Neural basis and biological function of masking by light in mammals: Suppression of melatonin and locomotor activity

Light influences mammalian circadian rhythms in two different ways: (1) It entrains endogenous oscillators (clocks), which regulate physiology and beh avior; and (2) it affects directly and often immediately physiology and beh avior (these effects are also referred to as masking). Masking effects of l ight on pineal melatonin, locomotor activity, and the sleep-wake cycle in m ammals and man are reviewed. They seem to represent a universal response in this group. The review reveals that the mechanism of photic inhibition of melatonin is fairly well understood, whereas only little is known about the influence of light on other circadian rhythm outputs, such as locomotor ac tivity.