Circadian rhythms

Circadian rhythms are physical, mental, and behavioral changes that follow a daily cycle. They respond primarily to light and darkness in an organism's environment. Sleeping at night and being awake during the day is an example of a light-related circadian rhythm. Circadian rhythms are found in most living things, including animals, plants, and many tiny microbes. The study of circadian rhythms is called chronobiology.

Overview

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. Circadian rhythms can influence sleep-wake cycles, hormone release, eating habits and digestion, body temperature, and other important bodily functions. Irregular circadian rhythms have been linked to various chronic health conditions, such as sleep disorders, obesity, diabetes, depression, bipolar disorder, and seasonal affective disorder.

Biological Mechanism

The circadian rhythms are controlled by the body's biological "clocks." This clock is in the brain, specifically in the suprachiasmatic nucleus (SCN) of the hypothalamus. It contains about 20,000 nerve cells. The SCN controls the body's temperature, thirst, urine production and blood pressure.

The SCN receives information about the amount of light through the eyes. The retina of the eye contains "classical" photoreceptors (rods and cones), which are used for conventional vision. But the retina also contains specialized ganglion cells that are directly photosensitive, and project to the SCN, where they help in the entrainment (synchronization) of this master circadian clock.

These cells contain the photopigment melanopsin, which is particularly sensitive to blue light. Melanopsin plays a vital role in the modulation and understanding of circadian rhythms.

Health Effects

Disruption to circadian rhythms, usually caused by jet lag or shift work, can interrupt your sleep and make you feel drowsy, irritable or disoriented. It can also lead to insomnia and other sleep disorders. Long-term disruption of circadian rhythms has been linked to obesity, diabetes, and other health problems.

See Also

• Chronobiology
• Suprachiasmatic nucleus
• Melanopsin
• Sleep disorders

References

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