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It's time to talk about sleep, after all it is World Sleep Day.

It's a day dedicated to celebrating sleep and highlighting the importance of sleep health.

This year, the event falls on 15 March and the theme is "sleep equity for global health".

This time the event is focussed on the differences in sleep health across different populations around the world.

Sleep is a really important part of our physical, mental and social wellbeing, just like eating well and being active.

But why do we sleep? And how can we help ourselves get the best night's sleep possible?

Read on to find out more.

What happens when we sleep?

For a long time, scientists could not agree fully on the function of sleep and it's been difficult to work out exactly why we do it and what happens to our bodies while we're catching some kip.

However, it's widely believed humans need to sleep in order to give our bodies - and, in particular, our brains - a chance to rest and recover.

We know for example that sleeping can help us feel more refreshed and alert when we wake up.

In fact, there are areas of the brain that are more active when we're asleep than when we are awake, so we know it's up to something.

"When we are asleep, our muscles relax and the body gets a good chance to rest. It's the same for our mind," clinical psychologist Dr Claire Halsey told Newsround.

There are two key activities that research suggests happen in our brains when we sleep:

1. Strengthen connections

Our brain cells build up new connections with other parts of the brain as we go through life, as we learn things and make memories, for example.

While we are asleep, it is believed that important connections are made stronger and ones that aren't as important are removed.

"All the important things we did and learnt during the day are stored and organised, and things we don't need are washed away ready for new experiences next day," explains Dr Halsey.

A bit like someone might prune bushes in the garden to get rid of dead leaves to help the rest of the healthy bush grow, the brain almost prunes its connections while we sleep.

This helps all sorts of things like memory and creativity.

2. Get rid of rubbish

Sleep is also a good chance for our brain to get rid of rubbish that it doesn't want.

Dr Halsey says: "Although we may not know it, sleep gives our brains the opportunity to take a break and tidy up."

A study by researchers at the University of Rochester Medical Centre in New York showed the brain may have its own cleaning system that washes away waste while we sleep - almost like an overnight freshen up.

Why do we need sleep?

While scientists continue to look at exactly what happens when our bodies sleep, studies have also been done that focus on what happens when we don't get enough sleep, which can help to explain why we need it.

"It's not easy to discover what goes on during sleep. We often find out from testing people who have not slept to see what happens. We know from these tests that sleep is important to our health as well as learning and coordination," Dr Halsey explains.

1. Help our genes behave

When we sleep well, we're actually helping our genes, which are really important as they build proteins in our bodies and help different cells to function.

Research has shown that a lack of sleep can change the way genes in our bodies behave.

Scientists from Surrey University found genes involved in inflammation increase their activity when we don't get enough sleep. They believe it is because the body is under stress, so we need to get enough kip to make sure the genes in our bodies behave normally.

2. Helps us to think

Having a good night's sleep usually means we're more alert and ready to learn. Sleep also helps us to react quickly and be well co-ordinated.

When you haven't had enough sleep, you can feel a bit dozy and slower, and it can be hard to think completely straight.

Research has also shown there could be a scientific reason for that too, as parts of the brain could actually be "asleep" (or inactive) while you're awake, if you haven't had enough rest.

Not only that, but getting enough sleep helps our mood too. If you don't get enough, you might find yourself being a bit grumpy or irritable.

3. Maintains our day-to-day health

Aside from helping to reduce the risk of more serious medical conditions, the right amount of sleep helps us to fight off more common illnesses.

"If you seem to catch every cold and flu that's going around, your bedtime could be to blame. Prolonged lack of sleep can disrupt your immune system, so you're less able to fend off bugs," the NHS says.

4. Helps our body's natural clock

Our bodies have a natural rhythm, or internal clock, that helps it to carry out important functions and processes.

However, not having enough sleep can throw this off.

Doing this can affect things like our metabolism, which describes the chemical processes in the body that help turn the food we eat into energy, so it's important to let our bodies rest as much as they need to.

Scientists are still trying to work out all the answers to exactly why we completely switch off our bodies for one-third of our lives, and what happens when we do.

"Why is sleep such as mystery? Sleep is a state of mind and body that's not completely in our control. We have to 'let' ourselves fall asleep and can't force sleep to happen, and this makes it extra puzzling," Dr Halsey says.

"While you're sleeping, plenty is going on to help you wake up ready and keen for a new day. Right now, scientists are still learning about how sleep does us so much good. It's exciting to think there are mysteries about sleep still to solve," says Dr Halsey.

One thing we know for sure is that it is definitely a good idea to catch the right amount of zzzs!

With thanks to clinical psychologist and broadcaster Dr Claire Halsey