Interim NIOSH Training for Emergency Responders: Reducing Risks Associated with Long Work Hours

Effects of Light on Circadian Rhythms

The light/dark cycle of the sun has a powerful effect on the circadian clock, sleep, and alertness. If you understand these effects, you can manipulate light exposure to help yourself sleep better at night and be more alert during the day. Keep in mind your circadian clock uses light and dark signals to predict what to do in the future: when to prepare you to be active and when to prepare you to sleep.

The circadian clock is most sensitive to light from about 2 hours before usual bedtime and through the night, until about 1 hour after usual wake-up in the morning (this is the sensitive period). Exposure to light during these times will affect when your body naturally gets sleepy and is ready to fall asleep.

Bright evening light 2 hours before bedtime will shift the time for sleep later, so you will tend to get sleepy and fall asleep later in the evening and will wake up later in the morning.

  • If you have trouble falling asleep, keep the light levels dim for the 2 hours before you want to go to sleep. You can wear dark sunglasses (wraparound ones work best) if it is hard to control the light in the area. That should help you fall asleep more easily.
  • If you are getting sleepy too early in the evening, you can go into a well-lit area to reduce the sleepiness. (However, if you are sleep deprived or fighting an infection, go to sleep early and catch up on needed sleep.)

Bright morning light will shift the time for sleep earlier, so you will tend to get sleepy and fall asleep earlier in the evening and will wake up earlier in the morning.

  • If you are waking up too early and cannot fall back to sleep, make sure you keep the lights very dim until the time you want to wake up.
  • If you cannot wake up early enough, go into a brightly lit area when you get up (for example, eat your breakfast outside or next to a sunny window).

If you have to get up in the middle of the night, keep light level very dim.

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Page last reviewed: April 1, 2020