Today I learned that a sudden jerk that you sometimes experience when you are just about to fall asleep due to a sense of free-fall or weightlessness is called a hypnic jerk.

If, while sleeping, I roll over to the edge of the bed, I feel free-fall & jerk up from sleep if I am about to topple down.
But in case I am comfortably lying down in the middle of the bed with no chance of toppling down, how do I feel weightlessness or free-fall when I am just about to fall asleep and jerk up, i.e. hypnic jerk?
And not just on the bed; this is also experienced if you are sleeping on the ground where u cannot possibly physically fall.

A bit more research in Falling sensation tells me its the ears & the eyes that sense the free-fall.

Sensation of falling can be induced when the eyes detect rapid apparent motion with respect to the environment.

How is this accounted for during sleep. Or is it something entirely different?

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    $\begingroup$ TIL the same thing...and also the meaning of the acronym TIL ;) $\endgroup$ – Nick Stauner Mar 14 '14 at 5:35
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    $\begingroup$ In the same way that people imagine smells or sights that they currently do not have: by recalling the experience from memory or making it up from memories of similar sensations. When you relax your muscles, you limbs move, you register this movement, correlate it to your relaxed state, draw the conclusion that if your leg "falls" you must be falling, your brain comes up with the fitting imagery. In the same way that you dream of the school bell when you hear your alarm clock and don't want to wake up. In the same way that you hug your teddy bear and dream of your girl friend. etc. $\endgroup$ – user3116 Mar 14 '14 at 15:28

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