Masking is known to affect a variety of circadian rhythms, making it difficult to use them as reliable markers of circadian phase position. Melatonin may be unique in that it appears to be masked only by (bright) light. Sleep and activity do not appear to influence the melatonin rhythm. By measuring the onset of melatonin production, a clearly demarcated event, we can reliably assess circadian phase position, provided blood is sampled under dim light (the dim light melatonin onset, or DL.MO). The DLMO has been useful in assessing the phase-shifting properties of bright light and in phase typing patients with chronobiologic disorders, such as winter depression.