Sleep is a normal, indeed essential part of our lives. But if you think about it, it is such an odd thing to do.

At the end of each day we become unconscious and paralysed. Sleep made our ancestors vulnerable to attack from wild animals. So the potential risks of this process, which is universal among mammals and many other groups, must offer some sort of evolutionary advantage.

Research in this area was slow to take off. But recently there has been a series of intriguing results that are giving researchers a new insight into why we sleep and what happens when we do it.

Why do I sleep?

Scientists simply don't know for sure. In broad terms researchers believe it is to enable our bodies and especially our brains to recover. Recently researchers have been able to find out some of the detailed processes involved.

During the day brain cells build connections with other parts of the brain as a result of new experiences. During sleep it seems that important connections are strengthened and unimportant ones are pruned. Experiments with sleep-deprived rats have shown that this process of strengthening and pruning happens mostly while they sleep.

And sleep is also an opportunity for the brain to be cleared of waste.

A group led by Prof Maiken Nedergaard at the University of Rochester Medical Centre in New York discovered a network of microscopic fluid-filled channels in rats that clears waste chemicals from the brain. Prof Nedergaard told BBC News whenher research was first published in 2013 that this process occurs mostly when the brain is shut off.

"You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can't really do both at the same time."