Circadian rhythms. 2: Shift work and health.

Health is associated with normal synchronised psychophysiological circadian rhythms. Synchronisation is endogenously controlled and environmentally modified according to homeostatic principles. Desynchronisation occurs when the circadian rhythms are outside their homeostatic range; this is known as a phase shift. If phase shifts are imposed chronically, as in shift work, then continued desynchronisation (homeostatic imbalance) results in illness.