What are circadian rhythms?
Circadian rhythms are physical, mental and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism’s environment. They are found in most living things, including animals, plants and many tiny microbes. The study of circadian rhythms is called chronobiology.

Are circadian rhythms the same thing as biological clocks?

No, but they are related. Our biological clocks drive our circadian rhythms.

What are biological clocks?

The biological clocks that control circadian rhythms are groupings of interacting molecules in cells throughout the body. A “master clock” in the brain coordinates all the body clocks so that they are in synch.

What is the master clock?

The “master clock” that controls circadian rhythms consists of a group of nerve cells in the brain called the suprachiasmatic nucleus, or SCN. The SCN contains about 20,000 nerve cells and is located in the hypothalamus, an area of the brain just above where the optic nerves from the eyes cross.

Does the body make and keep its own circadian rhythms?

Circadian rhythms are produced by natural factors within the body, but they are also affected by signals from the environment. Light is the main cue influencing circadian rhythms, turning on or turning off genes that control an organism’s internal clocks.

How do circadian rhythms affect body function?

Circadian rhythms can change sleep-wake cycles, hormone release, body temperature and other important bodily functions.

Do circadian rhythms have a genetic component?

Yes. Researchers have already identified genes that direct circadian rhythms in people, fruit flies, mice, fungi and several other model organisms.

Are circadian rhythms related to sleep?

Yes. Circadian rhythms are important in determining human sleep patterns. The body’s master clock, or SCN, controls the production of melatonin, a hormone that makes you sleepy. Since it is located just above the optic nerves, which relay information from the eyes to the brain, the SCN receives information about incoming light. When there is less light—like at night—the SCN tells the brain to make more melatonin so you get drowsy.

Are circadian rhythms related to jet lag?

Yes. Jet lag occurs when travelers suffer from disrupted circadian rhythms. When you pass through different time zones, your body’s clock will be different from your wristwatch. For example, if you fly in an airplane from California to New York, you “lose” thre hours of time. So when you wake up at 7 a.m., your body still thinks it’s 4 a.m., making you feel groggy and disoriented. Your body’s clock will eventually reset itself, but this often takes a few days.

Are circadian rhythms implicated in any health problems?

Yes. Circadian rhythms have been linked to various sleep disorders, such as insomnia and disrupted sleep-wake cycles. Abnormal circadian rhythms have also been associated with depression, bipolar disorder and seasonal affective disorder.

How do researchers study circadian rhythms?

Scientists can study circadian rhythms by studying humans or by using model organisms, such as mice or even algae. Basic researchers doing these experiments control the subject’s environment by altering light and dark periods and then look for changes in gene activity or other molecular signals.

How does circadian rhythm research contribute to human health?

Understanding what makes our biological clocks tick may lead researchers to treatments for sleep disorders, jet lag and other health problems. Learning more about the genes responsible for circadian rhythms will also enhance our understanding of biological systems and the human body.

Content created July 2008

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