Good question Becky, its not a pleasant dream really!
Basically when you’re in a deep REM sleep (the deepest part of sleep during a night), your muscles go into a special state of stiffness called atonia. This means that there are no brain signals going to these muscles, so you are actually paralysed while you sleep! This is to protect yourself so you don’t try and act out your dreams. But if you wake suddenly from a dream or because of a noise, your brain might not catch up so quickly and your muscles stay paralysed, only for a few seconds or so. This disconnect between your body and brain gives you a sensation of falling, so when then once your muscles kick in, you jerk out of the bed!
Great question Becky! and great answer Karen 🙂 I m only going to add a tiny bit to your answer. Sleep paralysis takes place every night in the typical sleeper to prevent him or her from acting out dreams during REM sleep. When this paralysis outlasts sleep, as it sometimes can, you get instances where your body is paralyzed or the also the feeling of falling out of a plane or off a cliff. Waking up from sleep paralysis can be extremely frightening, but rest assured that it is not uncommon and isn’t generally dangerous at all… it’s just your body trying to protect itself. Imagine dreaming about flying off a highrise building! It’s a good thing that your body goes into paralysis to prevent this such disasters from happening in real life! Hope that helps 🙂