Sleep: it’s one of the big questions in biology that scientists are still puzzling over.

Cats spend an average of 12 hours a day asleep. Some species of snake sleep 18 hours out of 24. And it seems that all animals spend more of their time asleep when they’re young, which is why babies seem to sleep all the time, but adults might spend much more time awake.

In this article, we’re going to be looking at some of the reasons our bodies need sleep, what we mean when we talk about the sleep cycle, and what happens to our brains at each stage of the night.

And in a future article, we’re going to be talking about dreams, so keep your eyes peeled for more info on that!

Why do we sleep?

The question of why we sleep is one that humans have been considering for thousands of years, as far back as Aristotle in 350 BCE, who wondered what we were doing when we sleep, and why.

In the last 100 years, with the invention of machines that allow us to record brain activity, we have finally been able to see what we are doing, but scientists are still uncertain about why exactly.

It used to be believed that sleep was a way for our bodies to save energy, but recent studies have shown that our bodies and brains are so active through the night, we actually don’t save much energy at all.

One thing we do know is that our immune system, which is how our body fights illnesses and repairs injuries, is highly active when we sleep. There’s a reason we’re always advised to get lots of rest when we’re poorly – our bodies are much better at fighting disease while we snooze!

Sleep also seems to play a really important role in brain development, and also brain “maintenance”. Sleep is like the brain caretaker – it comes in and tidies everything up, storing information we learned during the day that it thinks might be useful. That’s why the best thing to do before a big test is to try and get a good night’s sleep – after revising, of course!

If we don’t get enough sleep, then it can also have really negative effects on our speech, and our ability to concentrate. Most of us will have experienced this at some point in our lives. After a bad night it can feel like we’re unable to focus on anything. We might be forgetful, or grumpy, and we can have a hard time making decisions.

If we keep having a lack of sleep, then the part of our brain which is responsible for language, memory, and planning switches off completely. That’s why when you reach this stage you can have trouble speaking, and remembering things you’ve just been told.

This is why it’s so important to get plenty of sleep. Our brains need the chance to file away information, and if you’re interested in how exactly they do this, then keep reading for an explanation of what goes on in the brain when we sleep!

Sleep Cycles

When talking about sleep, scientists talk about the sleep cycle, which is divided into four different stages.

During the night, we tend to move between these stages in a similar way each time, moving up from one stage to the next, before returning to stage 1 and repeating the process.

These stages can be broken down into two different types: REM sleep, and non-REM sleep. REM stands for Rapid Eye Movement, and we’ll look at that in more detail below!

Stage 1

The first stage of sleep is very short, only lasting a few minutes. This is the stage where we’ve just fallen asleep, and it’s really just a stepping stone to stage 2.

During stage 1, our body temperature lowers slightly, and our muscles relax. Our eyes may move slowly from side to side during this stage, but we will most likely be unaware of this.

If we were able to monitor our brain waves at this stage, we would see them start to slow down, as our brain prepares itself for the work it needs to do while we sleep.

Stage 2

This is the first stage of “true” sleep, and it can last anywhere from 10 to 25 minutes on average.

During this stage, our heart rate has slowed down, our eyes are still, and we breathe much more slowly than when we’re awake.

Scans of our brains during this stage would show a really irregular pattern, as big, slow waves are mixed in with short bursts of activity, called sleep spindles.

Imagine it like a long distance runner in the Olympics. Instead of running really fast all the time, they spend most of the race at a medium pace, with big strides, and they only speed up to a sprint when they want to overtake someone, or right at the very end.

During these “brain sprints”, we briefly become much less aware of the outside world, which has led scientists to believe that this is when our brains begin to organise our memories for long term storage.

In other words, if you’ve learned something new during the day, this is when your brain starts to work on remembering it for the future.

Stage 3

The third stage is what we call “deep sleep”, and when we enter this stage, we become much harder to wake up.

When you’re younger, you spend more time in this stage – up to half an hour – because this is where the body releases hormones that help with growth, and repair any injuries we have suffered.

And if you’ve not slept well for a long period of time, then you’re likely to skip very quickly through stages 1 and 2 and spend longer in stage 3. Scientists therefore think that stage 3 is probably the most important stage for helping us feel less tired.

Stage 4 – REM

Stage 4 is completely different to the other stages.

During this stage, our eyes move quickly back and forth behind closed eyelids, which is why it’s called REM (Rapid Eye Movement) sleep.

Our body temperature, which cooled down during the first three stages of sleep, begins to rise again, and our heart rate and breathing nearly reach the same levels as during the day.

This means we’re breathing and pumping blood around our bodies almost as quickly as when we’re awake, despite the fact that our muscles are completely still at this point.

The part of our brain that controls our fight or flight response is actually twice as active during REM sleep as when we’re awake. For more information on fight or flight, check out this article exploring how it works!

REM sleep is thought to be the stage where the brain does most of its repairing and refreshing. When we reach REM sleep, our brains are able to process new information, storing it away and allowing it to understand it as well as remember it.

On average, we enter REM sleep between 3 and 5 times a night, and each time it increases in length. The first time it may only be a few minutes, but by the final time we achieve REM sleep in a night, it could be for up to half an hour.

How much sleep do I need?

You might have heard people say that humans need 8 hours of sleep every night in order to function properly. Well, this is actually a bit of a myth!

Everyone is unique, and that applies to sleeping patterns too. Some people will need lots of sleep, whilst others need less. Napoleon was said to have slept for only 4 hours a night. Teenagers, on the other hand, usually need closer to 10 hours, as their bodies are undergoing so many changes that they need plenty of time to recover.

There are even some scientists who think that the way we sleep, all in one big chunk at night, might not be the best way.

The most important thing is to figure out what feels right for you. If you notice that you feel tired a lot of the time, then it might be time to start heading to bed a little earlier.