Researchers have found that melatonin substantially reduced both sleep onset latency and the number of nocturnal awakenings in normal individuals as well as patients with diverse sleep pathologies. These findings have led to an increase in the number of individuals who ingest melatonin habitually to alleviate their sleep problem. Given the widespread use of melatonin, it is important to identify the factors capable of predicting physiological states while following a melatonin regimen. The result of this preliminary analysis showed that the number of hours spent outdoors is the best predictor of subjective sleepiness while under the influence of melatonin. Specifically, the more time individuals spend outdoors, the sleepier they are when administered melatonin.