Sleep

Ella - I wish I was a grown-up.
Me - Why?
Ella - Because then I can stay up later.
Me - You can stay up late sometimes.
Ella - But I want to stay up late all the time!
Me - Grown-ups need less sleep than kids, that's why they can stay up later.
Ella - Are grown-ups tireder?

I managed to stop myself from saying that most of them probably are - and I definitely am - and instead explained that kids need more sleep because they are still growing and growing takes lots of energy.