Abstract
Environmental light-dark alternation is the most potent zeitgeber of circadian rhythm in most organisms. Responses of the circadian system to brief light pulses and rectangular light-dark cycles have been extensively studied in many species. Under natural conditions, however, light intensity changes gradually throughout the day, and light-responsive neurons in the mammalian circadian system have response characteristics suitable for detection of gradual changes in light intensity during twilight. Several researchers have examined rhythm-entraining properties of artificial twilight and fluctuating light intensity cycles, and have stressed the importance of gradual transition between light and darkness for entrainment of circadian rhythms. But many questions about photic entrainment still remain to be answered. Further studies on entrainment of circadian rhythms will be useful for prevention and treatment of circadian rhythm-related disorders.
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