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Do We Sleep to Wash the Brain?

It is seven in the morning, you have not slept, and somewhere in the back of your mind sits a headline from 2013: sleep cleans the brain. It came from Nedergaard’s lab in Rochester, which found that amyloid, a protein whose build-up is linked to Alzheimer’s, washed out of sleeping mouse brains twice as fast as out of awake ones. If that holds for you, your night cost something. For it to hold, though, two things must be true: a sleeping brain has to clear waste faster than a waking one, and people have to work like mice.

In 2024, Nick Franks and Bill Wisden’s lab in London tested the first condition. It put a fluorescent dye into mouse brain tissue and measured how much had gone hours later. Sleeping mice had cleared less of it than awake ones, somewhere between 20% and 30% less, depending on who does the arithmetic.¹

Yet the two labs are not measuring the same thing. Franks argues that most studies claiming faster clearance in sleep watched fluid flowing into the brain from outside, a stand-in for clearance he calls “fallacious”, since clearance “is simply removal of substances from the brain.” Nedergaard says Franks’s dye only moved around inside the brain: “if you have a garbage can and you move it from your kitchen to your garage, you don’t get clean.”

So the question falls to the second condition, people, and there the evidence leans toward the headline. In Oslo, doctors injected an MRI tracer into the spinal fluid of patients already referred for that kind of scan. Seven were kept awake for a night while seventeen slept, and the next morning the sleepless group’s brains held more tracer. A night of recovery sleep did not close the gap. But seven people is a small sample, and the tracer only stands in for waste.²

Even if your night did slow the clearing, that would show sleep helps the job, not that the job is why we sleep. Nedergaard’s own picture is “turning on the dishwasher before you go to bed and waking up with a clean brain.” Nobody goes to bed just to run the dishwasher.³

The strangest turn came from Nedergaard’s own lab. In a 2025 study of naturally sleeping mice, clearance did not track how long or how deeply they slept. It tracked pulses of noradrenaline, about one every 50 seconds, that made blood vessels squeeze and relax, pumping fluid through the brain. The same pulses set off brief near-wakings, and the more near-wakings, the better the clearing. Franks and Wisden call it “a remarkable reversal.” One reading, mine and not theirs, makes the 2024 result less puzzling: if clearing peaks in the moments closest to waking, a fully awake brain might do it better still, which is what London saw. So on your groggy morning, believe the narrow thing: a lost night may have slowed your brain’s clearing. Nobody has shown that clearing is why you sleep.

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