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.