The feeling of falling off of something while we are asleep is a natural reflex that we have to prevent us from falling off of things.
The feeling of falling when we are asleep is known as a Hypnic Jerk.
The Hypnic Jerk which is also called hypnagogic jerk, is a normal reaction that can be caused by anxiety, caffeine, a dream, or discomfort of sleeping.
The falling when we are asleep feeling is a hypnic jerk which is the feeling that is triggered by a sudden muscle twitch, causing the feeling of falling while sleeping or dreaming.
It's a normal but sometimes scary thing to feel when asleep.
It really wakes me up whenever I get those Hypnic Jerks.