How sleep works
- Article
- 2021-02-03
Sleep is a rest period that alternates with vigilance. You have internal body clocks that control when you are awake and when your body is ready to sleep. These clocks have cycles of approximately 24 hours. The clocks are regulated by multiple factors, including light, dark and sleep schedules. Once you sleep, you go through the sleep phases during the night in a predictable pattern.
Sleep is important because it affects many of your body systems. Not getting enough or enough good quality sleep puts you more at risk for heart and respiratory problems and affects your metabolism and your ability to think clearly and focus on tasks.
Your sleep / wake cycle - How sleep works
Many factors come into play in preparing your body to fall asleep and wake up. Your body has several internal clocks called circadian clocks. These usually 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. Read more in our health topic Circadian Arrhythmias.
Your central circadian clock, located in your brain, tells you when it is time to sleep. Other circadian clocks reside in organs throughout your body. Your body's internal clocks are in sync with certain environmental signals. Light, darkness, and other signals help determine when you feel awake and sleepy. Artificial lighting and caffeine can interfere with this process by sending your body false signals of vigilance.
Urge to sleep
Your body has a biological need for sleep that increases after 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, stable. A compound called adenosine has been linked to this need for sleep. While you are awake, the adenosine levels in your brain continue to rise. The rising levels indicate a shift towards 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 and parasympathetic systems. This helps your central body clock to stay in tune with the day and night. Exposure to artificial light disrupts this process.
The light-dark cycle affects when your brain makes and releases the hormone melatonin. Melatonin travels to 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 rising sun, your body releases another hormone called cortisol. Cortisol naturally prepares your body for awakening.
Exposure to bright artificial light in the late evening can disrupt this process and prevent your brain from releasing melatonin. This can make it more difficult to fall asleep. Examples of bright artificial light are 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.
Problems with your sleep / wake cycle and circadian clock
Some people have problems with their sleep / wake cycle, which means that their brain doesn't keep them awake or asleep at the right times. Examples are:
- Insomnia. People with insomnia have trouble falling asleep, staying asleep, or both. As a result, they may be getting too little or not enough good quality sleep. They may not feel refreshed when they wake up.
- Narcolepsy. Narcolepsy causes episodes of extreme daytime sleepiness. The condition can also cause muscle weakness.
Sometimes your central circadian clock isn't properly aligned with your sleep time. Examples are:
- Jet lag. Many people have trouble adjusting their sleep to 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.
Sleep stages and stages
When you sleep, you go through two stages of sleep: rapid eye movement (REM) and non-REM sleep. The cycle starts every 80 to 100 minutes. Usually there are four to six cycles per night. You may wake up briefly between cycles. Sleep studies use sensors to record eye movements and brain activity, which are used to classify sleep phases and stages.
Non-REM sleep
Non-REM sleep has three phases, defined by brain activity measurements performed in sleep studies.
- Phase 1. This phase is the transition between waking and sleeping.
- Phase 2. When you reach phase 2, you are asleep.
- Stage 3. This stage is called deep sleep or slow sleep, after a particular pattern that appears in measurements of brain activity. You usually spend more time in this phase at the beginning of the night.
Rapid Eye Movement (REM) sleep
During REM sleep, your eyes vibrate and your brain is active. The brain activity measured during REM sleep is comparable to the brain activity during the hours that you are awake. Dreaming usually happens during REM sleep. Your muscles normally go limp to prevent you from fulfilling your dreams. You usually have more REM sleep later in the night, but at colder temperatures you have less REM sleep. This is because your body does not regulate its temperature properly during REM sleep.
Why sleep is important
Sleep plays a vital role in good health and well-being throughout your life. How you feel while awake depends in part on what happens while you sleep. During sleep, your body works to support healthy brain function and maintain your physical health. In children and teens, sleep also supports growth and development. Not getting enough sleep over time can increase your risk of chronic health problems. It can also affect how well you think, respond, work, learn, and get along with others. Find out how sleep affects your heart and circulation, metabolism, respiratory and immune systems, and how much sleep is enough.
Heart and circulation
When you fall asleep and enter non-REM sleep, your blood pressure and heart rate drop. During sleep, your parasympathetic system controls your body, and your heart doesn't work as hard as when you are awake. During REM sleep and upon awakening, your sympathetic system is activated, raising your heart rate and blood pressure to the usual levels when you are awake and relaxed. A sharp rise in blood pressure and heart rate on awakening has been associated with angina pectoris and heart attacks.
People who don't sleep enough or who wake up often may be at higher risk for:
- High bloodpressure
- Ischemic heart disease
- Obesity
- Stroke
Hormones
Your body produces different hormones at different times of the day. This could be related to your sleep patterns or your circadian clocks. In the morning, your body releases hormones that promote alertness, such as cortisol, which help you wake up. Other hormones have 24-hour patterns that vary throughout your life; For example, in children, the hormones that tell the glands to release testosterone, estrogen, and progesterone are produced in pulses at night, and the pulses get bigger as puberty approaches.
Metabolism
The way your body handles fat depends on several circadian clocks, including those in the liver, fat, and muscle. For example, the circadian clocks ensure that your liver is prepared to digest fats at the right times. Your body can treat fat differently if you eat at unusual times.
Studies have shown that insufficient quality sleep can lead to:
- Increased levels of hunger control hormones, including leptin and ghrelin, in your body
- Reduced ability to respond to insulin
- Increased consumption of foods, especially fatty, sweet and salty foods
- Reduced physical activity
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