Starting at about age 11, a child's internal clock gets a bit screwed up, to put it mildly. Most children go from being the family's early risers to needing a marching band through their bedrooms to get them out of bed.
This isn't (just) stubbornness : this is biology.
Most parents and doctors used to think that the later bedtimes and comfortable waking times were determined mainly by social factors, instead of biological ones, but recent research has led to the possibility that the trend towards night-owlishness and late rising is a result of changes in the young person's circadian clock. Indeed, teens tend to fall asleep later, need more sleep than when they were children, and it can be said that they have truly reached maturity when they stop sleeping in every chance they get. This seems to be related to the fact that the brain's circadian clock, which is mostly controlled by melatonin, switches time zones and keeps the child feeling awake until later in the evening...and sleepy later in the morning.
This isn't surprising when we consider the things going on in the young person's body. During the teen years, the brain overproduces gray matter, the thinking, sensory-processing part of the brain. The extra neurons are trimmed off as needed. Thus the brain is essentially rewired during the teens. The body, besides growing out of clothes every three weeks, is having a constantly changing hormone cocktail to deal with. And, because puberty is happening earlier and earlier, this is all happening to less and less mature people.
The kids need all the help they can get.
How this plays out in our home schools varies as much as the families themselves. In our family, I've tried to use our middle son's tendencies towards sleep deprivation experiments for our benefit. Ken goes to bed late on school days, and he gets up later than anyone else. This gives me time to spend with his two younger siblings, which means that the younger two get more attention, and Ken gets his own special time also, though he has some science activities with his younger brother. It's not standard, but it works, and Ken is less of a zombie.