Effects of Light on Circadian Rhythms

The light/dark cycle of the sun has a powerful effect on the circadian clock, sleep, and alertness. Your body’s circadian clock responds to light, as a signal to be awake, and dark, as a signal to fall asleep. Increase your amount of light during the day to be more alert. Darken your bedroom room at night to sleep better.

The circadian clock is most sensitive to light

  • about 2 hours before usual bedtime
  • through the night
  • about 1 hour after usual wake-up time in the morning

Bright light in the evening, 2 hours before bedtime, will make you feel sleepy and fall asleep later.

  • If you have trouble falling asleep, dim the lights 2 hours before you want to go to sleep.
  • If you get sleepy too early in the evening, go into a well-lit area to feel more awake.

If you’re not getting enough sleep or feeling fatigued, go to bed earlier to catch up on needed sleep.

Keep the lights dim to help you fall back to sleep if you

  • wake up in the middle of the night
  • too early in the morning

Bright light in the morning will make you feel sleepy and fall asleep earlier in the evening.

  • Getting bright light shortly after waking up may help you feel more alert.

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Page last reviewed: April 1, 2020