With the development of accurate and sensitive assays for measuring melatonin in plasma and saliva, it has been possible to advance our understanding of human chronobiology. In particular, the dim light melatonin onset (DLMO) is expected to have an increasingly important role in the diagnosis of circadian phase disorders and their treatment with appropriately timed bright light exposure and/or low-dose melatonin administration. The phase angle difference (PAD) between DLMO and mid-sleep can be used as a marker for internal circadian alignment and may also be used to differentiate individuals who are phase advanced from those who are phase delayed (a long interval indicates the former and a short interval indicates the latter). To provide a corrective phase delay, light exposure should be scheduled in the evening and melatonin should be administered in the morning. To provide a corrective phase advance, light exposure should be scheduled in the morning and melatonin should be administered in the afternoon/evening. The study of patients with seasonal affective disorder (SAD), as well as individuals who are totally blind, has resulted in several findings of interest to basic scientists, as well as psychiatrists and sleep specialists.