During sleep, the cerebrospinal fluid that cushions the brain helps to remove waste that has built up during the day. In a 2019 study, Lewis and colleagues showed that CSF flow during sleep follows a rhythmic pattern in and out of the brain, and that these flows are linked to changes in brain waves during sleep.

That finding led Lewis to wonder what might happen to CSF flow after sleep deprivation. To explore that question, she and her colleagues recruited 26 volunteers who were tested twice — once following a night of sleep deprivation in the lab, and once when they were well-rested.

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Sleep-deprived participants performed much worse than well-rested participants on these tasks, as expected. Their response times were slower, and for some of the stimuli, the participants never registered the change at all.

During these momentary lapses of attention, the researchers identified several physiological changes that occurred at the same time. Most significantly, they found a flux of CSF out of the brain just as those lapses occurred. After each lapse, CSF flowed back into the brain.

“The results are suggesting that at the moment that attention fails, this fluid is actually being expelled outward away from the brain. And when attention recovers, it’s drawn back in,” Lewis says.