Because your body's internal clock is just as good, if not better, than the contraption shrieking atop your nightstand.
At the center of your brain, a clump of nerves -called the suprachiasmatic nucleus- oversees your body's clock: the circadian rhythm. It determines when you feel sleepy and when you feel bright-eyed. It controls your blood pressure, your body temperature, and your sense of time. It turns your body into a finely tuned machine.
That machine happens to love predictability. Your body is most efficient when there's a routine to follow. So if you hit the hay the same time each night and awake the same time each morning, your body locks that behavior in. And that's where things get sciency.