We're all familiar with the feeling of zoning out, especially when we're sleep-deprived. A new study suggests these brief wanderings of attention are our brain's attempts to catch up on maintenance that usually happens while we snooze.

The study, from researchers at the Massachusetts Institute of Technology (MIT), used brain measurements taken by both electroencephalogram (EEG) caps and functional magnetic resonance imaging (fMRI) scanners.

Periods of zoning out – or "attentional failures", in the words of the study – were accompanied by a wave of cerebrospinal fluid (CSF) flowing out of the brain, before it returned a second or two later.

The patterns matched the waves of CSF that usually happen during deep sleep. The thinking is that this nightly fluid flow helps wash away waste products that build up during the day.

"If you don't sleep, the CSF waves start to intrude into wakefulness where normally you wouldn't see them," says MIT neuroscientist Laura Lewis.

"However, they come with an attentional trade-off, where attention fails during the moments that you have this wave of fluid flow."

The study participants were each tested twice: after a night of restful sleep and after a night in the lab with no sleep at all. Unsurprisingly, their cognitive performance during the study tests was generally worse when they hadn't gotten any shut-eye the night before.

While zoning out occasionally happened after a full night's slumber, it was much more common after the participants had stayed awake all night. It's almost as if the brain is trying to catch up with a burst of microsleep, at the temporary cost of your mind's focus.