Phase-Shifting Effect of Light and Exercise on the Human Circadian Clock.

Abstract

Abstract : In animals, the major environmental signal responsible for the entrainment of circadian rhythmicity to external time is the light-dark cycle. While for many years, it was thought that light did not play an important role in synchronizing human rhythms, and that social cues were the primary entraining agent, extensive evidence obtained during the past decade using light of greater intensity than in earlier studies has indicated that the light-dark cycle is also a major zeitgeber for human circadian rhythmicity. The evolution of concepts regarding zeitgebers for non-human mammalian rhythms ran in many ways opposite to that occurring in the field of human rhythms. Indeed, social and/or behavioral cues were long thought to be ineffective as zeitgebers in rodents and other mammals, but evidence has accumulated over the past few years to indicate that behavioral changes are indeed capable of inducing shifts in circadian rhythms. Specifically, stimuli which cause an alteration of the rest activity cycle, either by eliciting activity (i.e. locomotor activity in rodent studies) during the normal rest period or by preventing activity during the normal active period, result in phase shifts of circadian rhythms. These findings implied that physical activity during the usual rest period (i.e. nighttime) as well as sleep occurring during the normal active period (i.e. daytime) could alter the phase of human rhythms.