Sleep is something we cannot do without, and a good night’s rest is vitally important for our health and wellbeing. As humans, most of us go through various stages of sleep every night.
But how exactly does this phenomenon work?
Let’s go through each of these stages to see what differentiates them, so that we can understand exactly what happens when we sleep.
Understanding the Sleep Cycle
During sleep, our body goes through five stages. A sleep cycle refers to the period of time it takes for an individual to progress through the various stages of sleep. Stages 1, 2, 3 and 4 are non-REM sleep, followed by REM (rapid eye movement) sleep.
Each cycle lasts around 1½ hours and we need to experience all four stages in order to wake up rested. When we first fall asleep we enter non-rapid eye movement (NREM) sleep. NREM is divided into three stages: NREM1, NREM2 and NREM3, each stage gradually getting ‘deeper’.
During the first sleep cycles, your body relaxes and your blood pressure and heart rate drop and we go through complex changes in brain activity. These stages advance on a perpetual basis from 1 through REM then start again with stage 1.
A complete sleep cycle averages 90 to 110 minutes, with each stage lasting between 5 to 15 minutes. The initial sleep cycles every night have comparatively short REM sleeps and long periods of deep sleep. However, later in the night, REM periods increase and deep sleep time decreases.
When we sleep, we do not go straight from deep sleep to REM sleep. Instead, a sleep cycle goes through the stages of non-REM sleep from light to deep sleep, then goes back from deep sleep to light sleep, ending with time in REM sleep before starting over in light sleep again.
This is what the order looks like:
Stage 1 (light sleep) –> Stage 2 (light sleep) –> Stage 3 (deep sleep) –> Stage 2 (light sleep) –> Stage 1 (light sleep) –> REM
Here’s what happens in your body during each phase of sleep:
This is a light stage of sleep where you drift in and out of sleep and can be awakened easily. Eye movement slows and muscle activity falls. This introduction to sleep is relatively brief, lasting up to seven minutes.
At this stage, eye movement ceases and brain waves slow down with only a sporadic burst of rapid brain waves. The body starts to prepare for deep sleep, as the body temperature starts to fall, and the heart rates slows down.
At this stage, we reach a stage of deep sleep. At this stage, it is more difficult to arise from, and you may feel quite disorientated if someone were to wake you up from this stage of sleep. This is where people tend to experience different phenomena such as sleepwalking, night terrors, talking during one’s sleep, and bedwetting. These tend to occur during the transitions between non-REM and REM sleep.
Deep sleep continues at this stage of sleep, and people who are roused from this state feel disoriented for a few minutes. This is the end of NREM sleep, and we enter a stage which is known as rapid eye movement (REM) sleep.
Deep sleep provides the most restorative sleep of all the sleep stages. If you take a nap during the day that is long enough to fall into deep sleep, you’re going to have more difficulty falling asleep at night due to the fact that you have reduced your need for sleep.
During deep sleep, HGH (human growth hormone) is released by the brain and repairs and restores your body and muscles from the stress you’ve gone through on the day. It is also at this stage that your immune system restores itself.
This stage of REM sleep is similar to wakefulness or drowsiness. It is during the REM stage of sleep that we dream. Eyes remain closed but move rapidly from side-to-side, perhaps related to the intense dream and brain activity that generally occurs during this stage.
After REM sleep, we go back to stage 1 of light sleep and start a new cycle. As time goes on, people spend more time in REM sleep and correspondingly less time in deep sleep.