Human phase response curve to a 1 h pulse of bright white light

Key points The human circadian pacemaker generates near-24-h rhythms that set the timing of many physiological, metabolic and behavioural body rhythms, and is synchronized to environmental time primarily by the 24 h lightdark cycle. The magnitude and direction of the resetting response of the pacemaker to light depends on the time of day of exposure, and the change in responses over the day is summarized in a phase response curve (PRC). A previous PRC showed that a 6.7 h bright white light exposure maximally shifted the circadian pacemaker by over 3 h. We show that a PRC to a 1 h bright white light pulse maximally shifted the circadian pacemaker by similar to 2 h, despite representing only similar to 15% of the exposure duration. This study demonstrates that the circadian pacemaker is sensitive to short-duration light pulses with a non-linear relationship between light duration and the amount of resetting. Abstract The phase resetting response of the human circadian pacemaker to light depends on the timing of exposure and is described by a phase response curve (PRC). The current study aimed to construct a PRC for a 1 h exposure to bright white light (similar to 8000 lux) and to compare this PRC to a <3 lux dim background light PRC. These data were also compared to a previously completed 6.7 h bright white light PRC and a <15 lux dim background light PRC constructed under similar conditions. Participants were randomized for exposure to 1 h of either bright white light (n= 18) or <3 lux dim background light (n= 18) scheduled at 1 of 18 circadian phases. Participants completed constant routine (CR) procedures in dim light (<3 lux) before and after the light exposure to assess circadian phase. Phase shifts were calculated as the difference in timing of dim light melatonin onset (DLMO) during pre- and post-stimulus CRs. Exposure to 1 h of bright white light induced a Type 1 PRC with a fitted peak-to-trough amplitude of 2.20 h. No discernible PRC was observed in the <3 lux dim background light PRC. The fitted peak-to-trough amplitude of the 1 h bright light PRC was similar to 40% of that for the 6.7 h PRC despite representing only 15% of the light exposure duration, consistent with previous studies showing a non-linear durationresponse function for the effects of light on circadian resetting.