What happens when we sleep? The different stages of sleep

Sleep helps us repair and restore our body and mind. It wipes away the fatigue experienced during the day, if restorative and adequate sleep is achieved, it will make you feel refreshed and alert the next day. Essentially, the body goes through repeated cycles of 4 stages, namely 3 non-REM (Rapid Eye Movement) stages and 1 REM stage, throughout the entire duration of sleep. Let us discover more on how sleep restores the body and discuss the recommended duration for sleep across all age groups.

The non-REM and REM Sleep stages

Once we doze, there are generally 4 stages in our sleep cycle. This cycle repeats itself multiple times during our entire sleep duration. The first 3 stages are what we call non-rapid eye movement (NREM) and the last stage is called rapid eye movement (REM) (1).

1^st stage NREM

This consist of light sleep with muscles relaxing and at this stage, the heart rate, breathing rates and eye movements begin to slow down. This stage will last for a few minutes.

2^nd stage NREM

Your heart rate, and breathing rates will slow down even more with muscles becoming more relaxed as you fall deeper into sleep. At this stage, eye movements will stop and body temperature will drop. This stage is typically the longest of the 4.

3^rd stage NREM

At this stage, the heart rate, breathing rate and brain activities will reach their lowest levels, with muscles relaxed. This is the most important stage in ensuring that we feel the most refreshed and alert after we wake up. This stage will be the longest at the first cycle and start to decrease in duration as the cycle repeats itself into the night.

Experts also term this as deep sleep or slow wave sleep. At this stage, the body is being primed to enhance its energy building capabilities, produce a hormone to help with tissue and muscle repair / growth. It also helps the body build up its immune system. As such, someone who has disrupted or inadequate deep sleep will feel much less refreshed and alert than someone who has (2).

4^th Stage REM

As the name states, in the Rapid Eye movement stage, our eyes will move back and forth instead of shuttering under the eyelids. The REM of the first cycle will typically occur about 1.5 hours after we fall asleep. As this stage, our heart rate, breathing rate and blood pressure will begin to rise. Dreaming will typically occur at this stage.

The interesting part about REM is that your limbs will be paralysed and this is said to prevent us from physically moving along with what we are dreaming. There are studies which link REM sleep with what is termed as memory consolidation. It is the process of clocking our latest learned experiences and episodes into our long term memories.

As mentioned earlier, these 4 stages will repeat themselves cyclically until we wake up. Each cycle is said to last for about 1.5 to 2 hrs.

The Glymphatic System

Our central nervous system (CNS) is highly active, which causes a lot of metabolic waste to build up during our waking hours. The glymphatic system acts as a brain-based waste disposal mechanism that helps to clear this cellular garbage.

Experts found that the glymphatic system is most active when we slept, and this can be an indication of the restorative nature of the sleep, where the glymphatic system is given the opportunity to clear the metabolic waste from the brain (3).

How much sleep do we need?

The national sleep foundation recommends different sleep duration for different age groups.

New born requires the most sleep and as we age, our recommended sleep hours tend to get lesser. As an adult from 26 to 64, the recommended duration is 7 to 9 hours.

Read more : Understand the Circadian Rhythm, Why is sleep important to us?