Effects of light exposure and sleep displacement on dim light melatonin onset

The purpose of the study was to induce in two different ways, a phase-angle difference between the circadian pacemaker and the imposed sleep-wake cycl e in humans, we intended to: (i) shift the circadian pacemaker by exposure to bright light and keep the timing of the sleep-wake cycle fixed; and (ii) keep the timing of the circadian pacemaker fixed by a constant light-dark cycle and displace sleep. We monitored dim light melatonin onset (DLMO), co re body temperature and sleep. DLMO was delayed significantly after 3 days of a 3-h delayed sleep-phase when compared with 3 days of sleep at a normal or 3-h advanced sleep-phase. The shifts in DLMO were not accompanied by sh ifts in body temperature, changes in waking-up time or by a change in the d uration of the first rapid eye movement (REM) sleep episode. Three days of light exposure in the morning or evening resulted in shifts in DLMO of simi lar magnitude, but this was accompanied by shifts in the rhythm of body tem perature, changes in waking-up time and in the duration of the first REM sl eep episode. We conclude that the changes observed after light exposure ref lect shifts in the circadian pacemaker. In contrast, we propose that the ch anges observed in DLMO after sleep displacement are not mediated by the cir cadian pacemaker. These results raise some doubts about the reliability of DLMO as a marker of circadian phase in cases of sleep disturbances. Finally , we initiate a search for changes in sleep that might be responsible for t he unexpected effects on DLMO.