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Sleep is just one of many types of consciousness we experience, and sleep itself comprises several states of consciousness. Even when we’re sleeping, our brains and bodies continue to work.

Biological Rhythms

Sleep is affected by biological rhythms or periodic physiological changes. Biological rhythms are regular, periodic changes in a body’s functioning. There are three types of biological rhythms:

  • Circadian rhythms: biological cycles that occur about every twenty-four hours. Sleep follows a circadian rhythm. Hormone secretion, blood pressure, body temperature, and urine production also have circadian rhythms.
  • Infradian rhythms: biological cycles that take longer than twenty-four hours. For example, women’s menstrual cycles occur about every twenty-eight days.
  • Ultradian rhythms: biological cycles that occur more than once a day. Sleep follows an ultradian rhythm of about ninety minutes as well as a circadian rhythm. Alertness and hormone levels also follow ultradian rhythms.

Biological rhythms usually synchronize with environmental events such as changes in daylight. However, experiments have shown that many biological rhythms continue to have the same cycle even without cues from the environment. Such biological rhythms are endogenous, which means that they originate from inside the body rather than depend on outside cues.

Biological Clocks

Endogenous rhythms exist because the body has biological clocks that keep time. Biological clocks can be adjusted by environmental cues, such as changes in temperature.

In humans, the suprachiasmatic nucleus (SCN) is the main biological clock that regulates circadian rhythms of sleep. The SCN lies in the brain’s hypothalamus. When light stimulates receptors in the retina of the eye, the receptors send signals to the SCN. The SCN then sends signals to the nearby pineal gland, which secretes melatonin, a hormone that regulates the sleep cycle.

Jet Lag

Jet lag is the fatigue and disorientation air travelers feel after a long flight. Although traveling itself drains energy, the time change also contributes to fatigue. People experience jet lag when the events in their environment are out of sync with their biological clocks.

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