I have, on several occasions, felt a brief falling sensation as I drift off to sleep. This feeling is immediately followed by a sudden muscular jerk and total awareness.

This is an all too common experience, but the true nature and cause for this phenomenon is debatable. I would guess that the sense of falling is given to us by our brain's interpretation of a change in muscle tone. The whole process seems entirely involuntary to me, so I wonder what it is that triggers this to happen.