During sleep your body gets some much needed rest, but your brain remains very active. It processes information that you gathered while you were awake. It also continues to control your breathing, heart rate and other body functions.
The complex process of sleep involves multiple stages that make up a sleep cycle. The nature of your sleep is different in each of these stages. Each complete cycle lasts about 90 to 110 minutes. Most adults will go through four to six cycles in a full night of sleep.
Children have much shorter sleep cycles than adults. The sleep cycle of a one-year-old may last about 45 minutes. Once a child nears 10 years of age, the length of his or her sleep cycle is similar to that of an adult.
Your brain waves change as your body goes through these sleep stages and cycles during the night. These waves are fast and active during rapid eye movement (REM) sleep, or “R sleep.” They are slower during the stages of deep, non-REM sleep, or “N sleep.”
All of this activity can produce unusual behavior during sleep. These actions are signs of sleep disorders that are known as “parasomnias.” They involve undesirable events and experiences during the sleep period. Some of these disorders are more common during either N sleep or R sleep.
These are the unique aspects of each sleep stage:
N sleep (non-rapid eye movement sleep or NREM sleep)
When you first fall asleep you normally enter N sleep. This is the primary phase for about 75 percent of total sleep time in normal adult sleep and about 50 percent of an infant’s sleep time. Your heart rate, breathing and blood pressure all drop to levels that are lower than when you are awake. Some dreams may occur during this phase of sleep. They tend to be less intense than dreams that occur during R sleep.
N sleep is made up of these stages:
This initial stage is very brief. It occurs when you first fall asleep and again after awakenings during the night. Each episode tends to last for less than 10 minutes. This sleep is very light. A gentle whisper, a soft touch, or a slight sound can wake you up.
In this first stage your brain waves change from alpha waves of wakefulness to theta waves of sleep. Your eyes display slow, rolling movements. Your muscles begin to relax slightly. Stage N1 makes up about five percent of total sleep time in normal adult sleep.
You may have a sleep disorder that disrupts your sleep multiple times during the night. A common example is obstructive sleep apnea (OSA). The frequent awakenings cause you to enter stage N1 sleep more often. As a result stage N1 sleep makes up a much greater portion of your total sleep time.
This stage lasts about 30 to 60 minutes. It is not quite as easy to wake you up in this stage. Your eyes stop moving and your muscles relax more. There may be an appearance of high-amplitude, slow-wave brain activity, known as “delta waves.” Stage N2 makes up about 50 percent of total sleep time in normal adult sleep.
This is known as “slow-wave sleep” or “deep sleep.” It is harder to wake you up in this stage. There is an increase in delta waves. There may be some slight body movements toward the end of stage N3.
Slow-wave sleep tends to last for about 20 to 40 minutes in the first sleep cycle. It makes up about 20 percent of total sleep time in normal adult sleep. This stage may disappear in the later sleep cycles of the night. Stage N3 also may be absent from the sleep of older adults. Women seem to keep slow-wave sleep later into life than men.
These three parasomnias are common during slow-wave sleep in the first part of the night:
Each of these three disorders is more common in children. Stage N3 sleep in children tends to be longer than in adults. This amount of slow-wave sleep decreases greatly during the teen years. Children also tend to sleep more deeply than adults during stage N3. It can be almost impossible to wake up a child in the slow-wave sleep of the night’s first sleep cycle.
R sleep (rapid eye movement sleep or REM sleep)
Stage R sleep tends to be the final stage of the sleep cycle in normal adult sleep. It also can be called “active sleep” in infants. Your eyes tend to move rapidly during this stage. Your face, arm and leg muscles also can twitch. Your heart rate, breathing, and blood pressure all fluctuate. Your brain waves move in a fast pattern.
Most of your dreams occur during this stage. Your brain paralyzes many of your muscles during stage R sleep. This protects you by preventing you from acting out your dreams.
The first sleep cycle of the night contains an R-sleep period that may last for only a few minutes. Your R sleep then tends to get longer during each of the following sleep cycles. The longest period of stage R sleep may last for an hour near the end of the night. Stage R sleep makes up about 25 percent of total sleep time in normal adult sleep and 50 percent of total sleep time in the normal sleep of infants.
A newborn baby may go straight into stage R sleep after falling asleep. The baby’s arms, legs, and face may twitch and move restlessly. During the next few months of life, stage R sleep tends to decrease. When a child reaches three months of age, the normal pattern of N sleep and R sleep tends to take place. By age 10 a child has roughly the same percentage of stage R sleep as an adult.
People who have narcolepsy also go quickly into stage R sleep after falling asleep. These events are called sleep-onset REM periods (SOREMPs). Detecting them during a Multiple Sleep Latency Test (MSLT) helps doctors know that someone has narcolepsy.
Some parasomnias are common during stage R sleep. They are more likely to occur during the last half of the night when periods of stage R sleep are longer. These disorders include:
Kryger M, Roth T, Dement W, editors. Principles and practices of sleep medicine. 4th ed. Philadelphia: Saunders; 2005.
Lee-Chiong TL, editor. Sleep: a comprehensive handbook. Hoboken, N.J.: John Wiley & Sons; 2006.
Reviewed by David Kuhlmann, MD
Updated July 10, 2007