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Circadian rhythm
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The circadian rhythm is the body's internal clock. It runs about twenty-four hours, it lives in the brain, and it decides when we are awake and when we feel sleepy, taking its cue from the light around us.
That clock has an exact address: the suprachiasmatic nucleus, a small cluster of neurons in the hypothalamus. Information reaches it directly from the eye. As long as there is light, certain cells in the retina, the ganglion cells, keep it stimulated; when the light is gone, they reduce that stimulation.
With the sympathetic nervous system activated, sleepiness follows, and the pineal gland releases melatonin into the blood.
And there is not just one clock. The heart, the liver, the kidneys, the lungs, and the intestine each have their own, the peripheral clocks, and all of them run in sync with the central one. It is a central clock that every other clock in the house is set to.
In brief
The circadian rhythm is the brain's internal 24-hour clock that regulates alertness and sleep in response to light.
The pacemaker sits in the suprachiasmatic nucleus of the hypothalamus, and without light the retinal ganglion cells stimulate it less.
With the sympathetic system activated, sleepiness follows, and the pineal gland releases melatonin into the bloodstream.
Peripheral clocks also exist in the heart, liver, kidneys, lungs, and intestine, synchronized to the central one.
A central clock that every other clock in the house is set to.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Circadian Rhythm, StatPearlsPublished
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