The Stages of Sleep

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People spend nearly one-third of their lives asleep, yet many wonder what exactly happens during their nightly slumber. While sleepers appear peaceful, their body is actively cycling through four stages of sleep, each of which is vital for achieving the full benefits of a night of rest.

Learn more about how sleep works, the stages of sleep, and the answers to frequently asked questions about sleep mechanics.

What Are Sleep Stages?

Sleep is a dynamic time of rest in which the body alternates between two types of sleep and four sleep stages. During sleep, the body cycles through two types of sleep, non-rapid eye movement (NREM) sleep and rapid-eye movement (REM) sleep, which are further broken down into four sleep stages.

Each stage of sleep is associated with characteristic changes in the body that are important to staying healthy and waking up feeling refreshed:

Sleep Stage Description  Length
Stage 1 This initial stage occurs as a person begins to fall asleep. Sleep is very light during this period as the body begins to relax. 1 to 7 minutes
Stage 2 Before the body enters deep sleep, this stage of light sleep is marked by a drop in body temperature and a continued slowing of breathing and heart rate. 10 to 25 minutes
Stage 3 The deepest stage of sleep, this stage is when bodily muscles are fully relaxed, and heartbeat and breathing rates are at their slowest of the night. 20 to 40 minutes
Stage 4 Stage 4 sleep is when a person enters REM sleep, in which their breathing and heart rate become more rapid as their eyes begin to rapidly move left and right. 10 to 60 minutes

Sleep quality is determined by factors like how much time is spent in each sleep stage and how often they wake up during the night. Spending the right amount of time in each stage of sleep provides a plethora of benefits, from improving mood to reducing the risk of conditions like heart disease, diabetes, and depression.

Sleep Cycles

Sleep occurs in a series of cycles. One sleep cycle is the body’s progression through each sleep stage in a particular order, beginning with stage 1 and typically ending with a period of REM sleep.

Ordinarily, a person will go through four to six sleep cycles in a full night of sleep. Each sleep cycle lasts around 90 to 120 minutes. The first sleep cycle of the night may be slightly shorter, and the exact length of each cycle varies from person to person and over the course of the night.

REM Sleep vs. NREM Sleep

The first three stages of sleep are classified as non-rapid eye movement sleep. During these stages, the body progressively relaxes until reaching a deep and restful sleep. NREM makes up around 75% of the time people spend asleep and is thought to be the part of sleep important for rest and restoration.

The final stage of sleep is called rapid-eye movement sleep. Named for the quick eye movements seen during this stage, REM sleep accounts for around 20% to 25% of sleep time. Periods of REM sleep tend to get longer over the course of the night. REM is associated with dreaming, memory formation, and brain development.

Stage 1 Sleep

Stage 1 sleep is a short period when the body transitions from alertness to sleep. As a person nods off, they enter a light sleep that only lasts from 1 to 7 minutes per sleep cycle, or 5% to 10% of total sleep time. If a sleeper is awakened during this first stage of NREM, they may not realize they were actually asleep.

During this brief stage of sleep, brain waves begin to shift from their waking pattern while breathing and heartbeat slow and muscles begin to relax. Occasional muscle twitches are a normal part of this sleep stage, and the eyes may move slowly behind the eyelids.

Stage 2 Sleep

Stage 2 sleep is deeper than the first sleep stage, but sleep is still light enough for a person to be woken up easily. Sleepers spend around 45% to 55% of their sleep time in stage 2, more than any other stage of sleep. Stage 2 sleep is relatively short during the first sleep cycle of the night, lasting just 10 to 25 minutes, but gets longer as the night progresses.

As the body moves closer to deep sleep, breathing and heart rate continue to slow, body temperature drops, and muscles become even more relaxed. During this stage brain waves slow down with occasional short bursts of activity, some of which may help sleepers learn, form long-term memories, and resist being woken up.

Stage 3 Sleep

Stage 3 sleep is a period of deep sleep in which it is more difficult to be woken up. Younger adults spend around 10% to 20% of their time in stage 3 sleep, but this percentage decreases with age. Stage 3 lasts about 20 to 40 minutes in early sleep cycles and decreases in length as the night progresses.

During stage 3 sleep breathing, heart rate and blood pressure are at their lowest as the body reaches its most relaxed state of the night. This stage is also called “slow wave sleep” because of the dominance of very slow brain waves called delta waves.

Researchers believe that deep sleep is important to feeling refreshed the next day. This stage is also important for supporting memory, metabolism, and the immune system. During stage 3, the body also builds and repairs bones, muscles, and other tissues.

REM Sleep

REM sleep is the last sleep stage and typically begins around 90 minutes into the first sleep cycle. This stage makes up around 18% to 23% of sleep time. The amount of REM in a given sleep cycle changes over the course of the night, ranging from 10 minutes at the beginning of the night to an hour near the end.

In a departure from NREM patterns, breathing, heart rate, and blood pressure increase during REM. Brain waves also shift to look more like daytime patterns than those seen in deep sleep. At the same time, most muscles in the body become temporarily parallyzed and unable to move. Research suggests that losing muscle control and reflexes may prevent people from acting out their dreams.

Most dreaming takes place during REM sleep. While researchers are still learning about the purpose of REM sleep, many believe that this stage is important for brain development, retaining memories, and pruning unneeded connections between brain cells.

Frequently Asked Questions About Sleep Stages

What Stage of Sleep Do You Dream In?

While dreams can happen during any stage of sleep, most dreaming happens during REM sleep. Dreams in REM sleep tend to be more vivid and bizarre, with content that is often disconnected from reality.

What Stage of Sleep Do You Sleepwalk In?

Most sleepwalking happens in stage 3 NREM sleep. Sleepwalking is categorized as a parasomnia, which is a term for conditions involving abnormal movements and sounds that a person makes while sleeping. Other parasomnias that happen most often during stage 3 sleep include sleep talking and night terrors.

How Does Disrupted Sleep Affect Your Sleep Stages?

It is normal to wake up briefly several times during sleep and sleepers are usually unaware of these short arousals. However, too many disruptions can affect sleep quality and cause a sleeper to wake up feeling unrefreshed.

Sleep disruptions can also affect a person’s sleep stages. Whether from missing sleep or waking up too often during the night, the body makes up for lost sleep by spending more time in deep NREM sleep. After longer periods of sleep loss, a person may also experience an increase in REM sleep.

What Factors Affect Your Sleep Stages?

Sleep deprivation is only one of many factors that can alter how the body transitions through sleep stages. Other causes of changes in a person’s sleep cycles include:

  • Age: Sleep stages shift as people get older. As sleepers age, they spend less time in deep stage 3 sleep and more time in lighter sleep stages. This change in sleep stages may be one reason why older adults wake up more often during the night.
  • Mental health: Stress, anxiety, and depression can make it more difficult to fall asleep and cause a person to spend less time in deep sleep and more time in REM sleep.

Diet: Certain food and drinks, including those that contain caffeine and alcohol, can reduce the time a sleeper spends in stage 3 sleep. While alcohol also reduces REM sleep early in the night, the body compensates with more REM sleep as the night goes on and the effects of alcohol wear off.

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