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. The study showed that the clearance of the dye was reduced by about 30% in sleeping mice, and 50% in mice that were under anaesthetic, compared with mice that were kept 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,” said Franks. “We found that the rate of clearance of dye from the brain was significantly reduced in animals that were asleep, or under anaesthetic.” The researchers predict that the findings will extend to humans as sleep is a core need shared by all mammals. Prof Bill Wisden, the interim director of the UK Dementia Research Institute at Imperial College London and co-lead author, said: “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 have relevance for dementia research due to the increasing evidence of a link between poor sleep and Alzheimer’s risk. It has not been clear whether lack of sleep might cause Alzheimer’s, or whether it is simply an early symptom. Some had hypothesised that without enough sleep, the brain may not be able to clear toxins effectively, but the latest research raises doubts about the plausibility of this explanation. “Because that idea has held such sway, it’s probably increased people’s anxiety that if they don’t sleep they’ll be more likely to develop dementia,” said Franks. Wisden said: “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.” He added: “The other side to our study is that we have shown that brain clearance is highly efficient during the waking state. In general, being awake, active and exercising may more efficiently clean the brain of toxins.”