Overview of Entrainment of Circadian Rhythms in Sleep

Circadian rhythms can be confusing to understand. When learning about the basic concepts, one is particularly important: entrainment. What is entrainment of circadian rhythms and how might it impact sleep? Learn ways that entraining occurs and how light exposure and melatonin may ease insomnia by affecting sleep timing.

A woman sleeping in her bed

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Circadian Rhythms and Sleep and Wakefulness

Circadian rhythms describe the patterns of body functions that follow a nearly 24-hour cycle. These include body temperature, hormone fluctuations, and the timing of sleep and wakefulness.

Built into the genetics of every cell in the body is a clock that times internal processes to the external environment. This optimizes function to resource availability. Virtually every known organism on the planet has similar mechanisms. Though programmed into our genes, this internal clock may not precisely reflect the length of the geological day. In other words, our internal clocks are off.

Instead of running at 24 hours, most of our internal clocks are operating at a slightly longer interval. (Interestingly, there are rare people who actually run a little short.) The amount of this difference between the internal clock and the external day-night length varies. It can be off as little as a few minutes each day—or sometimes even longer.

The circadian rhythm profoundly influences the desire for sleep and works as an alerting signal to sustain wakefulness. The innate difference in timing with a clock that runs long would cause an individual to want to stay up a little later each night and wake a little later each morning. Something is needed to reset this tendency, and that's where entrainment comes in.


Entrainment is the synchronization or alignment of the internal biological clock rhythm, including its phase and period, to external time cues, such as the natural dark-light cycle. In simple terms, it is the way that our internal clocks are reset to reflect the natural periods of day and night that occur in our environment. Entrainment can impact the overall timing of sleep and wakefulness. It may also have a role in limiting the overall length of sleep episodes.

How Entrainment Occurs and Why It May Not

Entrainment occurs most often through light exposure affecting the suprachiasmatic nucleus (SCN) located in the hypothalamus of the brain. The SCN is known as the master of the bodies clock. Sunlight and darkness trigger the release of hormones in your body through the hypothalamus-pituitary-adrenal axis.

Upon awakening, morning sunlight signals the release of cortisol from the adrenal glands, bringing an end to the period of sleep and begin the process of sustained wakefulness. As night falls, our body produces melatonin that initiates that sleepy feeling, and helps us stay asleep through the night.

Morning light can reset the timing of sleep, moving it slightly earlier. As a result, the desire for sleep shifts slightly earlier as well, which may ease insomnia. Without light perception, as occurs in the totally blind, circadian disorders may develop. Melatonin may be helpful in this population as an external signal to initiate sleep-promoting processes. Unfortunately, melatonin can be a relatively weak sleep aid among the sighted and light exposure may have a more important role.

Disorders Associated With Loss of Entrainment

When entrainment of internal processes to the external environment becomes disturbed, certain circadian rhythm sleep disorders may result. These include:

In addition, some people experience symptoms of jet lag when travel across multiple time zones desynchronizes the internal rhythms to the external environment.

Depending on the timing of the internal tendency towards sleep or wakefulness, insomnia and daytime sleepiness may frequently result.

If you believe you may be suffering from symptoms of a circadian rhythm disorder, try to keep a regular sleep schedule and get 15 to 30 minutes of sunlight upon awakening. Avoid napping during the day and go to bed when you feel sleepy. Try to obtain sufficient hours of rest, for most adults, this means 7 to 8 hours of sleep nightly.

If you continue to struggle, speak with a sleep doctor about additional treatment options.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading
  • American Academy of Sleep Medicine. International classification of sleep disorders, 3rd ed. American Academy of Sleep Medicine, 2014.

  • Kryger MH, Roth T, Dement WC, eds. Principles and Practice of Sleep Medicine. 6th ed. Elsevier; 2017.

By Brandon Peters, MD
Brandon Peters, MD, is a board-certified neurologist and sleep medicine specialist.