Work-rest schedules during long duration space missions involve several factors which could disrupt sleep and circadian temporal organization: 1) substantial displacement of sleep with respect to mission control time, due to two-shift operations; 2) schedule changes, either planned or in response to unforeseen operational events; 3) social and light zeitgebers substantially different from those on earth; 4) pulses of hypergravity associated with launch and re-entry, and prolonged exposure to microgravity. Timed bright light exposures have recently received much attention as a possible countermeasure to accelerate adaptation to schedule changes. Four male subjects were therefore exposed to two sessions of eleven days of simulated weightlessness (6° head-down tilt bedrest) with six hour extensions of the scheduled waketime on days 3 and 4 (12 h phase delay). In a blind cross-over design, subjects were exposed to bright (>3500 lux) light for five hours on each of the two shift days and the following day, at times expected to accelerate adaptation to the phase delay (experimental group) or have no phase shifting effect (control group). Sleep was recorded polygraphically and the circadian system monitored by recordings of heart rate and rectal temperature (2 min samples), and urinary excretion of hormones and electrolytes (3 hourly samples during wake). Only the rhythms of 6-hydroximelatoninsulphate and potassium excretion showed significantly accelerated adaptation in the experimental protocol. Different rhythms adapted to the 12 h delay at different rates, comparable to those observed in ambulatory subjects after time zone shifts. Sleep was shorter in simulated weightlessness than in normal ambulatory age-matched controls, consistent with the shorter sleep durations characteristic of space flight. In summary, these results confirm the disruptive effects of wake/rest schedule shifts on sleep and circadian rhythms during simulated weightlessness. Contrary to our initial hypothesis, five hour exposures to bright light finishing at the time of the circadian temperature minimum were no more effective at accelerating adaptation to a 12 h schedule delay than exposures coinciding with the temperature maximum. We conclude that, while bright light may accelerate adaptation to work/rest schedule delays under simulated weightless conditions, any such effect seems to be largely independent of the timing of the light exposure.