A good night's sleep really does clear the mind as the brain flushes toxic material out of its cells, new research has shown.
The findings suggest a new biological purpose for sleep and indicate that waste disposal may underlie its restorative properties. There could also be far reaching implications for understanding and treating diseases such as Alzheimer's.
"This study shows that the brain has different functional states when asleep and when awake," said US researcher Dr Maiken Nedergaard, from the University of Rochester.
"In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness."
The findings, published in the journal 'Science', show that the brain's unique method of cleansing itself – known as the glymphatic system – is highly active during sleep.
As we slumber, it clears away toxins that would otherwise build up and trigger neurological disorders such as Alzheimer's disease.
The scientists also found that during sleep the brain's cells reduce in size to allow waste to be removed more effectively.
The purpose of sleep has vexed both philosophers and scientists since ancient Greek times. From an evolutionary perspective, sleep is a puzzle. Virtually every animal species needs some form of sleep. Yet being asleep has significant drawbacks, such as leaving an animal at the mercy of predators.
Recent research has shown that sleep can help the brain store and consolidate memories, but these benefits are not thought to outweigh its disadvantages. This has led scientists to suspect that sleep must have a more essential biological function.