You really should do more reading honestly. Sleep is when the wear and tear of activity is repaired and reversed, and is to a large extent regulated by molecules that amount to metabolic waste, which accumulate during consciousness, and are cleared away during sleep.

As far as my deep dives into the science of sleep have suggested, any deficits in sleep, either qualitative or quantitative, result in essentially minuscule quantities of irreversible kinda-neurodegeneration. "kinda" because it's not neuronal death, just the failure to maintain important connections and peak performance properly.

The important connections ... are important. By definition they will not be affected that much.

Without much drama (like the "kinda-neurodegeneration" wording), this can be easily summarized as: optimal amount of sleep = you're at peak performance, otherwise you're working at less than peak mental performance.