Light, social zeitgebers, and the sleep–wake cycle in the entrainment of human circadian rhythms

Abstract The availability of sufficient zeitgebers is essential for entrainment of circadian rhythms. Bright light is an important human zeitgeber. However, social cues acting through the sleep–wake cycle may also be important in the entrainment of human rhythms. Current research suggests that humans receive inadequate and sporadic exposure to bright light. Social cues, although less powerful, may be an important form of entrainment for contemporary humans, and also serve to augment the effects of available bright light. Both light and social zeitgebers should be investigated for their roles in the genesis, prevention, and treatment of alterations in health.