Delayed Sleep Phase Syndrome (DSPS) describes patients who cannot fall asleep until several hours past their preferred bedtime and have extreme difficulty arising in the morning in order to attend to normal activities (e.g., school, work). This differs from traditional sleep-onset insomnia in that DSPS patients can fall asleep quickly and obtain a normal 8-hr sleep episode, but only at a much-delayed clock hour (e.g., 3 am to 11 pm). Prevalence estimates range from 0.13 percent to 7 percent. Peak onset is in the second and third decades. Possible consequences of this disorders include increased risk for accidents, impaired learning, higher use of alertness-enhancing substances, and lower productivity. The specific aims are: 1. To confirm the hypothesis that the circadian phase, as assessed from core body temperature and salivary melatonin, will be delayed, with week-to-week stability, in patients with Delayed Sleep Phase Syndrome (DSPS) relative to normal-sleeping, age- and gender-matched control subjects. 2. To test the hypothesis that DSPS patients will display the same internal phase relationships between their habitual bedtimes and wake-times, and the circadian phase markers of the minimum of core body temperature, the dim-light melatonin onset, and the fitted maximum of melatonin, but will have a larger amplitude of core body temperature and melatonin rhythms. 3. To test the hypothesis that with a sufficient increase in sleep homeostatic drive, given by the sleep loss incurred during a constant routine protocol, patients with DSPS syndrome will be able to obtain a normal sleep episode at a clock time 3 hours earlier than their habitual, delayed schedule. However, unlike normal control subjects, patients with DSPS return to their baseline pattern of inability to fall asleep at this earlier hour, on a second recovery night of sleep following the constant routine. The proposed investigations fulfill the criteria for the R21 mechanism by offering innovation (e.g., first measurement of temporal stability of phase delay, first measurement of full circadian and sleep parameters), bringing approaches new to an area (e.g., constant routine protocol), and initiating the first step in programmatic research for this investigator (basic mechanisms, diagnostic protocols, and evidence-based treatments for circadian phase disorders, beginning with DSPS).