Both light and melatonin, appropriately timed, have been shown to phase-shift human circadian rhythms. In addition, both light and melatonin have acute physiological and behavioural effects. Depending on the dose, melatonin can reduce core body temperature and induce sleepiness. Conversely, light at night increases body temperature and enhances alertness and performance. The acute and phase-shifting effects of light and melatonin have justified their investigation and use in the treatment of circadian rhythm sleep disorders. Melatonin is the treatment of choice for blind people with non-24 h sleep/wake disorder. Current research is directed towards optimizing these therapies with respect to time of administration, dose and formulation of melatonin, intensity, duration and spectral composition of light. Our studies in totally blind people with non-24 h sleep/wake disorder have shown that, in addition to improving sleep, daily administration of melatonin can entrain their free-running circadian rhythms. The ability of melatonin to entrain free-running rhythms depends, in part, on the time of melatonin administration relative to the subject's circadian phase. Subjects who were entrained by melatonin began their treatment in the phase advance portion (CT 6-18) of the published melatonin phase-response curves (PRCs), whereas those who failed to entrain began their melatonin treatment in the delay portion of the PRC. Whether the effect of light on the human circadian axis can be optimized by altering its spectral composition has been investigated. Recently, it was demonstrated that light-induced melatonin suppression in humans is sensitive to short wavelength light (420-480 nm; lambda(max) approximately 460 nm), a response very different to the classical scotopic and photopic visual systems. Whether other nonvisual light responses (e.g. circadian phase resetting) show a similar spectral sensitivity is currently being studied.