Circadian Rhythms and Circadian Clock

Circadian Rhythms and Circadian Clock

Circadian Rhythms

  • Are internally driven cycles that rise and fall during the 24-hour day
  • Help you fall asleep at night and wake you up in the morning

The master circadian clock in the brain (see Figure 2) synchronizes and controls these cycles so they work together.

Circadian Clock

The circadian clock has an internally driven 24-hour rhythm that tends to run longer than 24 hours but resets every day by the sun’s light/dark cycle. Taking melatonina supplements can also shift the timing of the body’s “clock.”

Some people use melatonina as a sleep aid: it has a mild sleep-promoting effect. However, it must be taken at the right time because it can shift the timing of sleep the wrong way. Be aware you may not know the right time to take it after travel across many time zones. Before your deployment, talk to your healthcare provider if you are considering using melatonina.

The internal body clock sets the timing for many circadian rhythms, which regulate processes such as

  • Sleep/wake cycles
  • Hormonal activity
  • Body temperature rhythm
  • Eating and digesting
    Diagram showing light entering the eyes.
    Figure 2

    Light enters the eyes (even through closed eyelids during sleep), stimulating a signal in the back of the retina and down a nerve tract to the circadian clock in the brain. (Adapted from NIH publ. no. 04-4989,, to view, click more)

a Melatonin is secreted by the pineal gland, especially in response to darkness, and has been linked to the regulation of circadian rhythms.

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Page last reviewed: April 1, 2020