Why do some people jump or jerk before they fall asleep

A. This is a very common sleep phenomenon. As you’re peacefully drifting off to sleep you suddenly find yourself violently jumping, pulling you temporarily back into a more conscious state. The technical term for this is a hypnic jerk.

Q. What causes the hypnic jerk

A. Well although humans spend around a third of their lives sleeping, many aspects of our resting behaviour are still shrouded in mystery and are hotly debated by the experts. There are a few theories as to what causes the hypnic jerk.

Q. And one theory is

A. That the hypnic jerk is a natural step as your body moves from being alert to being asleep. Drifting off to sleep involves some physiological changes: a change in breathing rate, temperature and muscle tone. This theory suggests that the hypnic jerk is just a by-product of the change in muscles tone.

Q. Any others

A. Yes, another one has an evolutionary explanation. In this theory the brain interprets your relaxing muscles as a sign that you're falling down and tells your arms and legs to grab at something quick to keep you upright. In this case the hypnic jerk is a survival mechanism from the days when we were reptiles, didn't have the luxury of a bed and life was generally a lot more dangerous.

This theory may also account for the falling like dreams that some people report they have before a hypnic jerk pulls them into a more conscious state. Although of course, since dreams only really occur during a later sleep phase, known as REM, (rapid eye-movement), these falling dreams are more similar to hallucinations.

Q. Should I be worried about having hypnic jerks

A. No, they're perfectly harmless, although annoying, especially if you happen to thump someone sleeping with you.

Do you want to know why your body does something Ask The AnswerBank here.

by Lisa Cardy