Its normal. Theres a foot long medical term for it but I can't remember the name. I do remember some time ago this same thing came up in another thread and posts.

its the brains switching from wide awake to being asleep/unconscious. A person goes through different "cycles" when falling asleep. I don't remember them all but know REM is one of them. Anyway what usually happens is both the mental process of going to sleep matches the physical process of changing chemical levels, heart rate, breathing, and a chemicle that works like a paralyzing agent so that while in the REM cycle the person isn't sleep walking all over the place acting out their dreams. Sometimes when a person is over tired the mental aspects of going to sleep and the physical aspects are quite matching each other so mentally the person falls asleep first and then the body (this can happen with the body falling asleep first while mentally the person is still thinking of the days experiences and worries) any way the result is that the person experienese the "switching" from consciousness into unconsciousness like a drop of falling feeling. Which is where the term "falling asleep" came from. You can prevent experiencing this by getting plenty of sleep and rest so you don't get over tired and do something calming and relaxing before bed.