Have you ever wondered why your head suddenly jerks you awake just at the moment you are about to nod off?
It could be because you are stressed, tired or have irregular sleep patterns.
Professor Gaby Badre, a sleep expert at private hospital The London Clinic, calls the phenomenon hypnic myoclonia.
She explained to the Daily Mail, that normal sleep passes through three stages: light slumber, deep slumber and after 45-60 minutes the rapid eye movement (REM) stage begins.
But when we are stressed or tired, we can go into the REM stage much earlier.
It's here we experience the vivid images and dreams but, because your muscles are not fully relaxed as they should be, you twitch and jerk as if you are moving.
Prof Badre says the phenomenon is "perfectly normal" and nothing to worry about.
She said: "The best thing to do is try to adopt a regular sleep pattern."