Hi, it's Matt here. Welcome back to the podcast. In the winter of 2013, a Danish neuroscientist

named Myken-Nadegard was doing something that would have sounded on paper, almost embarrassingly mundane.

She was watching mice sleep. Her lab at the University of Rochester had been working on one

of neuroscience's oldest unanswered questions. The brain generates toxic waste products the way any

engine running at full throttle does, yet had no obvious way to take out the rubbish. Every other organ drains into the

lymphatic system. The brain sits sealed behind the blood-brain barrier, and for decades nobody could

figure out how it cleaned itself. Some researchers had quietly concluded that maybe it didn't, that maybe

the waste just accumulated. What Nedegard found when she looked at sleeping mouse brains really

looked with contrast enhanced imaging and fluorescent traces was something nobody had predicted.

The glial cells, the dense support cells packed tightly

around every neuron in the brain, shrank. During sleep, they physically contracted by roughly

60% of their waking volume. And in doing so, pulled apart from one another, opening up channels between them that had barely existed minutes before.

It was as if the brickwork of a wall had drawn inward, suddenly exposing the gaps between every brick.

Cerebrospinal fluid surged through these newly open channels channels like a cleaning crew let loose after closing

time, flushing out the debris of a full day's neural activity, beta amyloid, the molecular calling

card of Alzheimer's disease, tau protein, the scaffolding that tangles into dementia, cleared, removed, gone by morning.

The brain had built an entire private sanitation system operating exclusively at night

that science had completely missed. Two years later, her team put the mice in different

positions. Back, stomach, side, side sleeping produced

the most efficient transport. Front sleeping was the worst. Fluid accumulated rather than

draining, traces lingered. Waste sat instead of clearing. Back sleeping fell somewhere between the two. The brain's cleaning system

was not indifferent to the angle of the body it lived in. Nadergaard offered a hypothesis.

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