• change in sleep efficiency
  
• shift of circadian phase of melatonin secretion
  

• change in alertness and performance
  

The endogenous circadian pacemaker (the natural biological rhythm of a 24-hour cycle) is a major determinant of the timing of sleep and sleep structure in humans. There are considerable data from animals and humans suggesting that the properties of the circadian pacemaker change with advancing age. It has been hypothesized that these changes may underlie the sleep disruption and reduction in daytime alertness observed in the elderly. Recent studies have confirmed the impact of endogenous circadian phase on REM sleep (Rapid Eye Movement--the period of sleep associated with dreaming) and have revealed that high sleep efficiency can only be maintained when there is a unique phase-relationship between the sleep episode and endogenous circadian phase. This phase-relationship is such that even a small change in the relative timing of the circadian pacemaker and the daily sleep episode can have a large impact on an individual's ability to consolidate sleep throughout the night, especially in older individuals.

In this field-laboratory study, participants will first be monitored for 3 weeks while living at home on a self-selected sleep-wake schedule. They will then enter the laboratory for a 13-day study. The laboratory study begins with 3 baseline days living on their habitual schedule, and participants will be allowed to leave the hospital each day, returning in the evening. After this 3-day baseline, the initial circadian phase will be estimated in a constant posture (CP protocol). This is followed by a 4-day light treatment, when the participant will be exposed to approximately a 2 hour light session each evening. As in the baseline, the participant will be allowed to leave the hospital during the daytime hours, returning in the early evening. Following the 4-day treatment, a second CP will be conducted. After the CP, a 3-day laboratory follow up will take place (similar to the baseline), and this will be followed by an ambulatory follow-up, where the participant will be monitored with an actigraphy monitor for 3 weeks while living at home (as in the ambulatory baseline). Sleep will be polysomnographically recorded (oxygen saturation, electrocardiography, air flow, respiratory effort, limb movement, eye and jaw muscle movement, and brain electrical activity) each night in the laboratory, blood samples will be collected during each CP so that the phase of the circadian rhythm of melatonin secretion can be assessed, and activity monitoring will continue from the ambulatory baseline through the ambulatory follow-up. Tests of performance and alertness will be conducted during the times the participant is awake in the laboratory.