How Circadian Rhythms Affect One's Sleep Patterns

Get the facts about your body's internal clock

man sleeping
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Circadian rhythms are the changes in your body over the course of a day, including at night when you're sleeping. In the morning your body changes the levels of the hormones that create the feeling of being awake and alert. In the evening, these levels change again, making you feel sleepy. Circadian is a combination of two Latin words: circa meaning ‘around’ and dies meaning ‘day.’

The SCN – Your Body’s Clock

You body needs to know what time it is.

It uses cues like daylight and your own activity to figure out if it is day or night. Travel, night shifts and poor sleep habits can confuse your body’s clock and make good quality sleep difficult.

There is a tiny structure in the brain called the suprachiasmatic nucleus (SCN) inside the hypothalamus that controls your body’s clock. The SCN is located just above where both optic nerves come together. From the optic nerves, the SCN gets information about whether it is day or night. The SCN controls body functions associated with sleep, including body temperature, urine production (we make less during sleep so we don’t have to wake up to urinate) and changes in blood pressure. The SCN also controls the hormone melatonin.

Melatonin – Your Sleep Regulator

When the SCN detects light, it signals the pineal gland (which produces the hormone melatonin) to stop making melatonin. Melatonin is the hormone that makes you feel sleepy at night.

When there is less light, the SCN tells the pineal gland to make more melatonin and you feel sleepy.

Light and Sleep

Researchers have discovered that our body’s own circadian rhythms are based on a 25-hour cycle. Because a day is 24 hours, the SCN needs information about light to reset itself daily. This information can come in the form of daylight exposure or other cues associated with your daily sleep/wake cycle (such as your alarm clock, an evening bath or your sleep habits).

Interrupting Your Clock

Several things can confuse the SCN, making it difficult to get a good night’s sleep. These include:

  • Jet Lag: The time changes experienced in travel can disrupt your body’s clock. Your body stays on the time in the time zone it is used to. The SCN can only adjust at a rate of about one hour per day.
  • Night Shift: People working at night must override their body’s natural circadian rhythms. Reversing the normal sleep/wake cycle can result in drowsiness at work, an increased risk of accidents and workplace errors.
  • Blindness: People with total blindness may experience sleeping problems because they are unable to receive cues about light. As a result, their body clocks remain set to a 25-hour day. Medications and supplements may help people with blindness regulate their circadian rhythms.


National Institutes of Health; National Heart, Lung and Blood Institute. Your Guide to Healthy Sleep. NIH Publication No. 06-5271.

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