Why do we Sleep 💤💤💤

 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.

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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 when her 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."

What happens when I don't get enough sleep?

It seems that a lack of sleep alters the way in which the genes in the body's cells behave.

Researchers at Surrey University in Guildford have found that genes involved in inflammation seem to increase their activity. Dr Malcolm von Schantz, who is involved with the Surrey research, believes that the genes are responding to lack of sleep as if the body is under stress.

He speculates that in the distant past in times of stress our ancestors' bodies would prepare themselves for injury by activating these inflammation genes which would cushion the effects of attacks by wild animals or human enemies.

"It puts the body on alert for a wound but no wound happens," he told BBC News.

"This could easily help explain the links between sleep deprivation and negative health outcomes such as heart disease and stroke."

In modern times though preparing for an injury that never happens has no beneficial effect - in fact the consequent activation of the immune system might increase the risk of heart disease and stroke.

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Why is it hard to think when I am tired?

The expression "half asleep" might be an accurate description of what is going on in the brain when you are feeling slow-witted.

Research suggests that parts of the human brain may well be asleep when it is sleep-deprived. Studies on whales and dolphins show that when asleep they continue to use half of their brain to swim and come up to the surface for air.

A study on human patients, external showed that something similar goes on in our brains. As they became more sleep-deprived, parts of their brain became inactive while they were still awake.

What's more the local sleep areas move around the brain. So although when we go to bed we think one moment we are awake and then there is an abrupt change to sleep - it may well be more of a continuous process.