• Posted January 21, 2026

Why Your Brain Drifts After a Bad Night’s Sleep, Scientists Explain

Ever notice how hard it is to stay sharp after a rough night of sleep? 

A recent study published in the journal Nature Neuroscience points to a surprising reason why: The brain may briefly shift into a sleep-like cleaning mode, even while you’re awake.

Researchers at the Massachusetts Institute of Technology (MIT) say short lapses in attention after poor sleep owe to sudden movements of fluid in the brain, a process usually reserved for deep sleep.

That fluid, called cerebrospinal fluid (CSF), helps wash away waste that builds up during the day. 

During sleep, this cleaning system works without interfering with thinking. But when people are short on sleep, the study found, the brain may try to activate that system during waking hours, and attention suffers as a result.

"If you don't sleep, the CSF waves start to intrude into wakefulness where normally you wouldn't see them," senior study author Laura Lewis, an associate professor at MIT, said in a news release. "However, they come with an attentional tradeoff, where attention fails during the moments that you have this wave of fluid flow." 

Researchers studied 26 volunteers, testing each person twice: once after a night of sleep deprivation and once after a full night’s rest.

The next morning, participants completed attention tests while lying inside a functional magnetic resonance imaging (fMRI) scanner. Researchers tracked the movement of CSF in and out of the brain, as well as heart rate, breathing and pupil size.

During the attention tests, participants had to quickly respond to visual or sound signals. When sleep-deprived, they reacted more slowly and sometimes missed the signals altogether.

Each time attention failed, the same pattern appeared:

• Cerebrospinal fluid flowed out of the brain.

• Heart rate and breathing slowed.

• Pupils became smaller.

Once attention returned, the fluid flowed back in.

"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 explained.

Lead author Zinong Yang, an MIT postdoctoral associate, said the brain may be trying to recover from lost sleep by briefly switching into a sleep-like state.

"One way to think about those events is because your brain is so in need of sleep, it tries its best to enter into a sleep-like state to restore some cognitive functions," she said.

Researchers also found that these attention lapses are tied to areas outside the brain.

"What's interesting is it seems like this isn't just a phenomenon in the brain, it's also a body-wide event," Lewis added. "It suggests that there's a tight coordination of these systems, where when your attention fails, you might feel it perceptually and psychologically, but it's also reflecting an event that's happening throughout the brain and body," Lewis added.

Researchers did not identify the specific circuit involved, but they suspect a single control system is involved. That’s the noradrenergic system, which uses the chemical norepinephrine and is known to change activity during sleep.

More attention

Johns Hopkins Medicine has more on the effects of poor sleep.

SOURCE: Massachusetts Institute of Technology, news release, Jan. 20, 2026