The glymphatic system is the brain's cleaning network: it flushes waste away through the cerebrospinal fluid, including amyloid-beta, the protein linked to Alzheimer's. That happens almost exclusively during deep non-REM sleep, when brain cells shrink and flow increases sharply.
The glymphatic system — a contraction of glial and lymphatic — was described in 2012 and has since been a cornerstone of sleep science. Where the body has a lymphatic system to carry waste away, the brain uses this specialised network: cerebrospinal fluid flows in along the blood vessels, rinses through the tissue and carries accumulated waste away.
It explains why sleep isn't an off switch, but an active maintenance process.
During deep sleep the brain cells shrink, which enlarges the space between them. Through that space the cerebrospinal fluid can flow more freely and take more waste with it — like a dishwasher that only runs when the machine is empty enough.
During the day the brain is busy processing and performing; the cell shrinkage and the flow that goes with it hardly occur then. Only in deep non-REM sleep does the system switch to full power.
Forcing it isn't possible. Creating the conditions is: enough hours, an early and fixed bedtime so you don't miss the deep sleep in the first half of the night, a cool and dark room, and no alcohol in the hours before.
It is no accident that this list is exactly what a sleep spa does. The sauna, the cool room and the early dinner aren't there for relaxation alone.
Only in part. After a short night, deep sleep usually increases the following night, but that doesn't compensate one for one. Structurally sleeping too little is therefore something different from one bad night.
Alcohol gets you to sleep faster but suppresses both deep sleep and REM, especially in the second half of the night. Precisely the stage in which this cleaning process does its work gets less room.