How come when you just start to fall asleep, you get a jolt in the leg or arms? What is this called and what causes it?
That's called a hypnic jerk, and these jerks are a normal part of the body's transition from being awake and alert to going to sleep. Unfortunately, the scientific world is not united on what exactly causes it, but there are two hypotheses. The first is that the jerking motion is part and parcel of the physical changes that accompany going to sleep — your breathing slows down, your temperature drops, and your muscle tone changes.
The second hypothesis is that your brain interprets all of the signs of relaxation as a sign that you're falling and then shoots out a message to jerk your limbs to stay upright. This may seem more plausible to some people, since many experience a sensation or visions of falling as they drift into sleep. In fact, this might also be why you see some people jerking awake in the library after falling asleep over their textbooks.
Most people have hypnic jerks at some point, but many won't recognize them since they aren't strong enough to wake them up. There is something wrong, however, if you have periodic limb movement disorder (PLMD), what used to be known as myoclonic jerking. In this case, you don't just have one light jerk, but a series of jerks at precise time intervals that can go on for hours. Amazingly, many people who have myoclonic jerking may not realize it even though it can make getting quality sleep nearly impossible. PLMD is sometimes associated with other sleep disorders, such as restless leg syndrome. If you think you might have PLMD or related issues your health care provider can evaluate you to identify a cause and find out what you can do about managing the condition.
In all probability, however, you are experiencing a normal, occasional hypnic jerk: nothing to be concerned about — it's just annoying, as most jerks tend to be.Alice!