Circadian Rhythm Understanding Your Internal Clock - Sleep Mechanics: Sleep Disorders

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Circadian rhythm: Understanding your internal clock

Scientists have discovered that certain brain structures and chemicals produce the states of sleeping and waking. Understanding these control mechanisms helps doctors pinpoint what can go wrong and plan effective treatments.

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.

Mechanisms of your "sleep clock"

In the 1970s, the location of the internal clock in rodents was found to be the suprachiasmatic nucleus. This cluster of cells is part of the hypothalamus (see Figure 3), 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." These are as follows:

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.

Figure 3: The sleep/wake control center The pacemaker-like mechanism in your brain that regulates the circadian rhythm of sleeping and waking is thought to be located in the suprachiasmatic nucleus. This cluster of cells is part of the hypothalamus, the brain center that regulates appetite, body temperature, and other biological states.

Time cues. 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.

Your clock's hour hand

As the circadian rhythm counts off the days, another part of the brain acts like the hour hand on a watch. This clock is located in a cluster of nerve cells within the brain stem, the area that controls breathing, blood pressure, and heartbeat. Fluctuating activity in the nerve cells and the chemical messengers they produce seem to coordinate the timing of wakefulness, arousal, and the 90-minute changeover between REM and non-REM sleep.

Several neurotransmitters (natural brain chemicals that neurons release to communicate with adjacent cells) play a role in arousal. Their actions help explain why medications that mimic or counteract their effects can influence sleep. Adenosine and gamma-aminobutyric acid (GABA) are believed to promote sleep. Acetylcholine regulates REM sleep. Norepinephrine, epinephrine, dopamine, and the newly discovered hypocretin peptides — also known as orexins — stimulate wakefulness. Individuals vary greatly in their natural levels of neurotransmitters and in their sensitivity to these chemicals.

Last updated:	January 23, 2007