How Sleep Works
How Sleep Works

How Sleep Works Your Sleep/Wake Cycle

Many factors play a role in preparing your body to fall asleep and wake up. Your body has several internal clocks, called circadian clocks . These typically follow a 24-hour repeating rhythm, called the circadian rhythm. This rhythm affects every cell, tissue, and organ in your body and how they work. Learn more in our Circadian Rhythms Disorders page.

Your central circadian clock, located in your brain, tells you when it is time for sleep. Other circadian clocks are in organs throughout your body. Your body’s internal clocks are in sync with certain cues in the environment. Light, darkness, and other cues help determine when you feel awake and when you feel drowsy. Artificial light and caffeine can disrupt this process by giving your body false wakefulness cues.

Your body clock may be different from others

Most people’s natural circadian cycle is slightly greater than 24 hours. Some people naturally wake up early and some naturally stay up late. For example, it is natural for many teens to prefer later bedtimes and to sleep later in the morning than adults.

The rhythm and timing of the body clocks also decline with age. Neurons, or cells, in the brain that promote sleep are lost as part of normal aging. Certain conditions such as Alzheimer’s disease can also speed the loss of neurons. This makes it harder for older adults to stay asleep. Other factors, such as less physical activity or less time spent outdoors, also affect circadian rhythms. As a result, older adults usually sleep less and wake up earlier.

Screenshot of In Brief: your guide to healthy sleep

In Brief: Your Guide to Healthy Sleep

View the fact sheet, which tells you how to recognize if you or a loved one has a sleep disorder and has tips for getting better sleep.

Your body’s need for sleep

Your body has a biological need for sleep that increases when you have been awake for a long time. This is controlled by homeostasis, the process by which your body keeps your systems, such as your internal body temperature, steady.

A compound called adenosine is linked to this need for sleep. While you are awake, the level of adenosine in your brain continues to rise. The rising levels signal a shift toward sleep. Caffeine and certain drugs can interrupt this process by blocking adenosine.

Light-dark cycle

If you follow a natural schedule of days and nights, light signals received through your eyes tell your brain that it is daytime. The area of your brain that receives these signals, called the suprachiasmatic nucleus, transmits the signals to the rest of your body through the sympathetic system and the parasympathetic system . This helps your central body clock stay in tune with the day and night. Exposure to artificial light interferes with this process.

The light–dark cycle influences when your brain makes and releases a hormone called melatonin. Melatonin travels to the cells in your body through your bloodstream. The amount of melatonin in your bloodstream starts to increase in the evening and peaks in the early morning. Melatonin is thought to promote sleep. As you are exposed to more light, such as the sun rising, your body releases another hormone called cortisol. Cortisol naturally prepares your body to wake up.

Exposure to bright artificial light in the late evening can disrupt this process and prevent your brain from releasing melatonin. This can make it harder to fall asleep. Examples of bright artificial light include the light from a TV screen, a smartphone, or a very bright alarm clock. Some people use physical filters or software to filter out some of the blue light from these devices.

What happens when the sleep/wake cycle gets out of sync?

Some people have problems with their sleep/wake cycle, meaning that their brain does not keep them awake or asleep at appropriate times. They may have one of the following sleep disorders. 

  • Insomnia: People who have insomnia have trouble falling asleep, staying asleep, or both. As a result, they may get too little sleep or not enough quality sleep. They may not feel refreshed when they wake up.
  • Narcolepsy: Narcolepsy causes periods of extreme daytime sleepiness. The disorder may also cause muscle weakness.
Sleep diary

Sleep Diary

Print and use this sleep diary to record the quality and quantity of your sleep and daytime habits that may affect your sleep. You can then bring the diary with you to review the information with your doctor.

Sometimes, your central circadian clock is not properly aligned with your sleep time. This can happen if you have one of the following conditions.

  • Jet lag: Many people have trouble adjusting their sleep to fit a new time zone. This usually resolves within a few days.
  • Shift work disorder: People who work at night may have trouble sleeping during the day.
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