Q: Why do my muscles suddenly jerk just as I’m falling asleep?

Scientists call this phenomenon a hypnic jerk, which is just one example of a group of muscles twitches known as myoclonus. Myoclonic jerks or twitches also include things like hiccups. No one is quite sure what causes them. Some scientists believe they are caused by the natural transition of the nervous system as you move from wake to sleep. As the muscles begin to relax, the twitch occurs. Others believe it may be an evolutionary holdover that causes the brain to misinterpret the relaxation of the muscles as a sign you are falling. The brain sends a signal that causes the muscles to tense up to protect you against a fall. Such twitches are more common in children than in adults but are a normal physiological reaction and no cause for concern.