This study's first aim is to widen the knowledge of the characteristics of delayed sleep phase disorder (DSPS) by focusing on the circadian rhythms of appetite regulation factors and their phase relations to the cycles of sleep-wake, melatonin, cortisol and body temperature. This study's second aim is to assess the influence of forced morning awakening, as a daily struggle faced by DSPS patients, upon the synchronization of these variables in DSPS patients. The investigators hypothesize that the chronic incompatibility between the endogenous sleep-wake rhythm of the DSPS patients and the morning wakefulness, as a social demand, may impair the synchronization between the different rhythms, as findings indicate in normal subjects under jet lag. And finally, the third aim of the study is to assess the influence of successful treatment with melatonin upon the phase locations of circadian rhythms of studied measures and the synchronization between them. These measures will be assessed in a controlled study, for 36 hours (sampled every 2 hours) under three distinct experimental conditions: first, under free sleep-wake conditions (ad-libitum bedtime and arousal); second, under restricted sleep-wake conditions (enforced morning wake-up); and finally, after 12 weeks of melatonin treatment.