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Efficacy of a single sequence of intermittent bright light pulses for delaying circadian phase in humans

¹Division of Sleep Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston 02115; ²Harvard University, Cambridge, Massachusetts 02138; and ³Department of Integrative Physiology, Center for Neuroscience, University of Colorado, Boulder, Colorado 80309

Submitted 25 August 2003 ; accepted in final form 9 March 2004

It has been shown in animal studies that exposure to brief pulses of bright light can phase shift the circadian pacemaker and that the resetting action of light is most efficient during the first minutes of light exposure. In humans, multiple consecutive days of exposure to brief bright light pulses have been shown to phase shift the circadian pacemaker. The aim of the present study was to determine whether a single sequence of brief bright light pulses administered during the early biological night would phase delay the human circadian pacemaker. Twenty-one healthy young subjects underwent a 6.5-h light exposure session in one of three randomly assigned conditions: 1) continuous bright light of 9,500 lux, 2) intermittent bright light (six 15-min bright light pulses of 9,500 lux separated by 60 min of very dim light of <1 lux), and 3) continuous very dim light of <1 lux. Twenty subjects were included in the analysis. Core body temperature (CBT) and melatonin were used as phase markers of the circadian pacemaker. Phase delays of CBT and melatonin rhythms in response to intermittent bright light pulses were comparable to those measured after continuous bright light exposure, even though the total exposure to the intermittent bright light represented only 23% of the 6.5-h continuous exposure. These results demonstrate that a single sequence of intermittent bright light pulses can phase delay the human circadian pacemaker and show that intermittent pulses have a greater resetting efficacy on a per minute basis than does continuous exposure.

melatonin; circadian pacemaker; photoreception; phase shift

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