Why is it when kids get to a certain age they want to stay in bed all day and stay up all night!

when my oldest hit seventeen he just wanted to stop in bed all day and stay up late at night when he wasn't at work. My second oldest is eighteen and all he wants to do is stop up all night and stay in bed all day.

My 15 year old would love to stay up all night but he gets too tired at the weekend.
If i manage to have a lie in past nine am which is very very very rare i wake up with a headache.
Even when i was a teenager i couldn't stay in bed longer than 10am as i would have the dreaded headache.