You close your eyes and what feels like seconds later, your alarm wakes you up. The time between those two moments tends to feel like one continuous period of dormancy. 

In reality, a lot happens during sleep. The body goes through stages of alternating light and deep sleep. Missing out on some of these stages or not spending enough time in them can lead to worse sleep quality. 

Why Does Sleep Occur in Stages?

Why the heck are there sleep stages? As with a lot of other things about sleep, there are only theories. 

Scientists think that these stages play important roles in our health and function. During some stages, the body consolidates memory and helps you retain what you’ve learnt. In other stages, the body focuses on healing and tissue restoration. 

Whatever the reason for sleep stages, it is clear that they are important. Sleep disruptions that prevent you from properly cycling through the various sleep stages can affect your memory, energy levels, emotional health and physical health.  

The Four Stages of Sleep

Sleep is broadly divided into two stages: REM and NREM. That is, rapid eye movement and non rapid eye movement. 

Most of your sleep time is spent in NREM stages. This is when brain and body activity slows down. The rest of the time is spent in REM sleep, which is characterized by a spike in brain activity. 

There are three NREM stages and one REM stage. Note that the following stages typically repeat through the night with an average of 4-6 cycles that last 80-100 minutes each.  

NREM Stage 1 Sleep

When you close your eyes and drowse off, you immediately go into NREM stage 1 sleep. This is a transitional stage between wakefulness and sleep. 

It is the lightest stage of sleep and it’s fairly easy to wake up from it if something disturbs you. But it doesn't last long; usually 1-7 minutes. 

NREM Stage 2 Sleep 

As you fall deeper into sleep, you move into stage two NREM sleep. Your heart rate and breathing drops, and your body temperature drops as well. 

Brain waves also slow down, though there are occasional bursts in brain activity. These are called sleep spindles and scientists think they are important in memory consolidation. 

Stage 2 sleep lasts about 25 minutes but it gets longer with each successive cycle. 

NREM Stage 3 Sleep 

Stage three is also called deep sleep or slow wave sleep (SWS). The body falls further into deep sleep and it gets harder to awaken from sleep. 

If you wake up during this stage, you’ll feel groggy and foggy for quite some time. This is called sleep inertia and it’s why you should take a power nap longer than 20 minutes if you want to wake up feeling fresh. 

Even worse, not getting enough sleep time in stage 3 can impair your mental and physical health. That’s because this is the stage the body restores and heals damaged tissue. It’s also when muscle and bone growth occurs. 

Stage 3 sleep lasts between 20 and 40 minutes. It becomes progressively shorter with each cycle. 

REM Stage 4 Sleep 

REM stage 4 sleep is the polar opposite of stage 3. It is anything but restful. 

In REM sleep, brain activity rises to almost the same levels as when you are awake. Your body stays relaxed however due to self-induced paralysis. Only your breathing muscles and your eyes remain active. 

That’s why your eyes are able to dart about under your closed eyelids during REM sleep. It’s also where the name ‘rapid eye movement’ comes from. Most dreaming happens in REM sleep. 

REM sleep is thought to be important for memories, learning and mood. The first REM stage occurs about 90 minutes after you’ve slept and lasts only about 10 minutes. But as the night progresses, you spend more and more time in REM sleep. 

In fact, most people wake up in the morning while in the middle of REM sleep. 

What Can Affect Sleep Stages?

We’ve already mentioned how lack of enough time in particular stages or completely missing some stages of sleep is detrimental to your health. A number of factors can affect how you cycle through sleep stages. 

The most common is simply not getting enough sleep. If you sleep less than 6 hours a night, then your body doesn't get to spend adequate time in all stages and may even miss some of them. This is likely why sleep deprivation has a huge negative effect on memory, creativity, moods, muscle growth and overall health.

Sleep disorders can also affect how you cycle through sleep stages. Insomnia, for instance, can reduce how much time you spend in various stages. 

Here are other factors. 

• Sleep disturbances can interrupt sleep stages, which can prevent memory consolidation, body recovery and other important preferences. That’s why it’s important to have the right sleep environment. 
• Age also affects sleep stages. Older people tend to spend less time in REM and stage 3 NREM deep sleep. Time spent in Stage 1 and 2 sleep goes up. 
• Substances like alcohol and certain medication can affect sleep architecture. Alcohol, for instance, causes more deep sleep at the beginning of the night but as it wears off, you experience increased light REM sleep.   

Final Thoughts

Good sleep is not just about the number of hours you spend in bed. The quality of the sleep matters. Spending adequate time in all stages of sleep ensures you wake up feeling refreshed. It also protects your immune system, maintains mental acuity and helps your body recover.