It’s well-known that our focus diminishes when we’re short on sleep. But what causes this? It may stem from your brain momentarily losing attention as it attempts to rejuvenate itself.

While we sleep, our brain undergoes a cleansing process where cerebrospinal fluid (CSF) is cycled in and out of the brain, flushing out daily accumulated metabolic waste. If this does not occur, it could potentially harm brain cells.

Laura Lewis and her team at the Massachusetts Institute of Technology hypothesized that the drop in alertness following sleep deprivation could be the brain’s way of trying to recover while awake.

To test this theory, researchers asked 26 participants aged 19 to 40 to sleep adequately to feel refreshed, and two weeks later, they kept the same group awake all night in the lab.

In both circumstances, the team used MRI scans to record the brain activity of participants the following morning as they performed two tasks. These tasks required them to press a button whenever they heard a specific sound or noticed a cross on the screen transforming into a square, which occurred numerous times over 12 minutes.

As predicted, participants struggled to press the button more frequently when they were sleep-deprived compared to when they were well-rested. This indicates that insufficient sleep hampers concentration.

Crucially, analysis of the brain scans revealed that participants lost focus about two seconds prior to the CSF being expelled from the base of the brain, with CSF being reabsorbed roughly one second after attentiveness returned.

“If you envision the brain-cleansing process as akin to a washing machine, you fill it with water, run it around, and then you need to drain it. The lack of focus represents the ‘swishing’ stage during this cleaning process,” explains Lewis.

The findings imply that if the brain cannot cleanse itself during sleep, it resorts to doing so while awake, which compromises concentration, according to Lewis. “If this wave of fluid doesn’t materialize because you’ve been awake all night, your brain begins to sneak it in during the daytime, at the expense of your focus.”

While the exact cause of how this cleaning process leads to diminished attention remains unknown, pinpointing the brain circuits involved could illuminate ways to mitigate the cognitive repercussions of sleep deprivation, Lewis suggests.