To explore what might happen to this CSF flow in a sleep-deprived brain, Lewis, who is also a member of MIT’s Institute for Medical Engineering and Science, and her colleagues tested 26 volunteers on several cognitive tasks after they’d been kept awake in the lab and when they were well-rested. Using both electroencephalograms and functional magnetic resonance imaging, the researchers measured heart rate, breathing rate, pupil diameter, blood oxygenation in the brain, and flow of CSF in and out of the brain as participants tried to press a button when they heard a beep or saw a visual change on a screen.

Unsurprisingly, sleep-deprived participants performed much worse than well-rested ones. Their response times were slower, and in some cases the participants never noticed the stimulus at all.

The researchers identified several physiological changes during these lapses of attention. Most significant was a flow of CSF out of the brain just as a lapse occurred—and back in as it ended. The researchers hypothesize that when the brain is sleep-deprived, it “attempts to catch up on this process by initiating pulses of CSF flow,” as Lewis says, even at the cost of one’s ability to pay 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,” says Zinong Yang, a postdoctoral associate and lead author of a paper on the work. 

The researchers also found several other physiological events linked to attentional lapses, including decreases in breathing and heart rate, along with constriction of the pupils. They found that pupil constriction began about 12 seconds before CSF flowed out of the brain, and pupils dilated again after attention returned.

“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 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.” 

The researchers did not explore what this circuit might be, but one good candidate, they say, is the noradrenergic system, which regulates many cognitive and bodily functions through the neurotransmitter norepinephrine—and has recently been shown to oscillate during normal sleep.