Sleep and the brain’s night shift

The mouse experiments

Xie and colleagues reported in Science in 2013 that the interstitial space in the mouse cortex expanded by about 60 percent during natural sleep or anesthesia compared with wakefulness. That extra room reduced resistance to fluid movement, and tracer studies showed CSF influx and amyloid beta clearance both increased substantially during sleep. The effect appeared to be driven by a drop in noradrenaline signalling, the brain’s own alertness chemical, rather than by the absence of light or activity.

The human measurements

In 2019, Fultz and colleagues recorded healthy young adults falling asleep inside an MRI scanner while also recording EEG. During non-REM sleep they observed a repeating sequence: a slow electrical wave, followed by a dip in blood oxygenation and volume, followed by a surge of CSF into the fourth ventricle. The CSF waves were large and were not present in the same form during wakefulness.

This is one of the clearest demonstrations that the sleeping human brain has a macroscopic pumping rhythm, and that the rhythm is coupled to the brain’s blood vessels. It does not by itself prove that waste is removed faster in humans during sleep, but it provides the mechanism by which that could happen.

What this means in practice

Sleep is, for now, the only well-established way to engage this system at full strength, which is why every reputable brain-health organization lists sleep alongside blood pressure control, hearing, exercise and social connection. Research groups around the world are now asking whether the brain’s maintenance processes can be supported deliberately, and what the risks and benefits of doing so would be. Those are open scientific questions, and answering them responsibly means careful measurement, independent verification and patience.

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