The evidence suggests that sleep is the brain running scheduled maintenance, with different subsystems serviced in different stages. What this means is that the sleeping brain is not doing one thing. It runs several distinct processes in sequence, each scheduled into the stage where it is safe to run. REM handles the visual cortex and emotional memory. Slow-wave sleep consolidates declarative memory, replaying the day’s experience from hippocampus to cortex, and it is also when the physical housekeeping happens: the glymphatic system opens up to flush metabolic waste, growth hormone peaks, and immune activity is at its strongest, which is why lost sleep weakens the response to a vaccine. Underneath all of it, synaptic downscaling runs across the night to keep the system from saturating.
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