Bright Light as Chronobiological Countermeasure for Shiftwork in Space

Abstract

Work-rest schedules during long duration space missions involve se- veral factors which could disrupt sleep and circadian temporal organisation: (1) displacement of sleep due to two-shift operations; (2) planned or unplanned schedule changes due to operational require-ments; (3) social and light zeitgebers different from those on earth; (4) changes in the gravitational exposure. Timed bright light treat-ment has the potential to accelerating adaptation to schedule chan- ges. Four male subjects were exposed to two sessions of 11 d of simu-lated microgravity (6 head down tilt bedrest) with 6-h extensions of the wake period on 2 days (12-h phase delay). In a blind crossover design, subjects were exposed to bright light (> 3500 lux) for 5 h on each of the 2 shift days and the following day, at times either ex- pected to accelerate the adjustment to the phase delay (experimental condition) or to have no phase shifting effect (control condition). Sleep was recorded polygraphically, the circadian system was monito- by recordings of heart rate and body temperatur, and by collection of urine (electrolyte and hormone excretion). Only the rhythms of 6-hy-droxymelatoninsulphate and potassium excretions showed significantly enhanced adjustment under the experimental condition. Different rhythms adapted to the 12-h delay at different rates, comparable to those observed after time zone shifts. Sleep was shorter in simulated weightlessness than in normal ambulatory age-matched subjects, con- sistent with the shorter sleep durations characteristic of space flight. These results confirm the disruptive effects of wake-rest schedule shifts on sleep and circadian rhythms. Contrary to our ini- tial hypothesis, 5-h exposure to bright light finishing at the time of the circadian temperature minimum were not more effective at acce-lerating adjustment to a 12-h schedule delay than exposures coinci- ding with the temperature maximum. We conclude that, while bright light may accelerate adjustment to work-rest schedule delays, any.