Most of the vegetative, hormonal and behavioural functions of the human organism operate under the biological control of a circadian clock which responds to environmental and social stimuli, synchronizing the organism's physiology to daily and seasonal rhythms. The underlying anatomic structures are located in the suprachiasmatic nucleus and the pineal gland. Although the precise physiologic mechanisms involved are still under study, melatonin is known to play a major role. Normal function of the circadian clock is disrupted in jet-lag, night-shift work, and blindness as well as in rare cases of lesions to the pineal gland leading to a shift in biological rhythms including hormone secretion and control of body temperature, for example. Several signs of impaired function have been identified: various types of sleep disorders, memory and concentration impairment, dysphoria, asthenia, irritability. Seasonal recurrence of such signs and frequent depressive complications are also suggestive of a disorder in the circadian clock. Knowledge of specific clinical signs and biological parameters will undoubtedly lead to the discovery of other disease states dependant on the circadian clock and to the development of therapeutic strategies capable of regulating the organism's chronobiology.