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ok, that feeling happens to me a lot, and i finally figured out what it was:

link for more reading]

(copy and pasted this)
The general consensus among researchers is that, as your muscles begin to slack and go into a restful state just as you are falling asleep; your brain senses these relaxation signals and misinterprets them, thinking you are falling down. The brain then sends signals to the muscles in your arms and legs in an attempt to jerk you back upright. This misinterpretation that takes place in your brain may also be responsible for the “falling” dreams that accompany the falling sensation. These “dreams” are not really normal dreams, as they are not produced from R.E.M sleep, but rather more like a daydream or hallucination in response to the body’s sensations.

While this phenomenon happens to most everyone, studies have recently begun to link occurrences of “Hypnic jerks” to sleep anxiety, fatigue, and discomfort. People who are having trouble sleeping or can’t get comfortable in bed appear to experience the sensation more often throughout the night. It is especially more common with people who are trying to fight falling asleep or have deprived themselves of sleep for more than 24 hours.

i found that very interesting, how about you :'D