Circadian (pronounced sur-KAY-dee-an) rhythms regulate changes in mental and physical characteristics that occur during the course of a day. The word circadian, meaning "around a day," comes from the Latin words "circa" (around) and "diem" (a day).
Your body's biological clock controls most circadian rhythms. This clock is located in a region of the brain called the hypothalamus (pronounced hahy-puh-THAL-uh-muhs).
Signals from the hypothalamus travel to different regions of the brain that respond to light, 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. Changes in body temperature and blood pressure that occur during sleep are also controlled by the hypothalamus.
Because circadian rhythms are controlled by light, people who have some degree of blindness in both eyes have trouble sleeping. Many people with total blindness have lifelong (chronic) problems with sleeping because their eyes do not detect light. The failure of their eyes to detect light disrupts their circadian rhythms, leading to chronic sleep problems.
Melatonin supplements may improve the sleep of people with total blindness. However, these supplements can build up in the body and cause adverse side effects, and so their long-term use is not recommended.1
- National Institute of Neurological Disorders and Stroke. (2007). Sleep and circadian rhythms In Brain basics: Understanding sleep. Retrieved May 29, 2012, from http://www.ninds.nih.gov/disorders/brain_basics/understanding_sleep.htm#circadian_rhythms