Nearly everyone knows the feeling. After a night of poor sleep, it becomes harder to stay focused. Thoughts drift, reaction times slow, and mental clarity fades when attention is needed most.

New research from MIT sheds light on what is happening inside the brain during these brief lapses in focus. The study shows that when attention momentarily fails, cerebrospinal fluid (CSF) moves out of the brain, a process that normally happens during sleep and helps clear away waste that accumulates throughout the day. This cleansing activity is considered important for keeping the brain healthy and functioning properly.

When someone is sleep-deprived, the body appears to try to make up for lost rest by triggering bursts of this fluid movement during wakefulness. That compensation, however, comes with a significant downside: sharply reduced attention.

"If you don’t sleep, the CSF waves start to intrude into wakefulness where normally you wouldn’t see them. However, they come with an attentional tradeoff, where attention fails during the moments that you have this wave of fluid flow," says Laura Lewis, the Athinoula A. Martinos Associate Professor of Electrical Engineering and Computer Science, a member of MIT’s Institute for Medical Engineering and Science and the Research Laboratory of Electronics, and an associate member of the Picower Institute for Learning and Memory.

Lewis is the senior author of the study, published in Nature Neuroscience. MIT postdoctoral associate Zinong Yang is the lead author.

How Sleep Cleans the Brain

Sleep is essential for survival, yet scientists still do not fully understand why it plays such a critical role. What is clear is that sleep is necessary for staying alert, and that losing sleep reliably harms attention and other mental abilities.

One important function of sleep involves cerebrospinal fluid, which surrounds and cushions the brain. During sleep, CSF helps flush out waste that builds up during waking hours. In a 2019 study, Lewis and her colleagues found that this fluid moves in a rhythmic pattern as people sleep, closely linked to shifts in brain wave activity.

That earlier discovery raised a new question: what happens to this fluid system when sleep is disrupted? To find out, the researchers recruited 26 volunteers who completed testing twice, once after a night of sleep deprivation in the lab and once after being well-rested.

The following morning, participants completed a standard task used to assess the effects of sleep loss while researchers tracked a wide range of brain and body signals.

Measuring Attention and Brain Fluid Flow

During the experiment, each participant wore an electroencephalogram (EEG) cap to monitor brain activity while lying inside a functional magnetic resonance imaging (fMRI) scanner. The team used a specialized version of fMRI that could track both blood oxygen levels and the movement of CSF in and out of the brain. Heart rate, breathing rate, and pupil size were also recorded.

Participants completed two attention tests inside the scanner, one visual and one auditory. In the visual task, they watched a fixed cross on a screen that occasionally changed into a square. They were instructed to press a button whenever the change occurred. In the auditory task, the visual cue was replaced by a sound.

As expected, sleep-deprived participants performed significantly worse than when they were well-rested. Their reactions were slower, and in some cases they failed to notice the signal entirely.

When these brief attention failures occurred, the researchers observed multiple physiological changes happening at the same time. Most notably, CSF moved outward from the brain during the lapse and then flowed back in once attention returned.

"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.

The team believes this pattern reflects the brain’s attempt to compensate for missed sleep by activating a cleaning process that normally occurs at night, even though doing so temporarily disrupts attention.

"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," Yang says. "Your brain’s fluid system is trying to restore function by pushing the brain to iterate between high-attention and high-flow states."

A Brain and Body System Working Together

The study also revealed that attention lapses are linked to changes beyond the brain itself. During these moments, breathing and heart rate slowed, and pupils became smaller. Pupil constriction began roughly 12 seconds before CSF moved out of the brain and reversed after attention returned.

"What’s interesting is it seems like this isn’t just a phenomenon in the brain, it’s also a body-wide event. 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 says.

These findings suggest that a single control system may coordinate both attention and basic bodily functions such as fluid flow, heart rate, and alertness.

"These results suggest to us that there’s a unified circuit that’s governing both what we think of as very high-level functions of the brain – our attention, our ability to perceive and respond to the world – and then also really basic fundamental physiological processes like fluid dynamics of the brain, brain-wide blood flow, and blood vessel constriction," Lewis says.

While the researchers did not identify the specific circuit involved, they point to the noradrenergic system as a strong candidate. This system, which uses the neurotransmitter norepinephrine to regulate cognition and bodily functions, is known to fluctuate during normal sleep.

The research was supported by the National Institutes of Health, a National Defense Science and Engineering Graduate Research Fellowship, a NAWA Fellowship, a McKnight Scholar Award, a Sloan Fellowship, a Pew Biomedical Scholar Award, a One Mind Rising Star Award, and the Simons Collaboration on Plasticity in the Aging Brain.