What Actually Happens While You Sleep? REM and Sleep Cycles

We tend to think of sleep as one long state.

You get into bed, fall asleep, hopefully stay there for seven or eight hours and then wake up in the morning.

Except that isn’t really what happens at all.

While you’re lying there apparently doing absolutely nothing, your brain and body are actually pretty busy. Throughout the night, you move backwards and forwards through different stages of sleep, including lighter sleep, deep sleep and REM sleep.

At the same time, your internal body clock is quietly working away in the background, helping to decide when you should feel awake, when you should start feeling sleepy and even why jet lag can make you feel completely upside down.

And if sleep is something you struggle with, understanding a little more about what is actually happening can sometimes make the whole thing feel a lot less mysterious.

We don’t sleep in one long block

Sleep happens in cycles.

A typical sleep cycle takes somewhere around 90 minutes, although it varies, and most people will move through several of these cycles during the course of a night. NHS sources describe most of us having around four to six cycles during a normal night's sleep.

And here's something that quite often surprises people:

Waking up during the night is normal.

We can briefly wake as we move between sleep cycles and often have absolutely no memory of it the following morning.

That matters because once somebody becomes worried about their sleep, they can start monitoring every awakening.

I'm awake again.

Why am I awake?

I need to get back to sleep.

I've only got five hours and 12 minutes before the alarm goes off.

And suddenly something your brain would ordinarily have dealt with without much fuss has become an event.

Unfortunately, nothing wakes a brain up quite like urgently informing it that IT ABSOLUTELY MUST GO TO SLEEP NOW.

So, what are the different stages of sleep?

Sleep is broadly divided into non-REM sleep and REM sleep.

When we first drift off, we enter lighter stages of sleep. We're asleep, but we're still relatively easy to wake.

We then move into deeper sleep, sometimes referred to as slow-wave sleep. This tends to feature much more heavily during the earlier part of the night.

Then comes REM.

As the night progresses we cycle through these stages again and again. Interestingly, the balance changes as the night goes on: we generally have more deep sleep earlier in the night and more REM sleep towards the morning.

So the sleep you are getting at 5am isn't necessarily identical to the sleep you were getting at midnight.

What actually is REM sleep?

REM stands for Rapid Eye Movement. Those who have had sessions with me will have heard me mention this time and time again!

It gets its name because during this stage your eyes move rapidly behind your closed eyelids.

Your brain also becomes much more active. Dreams can be particularly vivid during REM and, while your brain is busy, most of your body's muscles become temporarily very still.

Which is probably fortunate given some of the completely ridiculous things our brains have us doing in dreams.

Running through airports that somehow turn into our old secondary school would be considerably more problematic if our legs joined in.

For adults, REM typically makes up somewhere around 20–25% of sleep.

The first period of REM usually happens around 90 minutes after falling asleep and tends to be relatively short. Later in the night, periods of REM become longer, meaning we get proportionately more REM during the second half of the night.

REM sleep is associated with things including learning, memory and emotional processing.

So sleep isn't simply your brain switching off for a few hours.

Quite the opposite.

Are we actually getting enough sleep?

This is where the UK figures get interesting.

The NHS generally recommends that healthy adults aim for around seven to nine hours of sleep a night, although individual needs obviously vary.

Yet a large YouGov sleep study of British adults found that 49% felt they weren't getting enough sleep.

Seven hours was the most commonly reported amount, but 13% of Britons said they were getting fewer than six hours per night. Women were also more likely than men to feel that they weren't getting enough sleep.

And more recent UK research suggests that it isn't only about the number of hours.

In 2025, the Mental Health Foundation surveyed 2,000 UK adults and found that the average person reported getting good-quality sleep on only three nights of the week.

Even more strikingly, 14% said that on no day in a typical week did they get the sleep they needed to function well.

So if you're occasionally lying awake convinced that everybody else is peacefully sleeping from 10pm until 7am while you're wondering whether you locked the back door, you are definitely not alone.

Sleep and our emotional wellbeing are closely connected

This is one of the reasons I talk about sleep so often in practice.

Poor sleep doesn't just leave us tired.

In that same UK Mental Health Foundation research, 38% of adults said poor sleep negatively affected their mental health at least once a week, while almost half reported that poor sleep had recently contributed to things such as feeling more irritable, overwhelmed, stressed or anxious.

Of course, it works in both directions.

Stress and anxiety can interfere with sleep, and poor sleep can then make us feel less able to deal with stress the following day.

And so the cycle continues.

Which explains why a problem that begins with I've had a couple of bad nights can sometimes turn into I'm now worrying about whether I'll sleep before I've even got into bed.

Then there's your body clock

Another important part of sleep is your circadian rhythm.

That sounds very scientific, but it's basically your internal body clock.

It follows a roughly 24-hour rhythm and helps regulate when you feel awake and when your body starts preparing for sleep. One of the most important influences on that clock is light. NHS guidance describes our circadian rhythm as particularly sensitive to changes between light and darkness.

As it becomes darker in the evening, your brain begins producing more melatonin.

Melatonin is a naturally occurring hormone involved in regulating your sleep cycle. Levels normally start rising after it becomes dark, peak during the early hours of the morning and fall again during daylight.

This is also why flying across several time zones can make you feel completely peculiar.

Your phone may confidently announce that it's 10pm, while your internal body clock is effectively saying:

Absolutely not. It's four in the afternoon and I have no idea why you've put pyjamas on.

Eventually your circadian rhythm catches up with the new light-dark pattern, but it can take a while.

My circadian rhythm most definitely doesn’t match up to today’s timings … those who know me well will know that it’s not unusual for me to start organising my wardrobe at midnight!

Your brain also builds up a need for sleep

Your circadian rhythm isn't the only thing involved.

The longer we stay awake, the greater our natural drive to sleep becomes. This is often referred to as sleep pressure.

Think of it as something that gradually builds throughout the day.

You wake in the morning with relatively little sleep pressure. As the hours pass, it increases. Eventually your brain becomes increasingly ready for sleep.

Then you sleep and that pressure reduces again.

It's one of the reasons why a very long late-afternoon nap can sometimes completely sabotage bedtime.

You might have been exhausted at 5pm, slept on the sofa for two hours and then found yourself wide awake at midnight wondering what's gone wrong.

You've essentially taken the edge off your sleep pressure.

The harder you try to sleep, the harder it can become

There is a strange contradiction with sleep.

We can create conditions that make sleep more likely, but we can't really force ourselves to sleep.

Yet when people start struggling, forcing it is often exactly what they try to do.

You wake at 3am.

You check the clock.

You try really hard to sleep.

You check again.

It's 3.23am.

Now you're calculating how much sleep you could still get if you fell asleep within the next seven minutes.

At 3.41am you're thinking about tomorrow.

At 4am you're wondering whether you've permanently forgotten how to sleep.

And somewhere along the way your brain has gone from briefly waking between sleep cycles to being fully alert because you've inadvertently told it that something important is happening.

Interestingly, the Mental Health Foundation's UK research found that 34% of adults felt worry and stress reduced the amount of control they had over their sleep.

That's why reducing the anxiety around sleep can sometimes be just as important as focusing on sleep itself.

Good sleep doesn't have to mean perfect sleep

This is probably the bit I'd most like people to take away.

Good sleep doesn't necessarily mean getting into bed at exactly 10pm, falling asleep immediately and remaining completely unconscious until your alarm goes off eight hours later.

Real sleep isn't like that.

We move between lighter and deeper sleep.

We dream.

We change position.

We briefly wake.

Some nights we sleep brilliantly and other nights we don't.

And our sleep can change depending on stress, hormones, illness, alcohol, medication, parenting, work, travel, age and dozens of other things happening in ordinary life.

One difficult night doesn't mean you've suddenly developed a sleep problem.

And waking briefly doesn't mean you've had a terrible night's sleep.

Sometimes simply understanding that sleep is a changing, cycling biological process, rather than eight hours of uninterrupted unconsciousness, can remove some of the pressure we place on ourselves to do it perfectly.

Your brain already knows how to sleep.

After all, it's been practising for your entire life.

Sometimes it just needs a little help getting out of its own way.

Sources I've used

For the UK figures, the Mental Health Foundation's October 2025 research is particularly useful because it's recent and surveyed 2,000 UK adults. Mental Health Foundation – UK sleep research

The YouGov Sleep Study gives the figures around typical sleep duration and the proportion of Britons who feel they're not getting enough. YouGov – The Sleep Study: sleeping patterns

For sleep cycles and normal night-time waking, I used Oxford Health NHS Foundation Trust. Oxford Health NHS – What happens when we sleep?

For REM specifically, North Bristol NHS Trust has a useful explanation of when REM occurs and how it changes through the night. North Bristol NHS Trust – REM sleep

And the NHS has a simple explanation of melatonin and the sleep-wake cycle. NHS – How melatonin works