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How do we sleep?
 The brain is  not resting. In fact, when we sleep, networks in the brain undergo very organised patterns of activity — and use almost as much energy as when we are awake.

We also know that during sleep, the brain orchestrates the manufacture of many different types of molecules — proteins, steroids, cholesterol, lipid rafts, human growth hormone, and many more. On the other hand, if you don't get enough sleep (as happens to many shift workers), you suffer increased risk of heart disease and some cancers.
Some sleep scientists claim the ultimate purpose of sleep is to prune or weaken brain connections that you have made while awake. This means that the next day, your brain won't be overloaded with irrelevant memories.

Other sleep scientists point to research that suggests that sleep replays and consolidates significant memories.

Some recent research points out that during sleep, some nerves send their electrical impulses in the opposite direction they normally do when you are awake. We're not too sure why they do this.

And other current research says that sleep gives the brain the opportunity to flush out the various metabolic waste products created during the day.

If you look at the brain, there's a lot of obvious anatomical stuff — the nerve cells, the glial cells that support the nerve cells, the arteries and veins, and the cerebrospinal fluid (or CSF) that keeps the brain afloat.

What's unappreciated is the space in between these structures — the gap, the void, the so-called interstitial space, that is filled with the wondrous liquid called CSF. This interstitial space makes up about 20 per cent of the volume inside your skull.
During sleep, the brain cells shrink, and so the interstitial space increases by about 60 per cent in volume. This means that in the case of the brain, when you sleep, the flow rate of the CSF increases by an astonishing 20 times. Suddenly, the waste products of brain metabolism that have built up during the day are whisked away by the raging torrent of cerebrospinal fluid when we are asleep.


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