Rethinking the brain's cleanup crew: Sleep does not detox brain as previously thought. Here's what does

A good night's sleep is commonly known as having a restorative effect, with the prevalent scientific explanation being that the brain washes off toxins during sleeping.

Though it has grown to be a widely accepted hypothesis in neuroscience, a new study raises the possibility that this could be wrong. The study discovered that fluid clearance and mobility in mice's brains were significantly decreased during sleep and anaesthesia.

“If you are sleep-deprived, countless things go wrong – you don’t remember things clearly, hand-eye coordination is poor. The idea that your brain is doing this basic housekeeping during sleep just seems to make sense,” Prof Nick Franks, a professor of biophysics and anaesthesia at Imperial College London and the study's co-lead said. However, they found only indirect evidence that the brain's waste-removal mechanism increases activity during sleep.

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The newest study, published in the journal Nature Neuroscience, employed a fluorescent dye to investigate mice's brains. This made it possible for scientists to assess the dye's rate of clearance from the brain directly and to see how rapidly the dye migrated from fluid-filled cavities, called the ventricles, to other parts of the brain.

The study found that dye clearance was lowered by roughly 30% in sleeping mice and 50% in animals under anaesthesia when compared to the mice who were awake.

“The field has been so focused on the clearance idea as one of the key reasons why we sleep, and we were of course very surprised to observe the opposite in our results. We found that the rate of clearance of dye from the brain was significantly reduced in animals that were asleep, or under anaesthetic,” said Franks.

The researchers expect that the findings will apply to humans because sleep is a basic requirement shared by all mammals. Prof Bill Wisden, interim head of the UK Dementia Research Institute at Imperial College London and co-lead author, stated, "There are many theories as to why we sleep, and although we have shown that clearing toxins may not be a key reason, it cannot be disputed that sleep is important."

The findings are relevant to dementia research since there is growing evidence of a relationship between poor sleep and Alzheimer's risk. It's unclear if sleep deprivation is an early sign of Alzheimer's or if it causes the disease. There was a theory that the brain couldn't remove poisons efficiently if it didn't get enough sleep, but the most recent findings cast doubt on this theory.

“Disrupted sleep is a common symptom experienced by people living with dementia. However, we still do not know if this is a consequence or a driving factor in the disease progression. It may well be that having good sleep does help to reduce dementia risk for reasons other than clearing toxins,” Wisden said.

The study also showedthat brain clearance is particularly efficient during the awake state. In general, being awake, active, and exercising may aid in the removal of toxins from the brain.