Not expert on this and thought very little bout this.

But slping is main to regenerate and fix the died tissue isn't it?
I know many ppl wan to slp lesser since we can slp eternally after life,
but the main thing is if we slp lesser, even if we can feel awake or not so slpy, it is still insufficient to provide enough recover to our body, and thus cause many bad effect.

Child would need slp more than 8, i think because they have weaker body system.
Adult 6-7, more abit would be better.
While i think elder people need more instead.