Gradual changes in environmental light intensity and entrainment of circadian rhythms

Environmental light-dark alternation is the most potent zeitgeber of circad ian rhythm in most organisms. Responses of the circadian system to brief li ght 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 mammalia n circadian system have response characteristics suitable for detection of gradual changes in light intensity during twilight. Several researchers hav e examined rhythm-entraining properties of artificial twilight and fluctuat ing light intensity cycles, and have stressed the importance of gradual tra nsition between light and darkness for entrainment of circadian rhythms. Bu t many questions about photic entrainment still remain to be answered. Furt her studies on entrainment of circadian rhythms will be useful for preventi on and treatment of circadian rhythm-related disorders. (C) 2000 Elsevier S cience B.V. All rights reserved.