This is where sleep comes in – it puts the brain into an “offline” state during which the synaptic strength accumulated during wakefulness can be surveyed through spontaneous electrical activity. And it ensures the process is uninterrupted by the external environment.

The process returns the system back to a more sustainable, baseline level while, at the same time, keeping a trace of important learning and memory information.

The hypothesis says slow-wave sleep plays a major role in achieving synaptic homeostasis or “renormalisation” by creating an environment that facilitates returning synaptic strength to a normal level.

via Feeling sleepy? Maybe your brain's too full.