Bright Light Exposure in the Morning and in the Afternoon after 6-h Advance Shift

Kurzfassung

The aim of the present study was to assess the time-of-day effect of bright light exposure on the resynchronization speed of circadian functions after a 6-h time advance. Eight healthy male subjects, aged 20 to 25 y, underwent a 6-h advance shift in our isolation facility during two periods each of ten days. The time shift was simulated on the 6th day by shortening the sleep period by 6h. In a cross-over design, subjects were exposed to bright light of 4 h duration either from 400 to 800 h or from 1300 to 1700 h local time on the two days following the shift. Body temperature was recorded continuously and urine was collected at 3-h intervals all day and night. Morning light exposure enhanced the resynchronization speed of temperature rhythm significantly only on day 7 compared with afternoon light exposure. The difference was 1.16 h, the absolute phase shift was 4.76 h and 3.6 h compared with baseline. Potassium excretion rate did not show any difference in acrophase shifts between conditions. Absolute shifts were 2.86 h (morning) and 2.53 h (afternoon) between baseline and the final day of each study phase.Circadian amplitudes were significantly reduced when compared to baseline, but no difference occured between light conditions. From the results of this study can be concluded that either bright light exposure does influence the circadian system less independentlyof the time of day than previously expected, or in conjunction with other potential zeitgebers (e.g., social, sleep-wake-cycle) the contribution of bright light is not as important as anticipated from results of other laboratories.