1.4: Circadian rhythm & sleep cycle

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Circadian rhythms, which are biological processes that develop in humans by age three, span approximately 24 hours and are driven by a circadian clock (depicted below). Without environmental cues, the human circadian cycle is actually 24.2 hours.
Human biological clock
The retinas house photosensitive cells containing melanopsin. These cells signal to the circadian (or biological) clock, located in suprachiasmatic nucleus in the hypothalamus, via the reticulohypothalamic tract. These signals provide information to the brain about the length of night and day, passing it on to the pineal gland, where melatonin is then secreted. This secretion, which is highest at night and decreases during the day, regulates the sleep-wake cycle, decreasing core body temperature and promoting sleep.  Melatonin regulates the rhythmicity of the circadian patterns of the body via yet-to-be-understood mechanisms.
Diagram of Melatonin and the biological clock

The sleep cycle

In the sleep cycle, Stages 1 and 2 range between wakefulness and sleep. Stages 3 and 4 include delta sleep, during which metabolic activity and brain waves slow, eye movements are absent, and muscle tone is atonic. Stage 5 is characterized by bursts of rapid eye movement and dreaming, increased cerebral blood flow, and generalized muscle atonia. 

  • Stage 1 (4 to 5% of sleep cycle)
    • Light sleep begins.
    • Muscle activity slows down.
    • Muscles twitch occasionally.
  • Stage 2 (45 to 55% of sleep cycle)
    • Breathing pattern and heart rate slow.
    • Body temperature decreases slightly.
  • Stage 3 (4 to 6% of sleep cycle)
    • Deep sleep begins.
    • Brain starts to generate slow delta waves.
  • Stage 4 (12-15% of sleep cycle)
    • Very deep sleep begins.
    • Breathing is rhythmic.
    • Muscle activity is limited.
    • Brain produces delta waves.
  • Stage 5 (20-25% of sleep cycle)
    • Sleepers experience rapid eye movement.
    • Brainwaves speed up.
    • Dreaming occurs.
    • Muscles relax.
    • Heart rate increases.
    • Breathing is rapid and shallow.