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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).