We are talking all about sleep for World Sleep Day on 15 March.
Why do we do it? And how can we help ourselves to get the best night's sleep possible?
Clinical psychologist Dr Claire Halsey told Newsround: "Being asleep means that your body - and mind- get very relaxed. So relaxed that you seem totally at rest, but that's not the whole story.
Read on to find out more.
Scientists have disputed why we sleep for a long time and it has been difficult to work out exactly why we do it and what happens to our bodies while we're catching some kip.
But it is widely believed that humans need to sleep in order to give our bodies - and, in particular, our brains - a chance to rest and recover.
There are in fact areas of the brains that are more active when we are asleep than when we are awake, so we know it's up to something!
Dr Halsey told us: "When we are asleep, our muscles relax and the body gets a good chance to rest. It's the same for our mind.
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 in the US showed that the brain may have its own cleaning system which washes away waste as we sleep - almost like an overnight freshen up.
Sleep is the golden chain that ties health and our bodies together.
While scientists continue to look at exactly what happens when our bodies sleep, studies have also been done that look at what happens if we don't get enough sleep, which can also help to explain why we need it.
Dr Halsey explains: "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."
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 showed that a lack of sleep can change the way that genes in our bodies behave.
Scientists from Surrey University found that 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, ready to learn and helps us react quickly and be well coordinated.
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 that there could be a scientific reason for that too, as parts of the brain could actually be 'asleep' (or inactive) whle 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 stave off more serious medical conditions, the right amount of sleep helps us to fight off more common illnesses.
The NHS says: "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."
4. Helps our body's natural clock
Our bodies have a natural rhythm and not having enough sleep can throw that off.
Doing this can affect metabolism and chemical processes in the body, so it's important to let our body rest as much as it needs 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 it.
Dr Halsey says: "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.
"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 do know for sure even now is that it is definitely good idea to catch the right amount of zzzs!
With thanks to clinical psychologist and broadcaster Dr Claire Halsey