for reprint requests and other correspondence: M. Ru¨ger, Centre for The costs of publication of this article were defrayed in part by the payment Brain Health, NYU Medical School, 550 First Ave., New York, NY 10016 of page charges. The article must therefore be hereby marked “advertisement” (e-mail: [email protected]). in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. http://www.ajpregu.org 0363-6119/06 $8.00 Copyright © 2006 the American Physiological Society R1413 R1414 DAYTIME VS. NIGHTTIME BRIGHT LIGHT EXPOSURE human psychophysiological variables (14, 15, 42, 43). The 0.155, not significant]}. Subjects gave written informed consent and fields of bright light application can range from clinical (light were paid for their participation. The medical ethics committee of therapy of Seasonal Affective Disorder patients and sleep Groningen University approved the protocol. disorder patients) to work (improving work environment for Time Isolation Facility shift workers) settings, and it is crucial to know to what extent immediate effects of bright light on human psychophysiology The protocol included either two stays of 1.5 days (experiment A: are time-of-day dependent. daytime bright light and dim light exposure) or three stays of 2.5 days For this reason, we compared two data sets of daytime and (experiment B: nighttime bright light, dim light, and extraocular light nighttime bright light exposure, 12 h out of phase with each exposure) each in the time isolation facility. Results from the com- parison of nighttime extraocular and ocular light exposure have been other, in humans and their effects on sleepiness, fatigue, and reported elsewhere (43). The facility, where neither daylight nor clock energy (psychological variables) as well as CBT, cortisol, and information is present, can host four subjects simultaneously in heart rate (physiological variables). Twenty-four subjects par- separate rooms. Subjects could read or study, listen to music, watch ticipated in the two studies and were exposed to4hof5,000 videos, or perform other nonphysical activities. Light sources present lux of bright light either between noon and 4:00 P.M.