Sleep does NOT clear brain of toxins - but exercise does, finds study

It's always such a relief to climb into bed at the end of a long day.

But the exact reason why humans need sleep is still a mystery, according to new research.

For a long time it has been thought the brain uses sleep to flush out toxins and get rid of damaging molecules.

However a new study now suggests the opposite may be true – and experts say being active might be more beneficial for 'cleaning' the brain.

A team from Imperial College London's UK Dementia Research Institute used a fluorescent dye to study the brains of mice, tracking how quickly the dye moved from one area of the brain to another before being eventually cleared.

They discovered the clearance and movement of fluid was markedly reduced during sleep and under anaesthetic.

Analysis revealed the clearance of the dye was reduced by about 30 per cent in sleeping mice and 50 per cent in mice that were under anaesthetic compared with mice that were kept awake.

Although the findings still need to be confirmed in humans, it casts doubt on the long-held belief that sleep clears toxins via the glymphatic system, a mechanism that flushes waste from the brain.

Previous research has suggested that sleep is important for preventing dementia as it is during this time that toxic proteins are cleared from the brain.

Study co-lead author Nick Franks, professor of biophysics and anaesthetics at Imperial College London, said: '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.'

The researchers also said the size of molecules may affect how quickly certain toxins move through the brain, and some compounds are cleared through different systems.

'As yet, we do not know what it is about these states that slows down the removal of molecules from the brain,' Professor Franks added.

'The next step in our research will be to try to understand why this occurs.'

Study co-leader Prof Bill Wisden, interim centre director of the UKDRI, 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.

'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.'

The findings were published in the journal Nature Neuroscience.