Your internal clock: controlled by your brain and external cues

You've probably heard the term "internal clock" and know it has something to do with how you feel at different times of the day. But do you know what brain areas control your internal clock? And what about the outside forces that affect it? This article, reprinted from Harvard Medical School's Special Health Report called Improving Sleep: A Guide to Getting a Good Night's Rest, explains the ins and outs of your internal clock.

Scientists have discovered that certain brain structures and chemicals produce the states of sleeping and waking.

A pacemaker-like mechanism in the brain regulates the circadian rhythm of sleeping and waking. ("Circadian" means "about a day.") This internal clock, which gradually becomes established during the first months of life, controls the daily ups and downs of biological patterns, including body temperature, blood pressure, and the release of hormones.

The circadian rhythm makes people's desire for sleep strongest between midnight and dawn, and to a lesser extent in midafternoon. In one study, researchers instructed a group of people to try to stay awake for 24 hours. Not surprisingly, many slipped into naps despite their best efforts not to. When the investigators plotted the times when the unplanned naps occurred, they found peaks between 2 a.m. and 4 a.m. and between 2 p.m. and 3 p.m.

Most Americans sleep during the night as dictated by their circadian rhythms, although many nap in the afternoon on the weekends. In societies where taking a siesta is the norm, people can respond to their bodies' daily dips in alertness with a one- to two-hour afternoon nap during the workday and a correspondingly shorter sleep at night.

In the 1970s, studies in rats identified the suprachiasmatic nucleus as the location of the internal clock. This cluster of cells is part of the hypothalamus, the brain center that regulates appetite and other biological states. When this tiny area was damaged, the sleep/wake rhythm disappeared and the rats no longer slept on a normal schedule. Although the clock is largely self-regulating, its location allows it to respond to several types of external cues to keep it set at 24 hours. Scientists call these cues "zeitgebers," a German word meaning "time givers."

Light. Light striking your eyes is the most influential zeitgeber. When researchers invited volunteers into the laboratory and exposed them to light at intervals that were at odds with the outside world, the participants unconsciously reset their biological clocks to match the new light input. The circadian rhythm disturbances and sleep problems that affect up to 90% of blind people demonstrate the importance of light to sleep/wake patterns.

Time. As a person reads clocks, follows work and train schedules, and demands that the body remain alert for certain tasks and social events, there is cognitive pressure to stay on schedule.

Melatonin. Cells in the suprachiasmatic nucleus contain receptors for melatonin, a hormone produced in a predictable daily rhythm by the pineal gland, which is located deep in the brain between the two hemispheres. Levels of melatonin begin climbing after dark and ebb after dawn. The hormone induces drowsiness in some people, and scientists believe its daily light-sensitive cycles help keep the sleep/wake cycle on track.

Have you had experiences where the zeitgebers listed above have had an affect on your sleep?

Julie K. Silver, M.D., is an assistant professor in the Department of Physical Medicine and Rehabilitation at Harvard Medical School. She is also the Chief Editor of Books for Harvard Health Publications.

Sleep
Need better sleep? Learn how to get it with Improving Sleep: A guide to a good night's rest. This special report from Harvard Medical School describes the factors that can disturb sleep, the latest in sleep research, and, most importantly, what you can do to get a good night's sleep. It also includes in-depth information on sleep disorders, including restless legs syndrome, narcolepsy, and sleep apnea.

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