What causes the sudden sleep twitches many of us experience?

Most people have experienced it: the sudden jolt as you drift off to sleep, sometimes with the sensation of falling.

Known as hypnic jerks, these involuntary muscle contractions occur as the body transitions from wakefulness into sleep, and are generally harmless.

Researchers think they may reflect a brief mismatch between the systems that relax muscles and those still monitoring the body's position.

They become more common with sleep deprivation, late caffeine and stress, which is the usual place to start if they are happening often.