It is commonly acknowledged that getting a good night’s sleep has restorative effects, and the prevalent scientific explanation for this is that when we sleep, the brain eliminates poisons.

New research, however, raises the possibility that this theory—which has come to dominate neuroscience—may not be accurate. The study discovered that during sleep and anesthesia, there was a noticeable decrease in the clearance and transport of fluid in the mice’s brains.

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

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

But according to Franks, there was only circumstantial evidence suggesting the brain’s waste-removal mechanism becomes more active while you sleep.

The most recent study examined mice’s brains using a fluorescent dye, and it was published in the journal Nature Neuroscience. This allowed them to assess the rate of dye clearance from the brain directly and to observe how rapidly the dye migrated from fluid-filled cavities, known as the ventricles, to other regions of the brain.

The results of the study demonstrated that, in comparison to mice kept awake, the clearance of the dye was decreased by roughly 30% in sleeping mice and by 50% in mice under anesthesia.

“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.”

Since sleep is a basic requirement that all mammals share, the researchers anticipate that the findings will also apply to humans.

Co-lead author and interim director of the UK Dementia Research Institute at Imperial College London, Prof. Bill Wisden, 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 growing body of evidence linking sleep disorders to an increased risk of Alzheimer’s disease makes the findings relevant to dementia research. Whether sleep deprivation is an early indication of Alzheimer’s or whether it is the actual cause of the disease has not been determined. The most recent research casts doubt on the viability of the theory that the brain cannot properly remove poisons if it is not getting adequate sleep.

According to Franks, “people’s anxiety that they’ll be more likely to develop dementia if they don’t sleep has probably increased because that idea has held such sway.”

According to Wisden,  “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.” he continued.