Due to orbital mechanics, current space shuttle programs routinely require astronauts to repeatedly shift their sleep-wake schedules to an earlier hour during their approximately 2-week missions. This is achieved in some missions by imposing a sleep- wake schedule on a day length averaging 23.5-h. NASA's planned exploration class mission to Mars, currently scheduled for 2018, will require crew members to adapt to the 24.65-h Martian solar day throughout their approximately 540 day stay on Mars. However, the plasticity of the human circadian pacemaker's intrinsic period, which has recently been shown to average 24.1- 24.2 h immediately after release from entrainment to the 24-h day and which is a key determinant of the pacemaker's ability to adapt to such non-24-hour schedules, has not been systematically investigated. Data collected in animal studies provides considerable evidence that the intrinsic circadian period is affected by past history. Since the ability of astronauts to sustain a high level of performance during exploration class missions will be critically dependent upon the ability of the human circadian pacemaker to adapt to such non-24-hour day lengths, we propose to determine whether the intrinsic period of the human circadian pacemaker can be influenced by prior entrainment to non-24 hour day lengths. Specifically, we propose to test the following hypotheses: i) that entrainment of the human circadian pacemaker to a shorter- than-24-hour sleep-wake schedule will shorten the period of the circadian pacemaker; ii) that entrainment of the human circadian pacemaker to a longer-than-24-hour sleep-wake schedule will lengthen the period of the circadian pacemaker; iii) that the phase angle of entrainment to an imposed non-24-hour day will be strongly related to the period of the circadian pacemaker. The results of the proposed studies will answer fundamental questions on the mechanisms underlying circadian entrainment in humans and lead to the development of space flight countermeasures such as pre-adaptation of astronauts to non-24-hour days prior to space flight.