Circadian Rhythms

Your Biological Clock

The word circadian (pronounced sir-KAY-dee-in) comes from a Latin phrase meaning "about a day". Circadian rhythms are physiological and behavioral characteristics that follow a daily, or circadian, pattern.

Our bodies display hundreds of these circadian rhythms -- a few of the most important are sleep and wakefulness, body temperature, blood pressure, and the production of hormones and digestive secretions.

Circadian rhythms are controlled by a "biological clock" in the brain -- a tiny clump of cells known as the suprachiasmatic nucleus, or SCN. The SCN is strongly influenced by the daily change between sunlight and darkness, with morning sunlight promoting early wakefulness and darkness setting the stage for sleep. When people choose to sleep, how well they control bedroom darkness, and workplace lighting also affect circadian rhythms.

In essence, we are "programmed" by our circadian rhythms to sleep at night and to be awake in the daytime. For most people, it's difficult to concentrate and maintain alertness between midnight and 7 a.m. and to sleep during the daylight hours. And it's hard to eat in the overnight hours -- when our digestive system essentially shuts down. However, some people are "birds of a different feather" - there are Larks and Owls.