We've covered sleep twitches, the unconscious twitches that people make while they're asleep. They're the result of the unconscious and conscious brain both being online at the same time. Sometimes people twitch themselves awake. This is preferable to another strange sleep tic — the sudden sensation of falling that can cause the person to wake in a panic. This isn't the dream of falling which happens when people are in deep sleep and a regular dream takes a nasty turn. It's a sudden physical sensation that wakes them up. And what accompanies it is a hallucination, not strictly a dream.
Sleep starts with a part of the brain called the reticular formation sending a signal down the spinal cord to relax the muscles and to inhibit responses to stimuli. A poke that you'd feel immediately when awake won't rouse you when you're asleep. The body dampens its own consciousness. That much, everyone agrees on. From there on in, there's dissent among scientists.
One group of scientists, led by Ian Oswald, noticed that the signal from the reticular formation is flipped in certain individuals. Instead of inhibiting muscle contraction, it can occasionally increase muscle contraction in response to almost no stimulus at all. This is called a hypnic jerk, and it's most commonly experienced by people with restless leg syndrome. When they jerk awake, the sudden change in position — splayed out without feeling any direct support under the hands or feet — can lead to a person reinterpreting the whole sensation as falling.
Other scientists believe that the sensation of falling comes from the act of relaxing itself, especially if the person is anxious or unable to get comfortable. As the muscles relax into sleep, the brain stays awake, monitoring the situation. The slackness of the muscles — and the fact that a person generally "settles" as their muscles relax — can be interpreted by the brain as a sudden sensation of the entire body falling. The brain then jerks the body awake.
And the hallucination? Well, many hallucinations are not the wild trips that movies make them out to be. Most people have experienced hallucinations. Hallucinations are just experience in which the brain takes a group of stimuli and misinterprets them into something that doesn't match up with reality. Plenty of people have seen, for example, a cat or dog following them out of the corner of their eye, and turned to see that it was just some trash hooked around a telephone pole. The brain makes a snap judgement and creates a picture, but that picture turns out to be wrong.
These hallucinations increase with stress (when the brain wants to make snap decisions more readily) and exhaustion (when the brain doesn't automatically process as much general information as it would under peak condition). Falling asleep while anxious, with a super-sensitivity to stimuli, or in an uncomfortable situation leads the brain to get a sudden emergency input (i.e. the body is falling) and cast around for some reason why the body might fall. It manufactures a half-dream that you remember when you're awake. Some people dream that they're walking and they've just slipped. (My brain always had me roller skating in these "dreams." Really, brain? That's the most likely reason you can think of for why I'd fall down?)
The phenomenon isn't well studied because it generally doesn't cause any problems. Only when people get restless enough to keep themselves awake regularly and for hours does it indicate sickness or a brain injury. So enjoy those sudden feelings of free-fall. They make getting into bed a safe adventure.