Replying to Internal Time Clock

Does anyone think they have the ability to fall asleep and wake up at an exact time with no alarm clock whatsoever? I really think i've done it more than once. It's almost an everyday ocurrence when i wake up for school or work. It's been brought up before in pop-culture in shows such as seinfeld where kramer says he has the ability. It's weird that your body knows not necessarily time in terms of a clock but knowing you've slept for a certain amount of hours and minutes and its time to wake up.