Several times in the last while, as I decide to do any meditation, communication with guides, any other metaphysical or spiritual thing, (call it what you will,) and certainly sometimes when I am intent an trying to notice when I've fallen asleep I will find I am aware of getting closer and closer to sleeping and of drifting off. This is typically obviously by the fact that the mind begins to produce senseless thoughts, images that make little sense and strings of unrelated and somewhat silly ideas in quick succession. I have no doubt in my mind at all that everyone does this as we fall asleep. I just happen to notice it because I've been training myself to notice it.
This is what meditators call 'monkey mind' and it's natural. It's what happens behind the scenes as we memorize our experiences. We only become aware of it when we silence our conscious minds.
Anyway, lately though, as I begin to really notice all this, and it of course becomes harder and harder to really stay conscious of it, (that of course is reaching the point of falling right into sleep,) I will very suddenly feel quite wide awake again. It's different though. Not a state of being really awake and thinking that perhaps I should get up, or roll over because I'm uncomfortable or anything. I can't actually explain how it feels but it's just a bit different from true wakefulness as we know it. Sometimes the rapidness with which I go from that state of drifting off and losing awareness, to suddenly becoming very aware again is startling. I've though before that something might have woken me up, such as perhaps a door slamming in the hallway, or a car starting loudly outside, but I never actually hear anything and it happens too often now to be any of that. One night when this happened I just knew that my own mind had woken itself up, if that makes a hint of sense. But why?
This does happen to almost everyone (or at least many people) and it can be a side effect of meditation- but it also can be because of a delayed reaction to a noise.
For some reason we don't actually perceive things when they happen (this has been shown in lab tests by brain scientists a few times) and this is most obvious when we sleep. If you want to, you can make a mean experiment (I've done it with both my husband and son, hee hee

) Wait until someone is deeply asleep, make a sudden noise and then count. They won't wake up at the moment you make the noise, but they will react (by moving, changing breathing rhythm or waking up.) If the noise is loud enough to wake them they'll do the whole startled awake thing- but not when the noise happens, approximately three or four seconds later.
Anyway, no worries.