Exactly why we sleep is still something of a mystery, but a newly-published study suggests one possibility: to physically clean our brains.
The idea comes following a study of mice that found brain cells shrinking during sleep, allowing around 60 percent more space in between them compared with waking hours. That allowed for a roughly ten-fold increase in the speed at which cerebral spine fluid flows through the brains in what’s now known as the glymphatic system.
A research team led by Maiken Nedergaard of the University of Rochester explored what benefits this might bring. They injected the mice with traces of amyloid beta proteins that have been linked to conditions such as Alzheimer’s disease and found these were “flushed out” faster from the brain when the mice were asleep.
Nedergaard believes that as well as removing toxins, the process may simply be clearing out waste products from the brain’s activity. Her theory is that the energy requirements of this process make it impractical while the mouse is conscious, using the analogy ” You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can’t really do both at the same time.”
The findings may spark further research to see whether the same process is taking place with humans and whether the results are different among people who are starved of sleep.
It could be an answer to the question of what physical reason (as opposed to psychological reason) there is for animals to sleep: as the BBC notes, it must be something important to make it worth the increased risk of being attacked by predators. However, sleep experts interviewed by the Guardian believe that even if the results are the same in humans, it’s likely to only be a part of the puzzle rather than a complete solution.
The research is more likely to have practical benefits in medicine: it’s possible that damage to the way the brain cleans itself of toxins may be a contributing factor to degenerative conditions.