Crash Course: A Guide to Sleep Stages and Cycles and What Happens When We Sleep

Sleep Cycles: A Guide to Sleep Stages and What Happens When We Sleep

There's more to sleep than the term "falling" asleep suggests

Have you ever been so tired that all you want is to fall asleep instantly when you go to bed?

Yet you lay in bed and you just can’t sleep?

Research into sleep stages and cycles show that there’s more to sleep than the term “falling” asleep suggests.

When we go to bed, we just don’t instantly fall into a deep, uninterrupted sleep.

Our sleep progresses from one phase to another. And then to another.

Even on those days when it seems you fell asleep right away, you still went through certain stages.


The good news is we don’t have to make an effort to drag ourselves through these sleep stages.

It all happens unconsciously. And the combined benefits of each stage helps our bodies to replenish and restore itself for the next.

So sit tight, and let’s talk about what happens to our bodies and brains when we sleep. 

What are the stages of sleep? 

There are four main cycles or stages of sleep.

The first three stages are what are known as non-REM sleep stages. The last stage is called the REM stage (more on these in a moment). 

It’s tempting to think that sleep starts from stage one (light sleep) to deep sleep (stage three) and then to stage four.

The reality is very different.

Most of the time, we move through stages 1 to 3 and back through 1 to 3 for a few cycles, before ending the sleep in stage 4.

So what are these stages I’m talking about? Let’s dive in.

Stage one

This is the first stage of sleep. At this stage, although you have your eyes closed, you’re also aware of your environment.

You have slow eye movement, your body begins to relax and your brain wave activity gradually slows down. 

While this is all happening, you’re also sensitive to your environment. And little distractions like a door slamming or high pitched voices can disrupt you.

You’re also likely to experience hypnic jerks at this stage of sleep.

Remember a time when you start to fall asleep and suddenly it feels you’re falling off a cliff, so you jerk yourself awake?  

Essentially your muscles contract. And your body or limbs jerk in what often happens in split seconds.

As disorienting as they can be, hypnic jerks occur at the start of sleep. And everyone at some point in their lives must have experienced it.

Although factors such as excessive caffeine intake, physical, and emotional stress can increase their frequency.


All of this happens so quickly between the state of wakefulness and sleep.

So quickly that if someone were to wake you up at this stage, you’ll claim to have never fallen asleep in the first place.

Proper sleep hygiene is key if you want to move past this stage.

Stage 2 

At this stage, your slow eye movements stop. You’re not easily disrupted and your brain wave activities are still winding down.

This stage is defined by sleep spindles and K complexes. Let me explain.

Sleep spindles are short but fast activities that wax and wane quickly.

Think of K complexes as big waves different from the background. And they are often a sort of response to sounds in the bedroom.

In general these two mechanisms – sleep spindles and K complexes- are the brain’s shield.

They protect you from distractions that will cause you to wake up during sleep. Your heart rate slows down and your body temperature reduces as well.

Stage 3 

This stage of sleep is where you enter into a deep sleep. The brain produces slow waves.

It’s unlikely that an external stimulus like noise will wake you up at this stage.

This is because your body tends not to respond to external arousal or stimuli. If you know someone who sleeptalks or sleepwalks, this is the stage of sleep where that happens.

It’s also the stage where the body restores itself through muscle and tissue repair.

This essential process enables us to develop stronger bones, improve exercise recovery, and boost immunity.

People experiencing jet lag, tend to have less sleep at this stage.

Stage 4

The transition from stage 3 to stage 4 leads you to what is referred to as REM Sleep. It’s also called Rapid Eye Movement.


As the name suggests, at this stage, the brain is active. It’s just like they’re when we’re awake. And often your eye movements become faster, swinging in different directions.

Remember those times when you’ve had deep vivid dreams that you wished never ended?

Well, this is the stage where this happens.

Studies into sleep learning have shown that this stage of sleep enables us to develop learning and fortify our memories. 

It’s likely the part of the brain responsible for storing memories is reactivated again as a network.

This ultimately aids in teaching the brain to remember later when we wake up.

How long do sleep cycles last?

The short answer: it varies. Let me explain. A baby’s sleep cycle is most likely not going to resemble that of an adult. 

You know how babies sleep for around 16 hours each day? Turns out that half of those hours are in the REM stage of sleep.

Stages one to three, as we discussed, are non-REM sleep (NREM). And those stages happen in a sort of progressive fashion. 

It starts with light, awake-ish sleep and then to less than deeper sleep. And then back to light, until later in the night you get to the REM stage.

You only get into REM stage of sleep after about 90 minutes.


When does deep sleep occur?

Deep sleep happens in the non-REM Stage 3.

Part of the reason is that not only are our brain waves slowed down. But also our bodies do not respond to external stimuli.

If you’ve ever skipped on sleep and finally you crash out on the couch, your sleep at this stage may be prolonged.

If you ever wanted to sleep more hours, this may be your chance.

When do we dream during our sleep?

Most dreams happen at the REM Stage 4 of sleep. At this stage, the brain waves are very active.

We breathe faster and inconsistently. Our heart beats faster. Our blood pumps faster. 

It’s likely that while you dream each night for about 2 hours, you don’t remember any of that. 

Depending on what happened during your day, your dream will vary too.

Stress, anxiety, depression, and fear, all tend to creep into our dreams in some way, shape, or form. Research is yet to identify precisely why we dream.