A new study published in the prestigious journal, Science, has found that the brain may wash away toxins built up over the day during sleep.
The research discovered "hidden caves" inside the brain, which open up during sleep, allowing cerebrospinal fluid (CSF) to flush out potential neurotoxins, like β-amyloid, which has been associated with Alzheimer's disease.
To reach their discovery, researchers injected mice's brains with a dye and monitored the flow while they were awake, asleep and anaesthetised (Xie et al., 2013).
One of the study's authors, Dr Maiken Nedergaard, explained the results: "We were surprised by how little flow there was into the brain when the mice were awake. It suggested that the space between brain cells changed greatly between conscious and unconscious states." For a long time the real physiological purpose of sleep has remained a mystery.
We know that lack of sleep causes all kinds of psychological problems like poor learning, decision-making and so on.
We also know that animals that are chronically deprived of sleep will eventually die: flies or rodents in days to weeks, humans within months or years.
Everyone who has ever enjoyed a blissfully good night's sleep knows just how restorative it can be, but the actual physiological process wasn't clear.
This study, though, suggests that the flushing out of toxins by the CSF may be central to sleep's wondrous powers.
The interstitial spaces in the mouse's brain took up only 14% of the brain's volume while it was awake. Yet, while it slept, this increased by almost two-thirds to take up fully 23% of the brain's total volume.
The difference might seem slight, but the actual physiological effects are profound.
During the day, the CSF mostly covers the surface of the brain. During sleep, though, the CSF is able to move deep inside.
The effect is that potential neurotoxins, like β-amyloid, are cleared twice as fast during sleep as during waking.
The results of this study - if they hold in humans - may help to explain why many neurological diseases, like strokes and dementia, are associated with problems sleeping.
It could be that lack of sleep, and restriction of the brain's cleaning system, may cause toxic metabolites to building up, leading to long-term damage ( via spring.org.uk ).