The restorative effect of a good night’s sleep is widely recognized and the popular scientific explanation was that the brain flushes out toxins during sleep.
However, new findings suggest that this theory, which has become a dominant view in neuroscience, may be wrong. The study found that the clearance and movement of fluid in the brains of mice was actually significantly reduced during sleep and anesthesia.
“It sounded like a Nobel Prize-winning idea,” said Prof Nick Franks, a professor of biophysics and anesthesiology at Imperial College London, and co-leader of the study.
“When you’re sleep deprived, lots of things go wrong – you don’t remember things clearly, hand-eye coordination is poor,” he added. “The idea that your brain does this basic housekeeping during sleep just seems to make sense.”
However, there was only indirect evidence that the brain’s waste disposal system increases 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 allowed them to measure the rate of clearance of the dye directly from the brain.
The study showed that the clearance of the dye was reduced by about 30% in sleeping mice, and 50% in anesthetized mice, compared to mice that were kept awake.
“The field is so focused on the clearance idea as one of the main reasons we sleep, and of course we were very surprised to observe the opposite in our results,” Franks said. “We found that the rate of clearance of dye from the brain was significantly reduced in animals that were asleep, or under anesthesia.”
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 detoxification may not be a key reason, it cannot be disputed that sleep is important is not.”
The findings have relevance for dementia research due to the increasing evidence of a link between poor sleep and Alzheimer’s risk. It is not clear whether lack of sleep can cause Alzheimer’s disease, or whether it is simply an early symptom. Some have hypothesized that without enough sleep, the brain may not be able to clear toxins efficiently, but the latest research casts doubt on the plausibility of this explanation.
“Because that idea has had such an influence, it’s probably increased people’s anxiety that they’re more likely to develop dementia if they don’t sleep,” Franks said.
Wisden said: “Disrupted sleep is a common symptom experienced by people with dementia. However, we still do not know whether this is a consequence or a driving force in the disease progression. It may well be that good sleep does help reduce dementia risk for reasons other than clearing toxins.”
He added: “The other side of our study is that we have shown that brain clearance is highly effective during the waking state. In general, being awake, active and exercising can clear the brain of toxins more effectively.”
