From Hamlet to Raymond Chandler, sleep has attracted the attention of artists the world over:

… die, to sleep
No more; and by a sleep, to say we end
The Heart-ache, and the thousand Natural shocks
That Flesh is heir to? ‘Tis a consummation
Devoutly to be wished. To die, to sleep,
To sleep, perchance to Dream; Aye, there’s the rub,
For in that sleep of death, what dreams may come,
When we have shuffled off this mortal coil,
Must give us pause.

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And it continues to do so:

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But for scientists it has never been easy to study, let alone to understand well-enough to propose clear and universal theories. But recent work published this week (Science 342, 373-377 October 18) by L Xie et al is offering some new ideas.

They come from studies of Alhzeimer’s disease, thought to be caused by the growth of plaques of beta-amyloid which block the action of enzymes and cause cell death.

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The new work shows that beta-amyloids are washed out through the circulation of cerebral-spinal fluid. In sleeping mice it goes ten times faster because cells contract to allow the fluid to slosh around more freely, like in a dishwasher,” M Nedergaard said in New York to The Times: “it washes everything outside the cells away. All fluid flows are dependent on how much resistance there is. In the brain it has to be about space. We measured the distance between cells – and found that as soon as we wake up, the cells swell.” It is why we die if we keep awake – we are accumulating waste product.”

In Oxford, R Foster is not so sure: “Memory consolidation, or clearance of beta amyloids, is what is going on in sleep. But is this the evolutionary purpose of sleep? No. We see sleep in organisms without any brain.”

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