Sleep Stages Explained

Pietari Nurmi
Dec 29 · 6 min read

Sleep can be divided into different stages that all serve their own purpose. Sleep stages differ by depth, yet all stages are important for balanced and refreshing sleep.

Sleep stages appear in repetitive cycles. During a one 90-minute sleep cycle the body goes through all of the different stages one by one. The exact length of the cycle is personal but on average there is time for 5–6 sleep cycles in a single night. Although the order of the sleep stages in the cycles is somewhat fixed, the time spent in each of the stages varies greatly throughout the night. In other words, sleep during the early parts of the night can be quite distinct from the sleep in the late night or the early morning. It might even serve a completely different purpose. This is one of the biggest reasons why it is so important to get a full night of sleep every day.

Light Sleep

Sleep cycle always begins with light sleep. Light sleep stages act as a bridge between wake and deep sleep. Nevertheless, light sleep is just as important as other stages of sleep. Although it has not yet been studied as comprehensively as other sleep stages, scientists believe that light sleep is especially important for storing and strengthening memories acquired during the day. The fact that most of the night is spent in light sleep proves that it is far more than just an intermediary stage between other “more important” sleep stages.

Deep Sleep

After light stages the sleep gets deeper. Unlike light sleep which can easily be interrupted, it is very difficult to wake up from deep sleep. The significance of deep sleep is often emphasized since it is the most physically restorative of the sleep stages. The majority of the nightly body maintenance processes take place during deep sleep. That is why it is important to get enough of it every night. The amount of deep sleep in the sleep cycle is highest in the early parts of night. Later in the night it gives room for REM sleep.

REM Sleep

REM sleep (rapid eye movement sleep) is quite exceptional compared to other sleep stages. It is characterized by vigorous eye movements behind closed eyelids. What makes REM sleep even more special is that the brain activity during this stage closely resembles the state of wakefulness, even though the person is fully asleep. This is also the reason why dreaming happens almost solely during REM sleep and the dreams can feel very vivid and lifelike.

During the REM stage the body goes into standby mode. All the muscles lose tone and the limbs go loose like boiled spaghetti. This way the body ensures that no matter how realistic the dreams get your muscles won’t react to them. You can safely dream about a running competition without having to fear that your legs will take you to a sleepwalking trip in the middle of the night.

REM sleep is essential for memory, learning, creativity and other higher cognitive functions. For example, among students the lack of REM sleep is associated with impaired learning results. Alcohol is one of the major suppressors of REM sleep which is why drinking, and regular alcohol use in particular, can be very harmful.

Sleep cycle ends with REM stage after which a new cycle begins. It is not uncommon to wake up for a brief moment between sleep cycles, but you usually fall asleep again so quickly you won’t even remember it in the morning. On average the sleep cycles follow the same pattern from light stages to REM sleep via deep sleep. However, different stages are more pronounced at different parts of the night. At the early parts of night deep sleep takes more space from the sleep cycle whereas REM sleep is almost nonexistent. As the night goes on the amount of REM sleep increases and deep sleep gets less prominent. That is why REM sleep and the associated cognitive functions, such as learning and creativity, are the first to suffer from sleeping too little.

Sleep stages during an average sleep cycle
One sleep cycle can host all of the different sleep stages but the time spent in each of the stages varies greatly throughout the night. It is not uncommon to wake up for a brief moment between the 90 minute cycles, but you usually fall asleep again so quickly you won’t even remember it in the morning.

Measuring Sleep Stages

During sleep the brain never shuts down completely, but the level electrical activity is different during each of the sleep stages. Therefore, the best way to measure the sleep stages is by monitoring the brain activity with EEG electrodes (electroencephalography). Polysomnography that combines several other techniques in addition to EEG provides even more accurate results. These measurement techniques are used primarily in research labs and hospitals. 

Consumer-level sleep trackers are way simpler. As they do not measure brain activity, they cannot really tell much about sleep stages. Nevertheless, some manufacturers claim that their devices can track sleep stages using activity and heart rate data, and many sleep tracker apps might show you, for example, how much deep sleep you had last night. Although these devices can accurately track many aspects of your sleep, they do not have the necessary means to track sleep stages very well on their own. They can offer rough estimates at best, so treat their assessments with caution. The best measurement tool for sleep quality are your own senses. If you feel rested and refreshed in the morning, your sleep stages should be well in balance.

Additional Reading

Genzel, L., Kroes, M. C. W., Dresler, M., & Battaglia, F. P. (2014). Light sleep versus slow wave sleep in memory consolidation: A question of global versus local processes? Trends in Neurosciences, 37(1), 10–19.

Walker, M. (2017). Why we sleep: Unlocking the power of sleep and dreams. Simon and Schuster.