What is it about a teenager that induces an at-least once weekly (if not daily) hibernation? Take a day off from school/ work. An adult will get up probably at the same time (his internal clock won’t let him sleep), or at the most an hour later. However, a teen with a day off, will undoubtedly “sleep in”. (Unless he has a trip planned, in which case he’ll get up before dawn.)
Is this sleep an “escape”? Is the world so much harder for him to face? Is it the lack of having an urgent list of “things that need to be done”? Is it perhaps the lack of knowledge that the adult feels:” A pity to waste time, it is limited”?
I do know, however, that if you want a teen to awaken on time for obligations, like school, the best method is to give the responsibility to them. Give them a LOUD alarm clock, and let them face the consequences of not waking up on time. It’s the only way that consistently works. And it saves you from having to nag.