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Bodily rhythms, melatonin and zeitgebers

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Why do you feel sleepy at night? Sleep is controlled by internal body clocks and external cues in the environment.

Bodily rhythms

Circadian rhythms last about 24 hours. The sleep-wake cycle is the best-known example; body temperature also follows a daily rhythm.
Ultradian rhythms last less than 24 hours and repeat during the day or night. The 90-minute sleep cycle is an ultradian rhythm.
These rhythms are driven by an internal 'body clock' in the brain.

Hormones: the pineal gland and melatonin

Melatonin is a hormone that makes us feel sleepy. It is released by the pineal gland in the brain.
When it gets dark, the pineal gland releases more melatonin, so we feel drowsy. When it is light, melatonin release drops and we wake up.
Information about light travels from the eyes to the body clock in the brain, which controls the pineal gland.

Zeitgebers

Zeitgebers (German for 'time givers') are external cues that reset our body clock each day.
Light is the most important zeitgeber. Others include meal times and social routines.
When zeitgebers clash with our body clock, sleep is disrupted. Jet lag: after flying across time zones, light comes at the 'wrong' time for the body clock. Shift work: night workers must sleep in daylight.
Tips: getting daylight in the morning helps reset the clock; screens and bright light late at night can delay melatonin release.
Worked example

After flying from London to New York, a traveller feels wide awake at midnight local time. Explain why using zeitgebers.

  1. Her body clock is still set to London time, where it is already morning.
  2. Light at the new location (a zeitgeber) has not yet reset her body clock or melatonin timing.

Answer: Her internal rhythm is out of step with the new light cues; it takes a few days for light to reset it.

Key idea

Circadian rhythms last about 24 hours (sleep-wake cycle); ultradian rhythms are shorter (the 90-minute sleep cycle). The pineal gland releases melatonin in darkness, causing sleepiness. Zeitgebers, especially light, reset the body clock; mismatches cause jet lag and problems for shift workers.

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