This study investigated the effects of melatonin on the sleep quality of healthy college students with aerobic exercise as a covariate, using a single-factor, repeated measures design. Twenty-seven students (11 men and 16 women) were administered either 6 mg of melatonin or a physically identical placebo over a period of 6 nights. These participants, age 18-22, were prescreened for unstable sleeping patterns and excessive intake of sleep-altering substances. Each morning, participants rated the previous night's sleep and the previous day's exercise activities on a sleep and exercise questionnaire. Findings indicate that certain components of sleep quality, including the ease of falling asleep and the depth and continuity of sleep, significantly improved (p < .05) in the melatonin condition. However, subjective ratings of overall sleep quality and the psychological aspects of post-sleep experiences showed no significant improvement. This pattern implies that melatonin may have a limited effect among a healthy population that is not chronically sleep deprived.