There are many theories about the purpose of sleep, one of them being the chance for the brain to clean out toxins that have collected during the day.
Well, maybe not. The brain’s ability to rid itself of toxins may actually be reduced during sleep.
Over the past decade, flushing out toxins has been the leading explanation for why we need our shuteye.
But a new study led by scientists at the UK Dementia Research Institute at Imperial College London measured toxin clearance and movement of fluid in the brains of mice, and showed it was markedly reduced during sleep and under anaesthesia.
Using a fluorescent dye they found 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.
According to the researchers, the latest findings are surprising and more work is now needed to understand exactly what’s happening, and why.
Professor Nick Franks, Professor of Biophysics and Anaesthetics at Imperial College London, and co-lead of the study, said: “The field has been so focused on the clearance idea as one of the key reasons why we sleep, that we were 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.
“As yet, we do not know what it is about these states that slows down the removal of molecules from the brain. The next step in our research will be to try to understand why this occurs.”
Professor Bill Wisden, Interim Centre Director of the UK Dementia Research Institute at Imperial, and study co-lead said: “Although we have shown that clearing toxins may not be a key reason why we sleep, it cannot be disputed that sleep is important.
“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.
“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.”
How does sleep reduce the clearance of toxins from the brain?
That’s the next step.