Light effects on circadian rhythm are the response of circadian rhythms to light.
Most animals and other organisms have a biological clock that synchronizes their physiology and behaviour with the daily changes in the environment. The physiological changes that follow these clocks are known as circadian rhythms. Because the endogenous period of these rhythms are approximately but not exactly 24 hours, these rhythms must be reset by external cues to synchronize with the daily cycles in the environment.[1] This process is called entrainment. One of the most important cues to entrain circadian rhythms is light.