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Finding that clearance of fluid in mice brains is lower in sleep and anaesthesia runs counter to dominant view in neuroscience

The restorative effect of a good night’s rest is widely recognised and the popular scientific explanation has been that the brain washes out toxins during sleep.

However, new findings suggest this theory, which has become a dominant view in neuroscience, could be wrong. The study found that the clearance and movement of fluid in the brains of mice was, in fact, markedly reduced during sleep and anaesthesia.

“It sounded like a Nobel prize-winning idea,” said Prof Nick Franks, a professor of biophysics and anaesthetics at Imperial College London, and co-lead of the study.

“If you are sleep-deprived, countless things go wrong – you don’t remember things clearly, hand-eye coordination is poor,” he added. “The idea that your brain is doing this basic housekeeping during sleep just seems to make sense.”

However, there was only indirect evidence that the brain’s waste-removal system ramps up activity during sleep, Franks said.

In the latest study, published in the journal Nature Neuroscience, researchers used a fluorescent dye to study the brains of mice. This allowed them to see how quickly the dye moved from fluid-filled cavities, called the ventricles, to other brain regions and enabled them to measure the rate of clearance of the dye from the brain directly.