Because the human body has a cycle of rest. Your cycle starts when you are a baby, and you go to bed early and wake up late, that is the ideal cycle. But when you are older you tend to go to bed late and wake up early because of school, and that's why it's so hard to wake up early on a different schedule because your body's use to a different one. But after a while it adjusts to it and it goes on and on as you continue your life.
According to what I've read, the human body works like a machine with set times for rest and activity, so it automatically wakes itself up according to its settings. When you are a baby, your body rests for about 18 hours a day and this amount of time decreases gradually as you grow and develop.