Circadian Rhythms

These are physical, mental and behavioral changes that follow an approximately 24-hour cycle, responding primarily to light and darkness in an organism’s environment. The study of circadian rhythms is called chronobiology.

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. SCN controls the production of melatonin, a hormone that makes us sleepy. When there is less light, like at night, the SCN tells the brain to make more melatonin so we get drowsy.

Circadian rhythms can influence sleep-wake cycles, hormone release, body temperature, and other important bodily functions. They have been linked to various sleep disorders, including insomnia. Abnormal circadian rhythms have also been associated with obesity, diabetes, depression, bipolar disorder, and seasonal affective disorder.

Clock genes contain instructions for making clock proteins, whose levels rise and fall in a regular cyclic pattern. This pattern in turn regulates the activity of the genes.

Many of the clock genes and proteins are similar across species, allowing researchers to make important findings about human circadian processes by studying the clock components of other organisms. They have identified genes that direct circadian rhythms in people, fruit flies, mice, fungi, bread mold, plants, and several other model organisms used for studying genetics.

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' 3 hours of time. So when you wake up at 7:00 a.m., your body still thinks it is 4:00 a.m., making you feel disoriented. Your body’s clock will eventually reset itself, but this often takes a few days. My circadian clock tells me this is a Happy Friday.