i'm sure all of you, like i, have had this experience in school. we normally would have to be dragged/threatened/thrashed out of bed, but on the day we are going out on a school picnic we are out of our beds in a flash at some unearthly hour; even if we managed to sleep only a couple of hours earlier!
in fact most of the time, if you've noticed, when we want to wake up for something really important or for something really exciting or something we are looking forward to, we manage to wake up just before the alarm goes off. and this is in stark contrast to the umpteen number of times when we sleep through the blaring which has managed to wake up the rest of the household and some neighbors too!
all of us have experienced this fascinating body clock within us at some point of time or the other. its only recently when i started travelling abroad that i realised that our bodies don't strictly follow the time according to the watches on our wrist but has its own internal clock which is ticking away (like a time bomb?). earlier, i could never appreciate 'jet lag' which is just our body clock trying to sync itself with the watch on your wrist.
i recently learnt that there is a technical term for this internal body clock - circadian rhythm.
this only further reinforces the ancient vedantic pronouncement that time exists only in the mind; and the true measure of time is the gap between thoughts. swami chinmayananda used to say that time can be measured only after the 'second' thought and that is possibly why time is measured in seconds and not in 'firsts'!
(click here for an interesting perspective of time from a vedic standpoint - http://www.templenet.com/beliefs/concept_of_time.htm)