70 %of all People experience this phenomenon just after nodding off
The general cause 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.
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.
Studies have recently begun to link occurrences of “Hypnic jerks” to certain factors :
> 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.
The sensation is completely normal for our bodies and is of little medical significance.
Our bodies go through several procedures of shutting down and preparing for an extended period of rest. “Hypnic jerking” is just one of them. It doesn’t appear to cause damage to body and poses no danger to our physical wellbeing. That may be true, but it could pose a significant danger since someone wake up thinking he just fell off of a building.
Therefore ,the lack of sleep from sleep anxiety or sleep deprivation confuses the muscles and the brain. The muscles continually attempt to relax and shut down for rest, while your brain remains awake creating continued “misinterpretations” of falling or loss of balance feeling while sleeping .