徹夜明けに「ボーッ」とする瞬間、脳では睡眠中の“洗浄”に似た動きが起きていた

Have you ever "spaced out" after staying up all night? In 2025, researchers at MIT kept 26 people awake for one night. Then they checked their brains while they did an easy test.

When people lost attention for a moment, a liquid in the brain flowed out. When their attention came back, the liquid flowed in again. This usually happens during sleep and may help clean the brain. So the tired brain may try to clean itself while awake.

After a sleepless night, many people experience brief moments when their mind goes blank. Researchers at MIT wanted to know what happens in the brain at these moments. They asked 26 volunteers to take an attention test twice, once after staying awake all night and once after a good night's sleep. During the test, the researchers recorded brain waves and used fast MRI scans.

When sleep-deprived participants failed to respond, cerebrospinal fluid flowed out of the brain, and it flowed back in when their attention returned. Their pupils also became smaller about 12 seconds before the fluid flowed out. Because such fluid waves usually appear during sleep, the researchers think the tired brain may be trying to clean itself while awake.

Most people know that sleep deprivation impairs concentration, but what happens in the brain during the moments when attention fails? A 2025 study in Nature Neuroscience by researchers at MIT examined this question in 26 volunteers who completed attention tasks once after a night without sleep and once after being well rested. Using simultaneous EEG and fast functional MRI, along with measurements of pupil size, heart rate, and breathing, the team found that attentional lapses in sleep-deprived participants were accompanied by a coordinated sequence of changes. Shortly after attention failed, cerebrospinal fluid flowed out of the brain, and it flowed back in as attention recovered.

Pupils began to constrict about 12 seconds before the outflow, and blood flow, heart rate, and breathing changed at the same time. Since large fluid waves are normally a feature of sleep and are thought to help clear waste, the researchers suggest that the sleep-deprived brain may try to perform this cleaning while awake, at the cost of attention. The study was small, it shows timing rather than cause, and fluid flow was estimated indirectly. Based on the tight timing, the team proposes that a single control system, possibly involving norepinephrine, coordinates these changes.