More than one-quarter of older people complain that they wake up frequently during the latter part of the night, with 15 percent reporting that they cannot sleep past 5 AM. Prior results show that early morning awakening also occurs in most healthy older people who have no sleep complaint, who are not depressed and who do not exhibit any evidence of age-related shortening of intrinsic circadian period. Studies funded by the current grant suggest that this age-related increase in sleep interruptions during the latter part of the night cannot be explained by an age-related change in circadian photic resetting sensitivity. Based on recent analyses, the investigators have found: 1) an age-related narrowing of the circadian phase interval over which the latter part of sleep can remain consolidated, leading to a marked age-related increase in the susceptibility of sleep to misalignment of circadian phase; and 2) an age-related increase in the variability of entrained circadian phase. It is hypothesized that these age-related circadian changes may account for a substantial component of the high prevalence of disrupted sleep and early morning awakening observed among older people. This implies that in older people, achieving and maintaining a precise (and narrow) relationship between circadian phase and the timing of sleep is crucial to attain sleep consolidation. The investigators' findings on the effect of light on human rhythms suggest that the optimum phase relationship between sleep and endogenous circadian rhythms in older people can be achieved by appropriately timed exposure to a novel light exposure pattern of brief, intermittent bright light pulses. Four testable hypotheses are proposed: (I) the phase of the melatonin and temperature rhythms and wake-time will occur at an earlier hour for older than for young adults living on self-selected sleep-wake schedules at home; (II) under base-line conditions on those self-selected schedules, wakefulness in the final third of the night will be higher for older subjects; (III) four brief exposures to evening bright light each day will be sufficient to establish and maintain entrainment of the sleep-wake cycle at an optimal circadian phase, resulting in reduced wakefulness in the final third of the night and enhanced daytime alertness; and (IV) a similar pattern of bright light exposure in the morning will not improve circadian phase misalignment.