Following abrupt phase shifts (real or simulated time zone changes, night shift work) there is desynchronisation between the internal circadian rhythms (including melatonin) and the external environment with consequent disturbances in sleep, mood and performance. In humans the pineal hormone melatonin has phase-shifting and resynchronising properties with regard to a number of circadian rhythms. Suitably timed melatonin administration hastened adaptation to phase shift and significantly improved self-rated jet lag in large numbers of time zone travellers. Preliminary results in night shift workers showed improved daytime sleep and night-time alertness. In simulated experiments, appropriately timed melatonin improved subjective sleep, alertness and performance and facilitated the readaptation of the melatonin rhythm following a rapid 9 h advance phase shift. Melatonin has also been assessed in circadian rhythm disorders with disturbed sleep (blindness and delayed sleep phase insomnia). Compared with placebo, melatonin significantly improved sleep and synchronised the sleep wake cycle in some blind subjects. Melatonin treatment significantly advanced the sleep onset time in delayed sleep phase insomnia. Taken together these findings suggest that melatonin is of benefit in facilitating adaptation to forced phase shifts and in conditions of circadian rhythm disturbance.