Can’t focus after a bad night’s sleep? Your dirty brain is to blame

We all know it can be hard to concentrate when you are sleep-deprived, but why does this happen? It may be because your brain is trying to refresh itself, causing momentary lapses in attention.

During sleep, the brain carries out a rinse cycle, where cerebrospinal fluid (CSF) is repeatedly flushed into the organ and out again at the base of the brain. This process clears out metabolic waste that has built up during the day – and that would otherwise damage brain cells.

Laura Lewis at the Massachusetts Institute of Technology and her colleagues wondered whether lapses in attention, which commonly occur after sleep deprivation, may result from the brain trying to catch up on rinsing itself when it’s awake.

To explore this idea, the researchers asked 26 people aged between 19 and 40 to get a good night’s sleep that left them feeling well-rested, then kept them awake all night in a lab two weeks later.

In both cases, the team recorded the participants’ brain activity using MRI scans the next morning, while they completed two tasks. During these tests, participants had to push a button whenever they heard a specific tone or saw a cross on a screen turn into a square. This occurred dozens of times over 12 minutes.

As expected, the participants failed to press the button substantially more often when they were sleep-deprived compared with when they were well-rested, meaning a lack of sleep made it harder to focus.

Crucially, when the researchers analysed the brain scans, they found that participants lost focus about 2 seconds before CSF was flushed out of the base of their brain. What’s more, CSF was drawn back into the brain about 1 second after attention recovered.

“If you think about the brain-cleaning process like a washing machine, you kind of need to put the water in and then slosh it around and then drain it out, and so we’re talking about the sloshing part occurring during these lapses of attention,” says Lewis.

The findings suggest that when the brain can’t clean itself during sleep, it does so when you’re awake, but this impairs concentration, says Lewis. “If you don’t have these waves [of fluid flowing] at night because you’re kept awake all night, then your brain starts to kind of sneak them in during the daytime, but they come with this cost of attention.”

Exactly why this cleaning process leads to a loss of attention remains unclear, but pinpointing the brain circuits that are responsible could reveal ways to reduce the cognitive effects of sleep deprivation, says Lewis.