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Most animals and other organisms have "built in clocks" in their brains that regulate the timing of biological processes and daily behavior. These "clocks" are known as circadian rhythms. They allow maintenance of these processes and behaviors relative to the 24-hour day/night cycle in nature. Although these rhythms are maintained by the individual organisms, their length does vary somewhat individually. Therefore they must, either continually or repeatedly, be reset to synchronize with nature's cycle.〔Kolmos, E. and S.J. Davis (2007). "Circadian rhythms: Rho-related signals in time-specific light perception." Current Biology 17(18): R808–R810.〕 In order to maintain synchronization ("entrainment") to 24 hours, external factors must play some role. Of the various factors that influence this entrainment, light exposure to the eyes is the strongest.〔Baehr, E.K., L.F. Fogg, et al. (1999). "Intermittent bright light and exercise to entrain human circadian rhythms to night work." American Journal of Physiology-Regulatory Integrative and Comparative Physiology 277(6): R1598–R1604.〕〔Hirayama, J., S. Cho, et al. (2007). "Circadian control by the reduction/oxidation pathway: Catalase represses light-dependent clock gene expression in the zebrafish." Proceedings of the National Academy of Sciences of the United States of America 104(40): 15747–15752.〕〔Warman, V.L., D.J. Dijk, et al. (2003). "Phase advancing human circadian rhythms with short wavelength light." Neuroscience Letters 342(1–2): 37–40.〕 ==Demonstrated effects== All of the mechanisms of light-effected entrainment are not yet fully known, however numerous studies have demonstrated the effectiveness of light entrainment to the day/night cycle. Studies have shown that: * The timing of exposure to light influences entrainment; as seen on the phase response curve for light for a given species. * * In diurnal (day-active) species, exposure to light soon after wakening advances the circadian rhythm, whereas exposure before sleeping delays the rhythm.〔〔Duffy, J.F., R.E. Kronauer, et al. (1996). "Phase-shifting human circadian rhythms: Influence of sleep timing, social contact and light exposure." Journal of Physiology-London 495(1): 289–297.〕 An advance means that the individual will tend to wake up earlier on the following day(s). A delay, caused by light exposure before sleeping, means that the individual will tend to wake up later on the following day(s). * The length of light exposure influences entrainment. * * Longer exposures have a greater effect than shorter exposures.〔 * * Consistent light exposure has a greater effect than intermittent exposure.〔 * * In rats, constant light eventually disrupts the cycle to the point that memory and stress coping may be impaired.〔Ma, W.P., J. Cao, et al. (2007). "Exposure to chronic constant light impairs spatial memory and influences long-term depression in rats." Neuroscience Research 59(2): 224–230.〕 * The intensity and the wavelength of light influence entrainment. * * Dim light can affect entrainment relative to darkness.〔Gorman, M.R., M. Kendall, et al. (2005). "Scotopic illumination enhances entrainment of circadian rhythms to lengthening Light : Dark cycles." Journal of Biological Rhythms 20(1): 38–48.〕 * * Brighter light is more effective than dim light.〔 * * In humans, a lower intensity short wavelength (blue/violet) light appears to be equally effective as a higher intensity of white light.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Light effects on circadian rhythm」の詳細全文を読む スポンサード リンク
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