When administered to humans the pineal hormone melatonin can phase shift a number of circadian rhythms. This property has prompted the investigation of exogenous melatonin in sleep disorders known to have an underlying chronophysiological basis (i.e. circadian rhythm sleep disorders). Both in field and simulated studies of jet lag and shift work suitably timed melatonin improved sleep and, in some cases, hastened readaptation of the circadian rhythms following the phase shift. Melatonin treatment has also been evaluated in the circadian sleep disorders: delayed sleep phase syndrome (DSPS) and non-24-hour sleep wake disorder. Compared with placebo, melatonin advanced the sleep period in subjects with DSPS. Melatonin also improved a number of sleep parameters in blind subjects suffering from non-24-hour sleep wake disorder.