What Do We Do When We Sleep?

  • 1st Revision: Darvina Magandran[Linkedin]
  • 2nd Revision: Anna Mazepa
  • 3rd Revision: Pranitha Ven Murali[Linkedin]

Short Explanatory Video

Have you ever considered what happens to our body when we are asleep?

Does sleep mean that our body is completely inactive?

What happens when we dream?

With those questions in mind, let's take a closer look at our sleep behaviour.

There are two types of sleep: non-REM and REM. REM stands for rapid eye movement, and describes a period of sleep where your eyes move randomly and you experience vivid dreams. However, REM sleep only occurs for short periods meaning that most of our time sleeping is spent in non-REM sleep.

Non-REM sleep can be further divided into three stages:

Stage 1

This is the transition stage from wakefulness to sleep, which usually lasts around 5 to 10 minutes. Everything starts to slow down during this light sleep stage; your muscles, eye movements, heart rate and breath. You can still easily be woken up.1-3

An interesting phenomenon that many experience is a sudden muscular contraction. You might feel or dream that you are falling. This is called hypnic jerk.4 To date, there is no understanding of why people experience hypnic jerks, but it is perfectly normal to have this muscle contraction and it is not related to any underlying health condition.

Stage 2

In this stage, you are still in light sleep, but it is the transition stage before your body falls into deep sleep. It usually lasts around 10 to 25 minutes. Your eye movements stop and your heart rate and body temperature drops further. The muscles are even more relaxed.

On a normal night, more of the repeated sleep cycles are spent in this stage.1

Stage 3

Now you enter the deep sleep stage, a period of time that is harder to wake up from. If someone wakes you up at this stage, you may feel disoriented for a while.3 In this stage, your heart rate and breath are at their lowest levels. Now, your body can repair itself - building up muscle, growing damaged/old tissues, repairing bones, and boosting your immune system.

REM Stage

REM sleep is quite different from non-REM sleep.

Your first REM period usually happens 90 minutes after you fall asleep and it lasts around 10 minutes. This phase is called REM (rapid eye movement) because when you enter this stage, your eyes will move rapidly from side to side, even though you are asleep and your eyelids are closed.

Unlike other sleep stages, when your brain activity is low, your brain is very active during this stage. Similarly, your heart rate, breath, and blood pressure rise close to waking levels. Although dreams can occur in other stages, most of your dreams, especially the intense ones, occur during REM sleep.

Interestingly, at this stage your muscles are not as active as your other body parts (such as the brain and heart). In fact, the muscles of your limbs are temporarily paralyzed to prevent you from acting out your dreams and sleepwalking.1

REM sleep is extremely important because our brain consolidates what it has learned and memorised throughout the day during this stage. It is the essential sleep phase that strengthens your learning, creativity, and other cognitive functions.

The three non-REM stages and REM stage form a cycle, and this sleeping cycle repeats throughout the night. In the last cycle towards morning, you tend to have the longest and deepest REM sleep.1

Factors affecting Sleep Stages


As you age, you get less non-REM (rapid eye movement) sleep, especially for deep sleep. This holds the same for REM sleep as well. Research shows that the percentage of REM sleep in the whole sleeping time is the highest in infants and babies, and it gradually declines as we age.2 Babies can have 50% of their sleep in the REM stage, while adults usually have only 20%.3


Many might think that drinking a little alcohol can help people fall asleep more quickly and have a very “deep” sleep. But don’t be deceived by the sleepy feeling after you drink alcohol. Alcohol seems to aid your sleep because of its sedative effect, which causes the brain to slow down and make you feel very relaxed and sleepy. However, even if the alcohol makes you fall asleep, it has a negative effect on your sleep quality. In fact, alcohol can disrupt and alter your normal sleep stages. Research has shown that long-term consumption of alcohol, especially in excessive amounts, is closely related to insomnia.4


To summarise, the three stages of non-REM sleep take you from being awake to falling asleep, and it is the best time for your body to relax and self-repair. REM sleep is important as it is the essential sleep phase that strengthens your learning, creativity, and other cognitive functions. There are factors that affect our sleep cycles, such as we age, we get less non- REM sleep. Also, alcohol may make you sleepy but in fact it is just the effect of alcohol which can actually disturb your sleep pattern. We have now considered what happens to our body when we are asleep.


  1. National Institute of Neurological Disorders and Stroke, 2019. “Brain Basics: Understanding Sleep” [Online]. Maryland: National Institute of Neurological Disorders and Stroke. Available from: https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-Sleep
  2. Cleveland Clinic, 2020. “Sleep Basics: REM & NREM” [Online]. Ohio: Cleveland Clinic. Available from: https://my.clevelandclinic.org/health/articles/12148-sleep-basics
  3. WebMD, 2020. “Stages of Sleep: REM and Non-REM” [Online]. New York: WebMD. Available from: https://www.webmd.com/sleep-disorders/sleep-1014. Sleep Foundation, 2022. “Alcohol and Sleep”
  4. [Online]. Washington: Sleep Foundation. Available from: https://www.sleepfoundation.org/nutrition/alcohol-and-sleep
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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