The brain has no conventional lymphatic vessels, so it relies on a different route for clearing the byproducts of neural activity. Cerebrospinal fluid (CSF) is driven through the brain parenchyma along the walls of penetrating arteries and exits along perivascular channels, carrying dissolved solutes with it. This network is called the glymphatic system, a name that combines glia and lymphatic because the flow depends on astrocytic endfeet that line the perivascular space.
The rate of this flow is not constant. It depends strongly on the size of the interstitial space, the narrow gap between brain cells. During wakefulness, noradrenergic signaling keeps that space relatively tight. During slow-wave sleep, noradrenergic tone falls, the interstitial space expands by roughly 60 percent, and the resistance to fluid movement drops. The result is that glymphatic clearance is substantially faster in slow-wave sleep than in wakefulness, even when the comparison is made at the same level of brain activity.
What gets cleared matters. Metabolic byproducts that accumulate during waking neural activity, including amyloid-beta and other proteins associated with neurodegenerative disease, are removed more efficiently during sleep. This is why losing slow-wave sleep specifically, rather than just losing total sleep time, is the more informative way to think about impaired clearance. A night that preserves total duration but fragments deep sleep still leaves waste removal incomplete.