Your Internal Sleep Clock

Circadian rhythm isn’t a new type of dance step; it has to do with our internal sleep clock. Learn about how your sleep clock works.

Your pattern of sleep and waking is run by the body’s internal sleep clock. Governed by light, your internal sleep clock tells you when it’s time to fall asleep and wake up. Your internal sleep clock, otherwise known as the circadian rhythm, runs on a 24-hour cycle. Disruptions to the circadian rhythm and your internal sleep clock can deprive you of sleep. This, in turn, can cause health problems by disrupting all the physiological, biological, and chemical functions that are affected by sleep.

Your sleep clock is influenced by light signals to the retina (the back of the eye), neural (nerve) pathways to a specific part of the brain that govern wakefulness and sleep, exhaustion and the length of time you’ve been awake, your natural circadian rhythm, and daylight-saving time and seasons.

Your Internal Sleep Clock: How It Works

Light comes into the eye through the retina, travels down a neural pathway into a part of the brain called the suprachiasmatic nucleus, and signals that it’s time to be awake, says Lisa Shives, MD, a sleep specialist at Northshore Sleep Medicine in Evanston, Ill., and spokesperson for the American Academy of Sleep Medicine. “That starts a whole cascade of neurotransmitters that wake you up and are involved in wakefulness.”

Sleep is also governed by conflicting forces. Dr. Shives says that light is one of the most powerful signals that tell your body to remain awake — however, a signal saying “stay asleep” (especially if you’re sleep-deprived) can override the fact that sunlight is shining all around you.

“One of the most powerful cues that you should sleep is what we call the homeostatic force that builds up,” says Shives. The longer you are awake, the more likely you are to get tired, so the urge to sleep builds over a 16-hour period. This need for sleep waxes or wanes throughout the day.

Your Internal Sleep Clock: Circadian Rhythm

The study of sleep is a relatively young field; research began in the United States during the 1960s and 1970s. The first biological clock was identified in fruit flies during the early ’70s. Clinically, sleep only became a relevant subspecialty in the 1980s but “didn’t officially achieve subspecialty status in the academic hierarchies of American medicine until 2006,” Shives says.

A circadian rhythm refers to the body’s internal sleep clock and all the physiological functions that revolve around it. Circadian rhythm “derives its name from Latin; it means ‘around the day.’ It’s a fancy term for the fact that human beings have an internal 24-hour clock,” says Shives.

Back in the 1930s, researchers conducted “cave studies” — volunteers were sequestered in caves and deprived of natural light — and speculated that the body’s internal clock revolved around a 25-hour cycle, says Shives. “But more recent studies show that it’s closer to 24 hours.”

If you listen to your body, you will be in tune with your circadian rhythm. Don’t hit the metaphorical snooze button and ignore your internal clock — it will keep you awake and productive or relaxed and ready for bed when you need to be.

By Clare Kittredge – Medically reviewed by Lindsey Marcellin, MD, MPH