What are circadian sleep rhythms?

Updated: Nov 05, 2019
  • Author: Pradeep C Bollu, MD; Chief Editor: Selim R Benbadis, MD  more...
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Circadian sleep rhythm is one of the several intrinsic body rhythms modulated by the hypothalamus. [17] The suprachiasmatic nucleus in the anterior hypothalamus sets the body clock to approximately 24.2 hours, with both light exposure and schedule clues entraining to the 24.2-hour cycle. [18] The retinohypothalamic tract allows light cues to directly influence the suprachiasmatic nucleus. Light is called a zeitgeber, a German word meaning time-giver because it sets the suprachiasmatic clock. Examples of other external zeitgebers are exercise, social activities, and mealtimes. [19]  A practical purpose has been proposed for the circadian rhythm, using the analogy of the brain functioning somewhat like a battery charging during sleep and discharging during the wake period.

The nadir of this rhythm is in the early morning. The downswing in circadian rhythm prior to the nadir is thought to assist the brain to remain asleep overnight for full restoration by preventing premature awakening. The morning upswing then facilitates awakening and through the day acts as a counterbalance to the progressive discharge of wake neuronal activity. After the circadian apex in the early evening, the downswing aids sleep initiation. This model explains the relatively steady cognitive function throughout wakefulness. [20, 7]

Body temperature cycles are also under hypothalamic control. An increase in body temperature is seen during the course of the day and a decrease is observed during the night. The temperature peaks and troughs are thought to mirror the sleep rhythm. People who are alert late in the evening (ie, evening types) have body temperature peaks late in the evening, while those who find themselves most alert early in the morning (ie, morning types) have body temperature peaks early in the evening. [21, 7]

Melatonin has been implicated as a modulator of light entrainment. It is secreted maximally during the night by the pineal gland. Prolactin, testosterone, parathyroid hormone, and growth hormone also demonstrate circadian rhythms, with maximal secretion during the night. [22, 2]

For excellent reviews of clinically applicable information on circadian rhythm disorders, please refer to Sack, 2007. [23, 24]

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