This may depend on their chronotype. When we were cave people some of us evolved to stay up later and guard the camp from predators, some of us got up early to

This may depend on their chronotype. When we were cave people some of us evolved to stay up later and guard the camp from predators, some of us got up early to hunt. You need to try and support that where possible, but not to overindulge it. If they are staying up until 2:00am to play computer games it may just be because their brain is more sensitive to light and it's actually that that's keeping them awake.