Your body has a natural clock, called a “circadian clock,” that helps you regulate your sleep. The word “circadian” refers to rhythmic biological cycles that repeat about every 24 hours. These cycles are also called circadian rhythms. Your circadian clock is strongly influenced by light, which is the reason why people living in different regions have different sleeping schedules. This is also the reason why people who work night shifts can have difficulty falling asleep or staying awake.2
At bedtime, when your drive to sleep is greatest, your sleep drive and circadian clock work together to allow you to fall asleep. After you have slept for some time, when your drive to sleep is lower, your circadian clock allows you to stay asleep until the end of the night.
Circadian rhythms regulate changes in the brain and body that occur over the course of a day. Your body’s biological clock controls most circadian rhythms. This clock is found in a region of the brain called the hypothalamus (pronounced hy-puh-THAL-uh-muhs). The hypothalamus affects sleep and arousal.
Light detected by special neurons in the eye sends signals to many areas of the brain, including the hypothalamus. Signals from the hypothalamus travel to different regions of the brain, including the pineal (pronounced PIN-ee-uhl) gland. In response to light, such as sunlight, the pineal gland turns off the production of melatonin, a hormone that causes a feeling of drowsiness. The levels of melatonin in the body normally increase after darkness, which makes you feel drowsy.
The change in melatonin during the sleep/wake cycle reflects circadian rhythms. During sleep, the hypothalamus also controls changes in body temperature and blood pressure.
Because circadian rhythms are controlled by light, people who have some degree of blindness in both eyes may have trouble sleeping. For more information about how vision affects circadian rhythms, visit the National Institute of Neurological Disorders and Stroke page on understanding sleep.2