You’re gently drifting off to sleep when suddenly your arms and legs convulse and you jolt yourself awake. Or, perhaps you’re relaxing in bed when, out of the blue, your dozing partner does an almighty twitch, scaring you half to death. This is called a hypnic jerk or sleep start. It is often accompanied by a sensation of falling or tripping over. An estimated 70 per cent of people will experience this at some point. So, what causes the hypnic jerk? And can certain factors make it more likely to happen? A minor misinterpretation The truth is, we don’t know exactly why it happens, but sleep researchers have some theories. As we transition from wakefulness to sleep, the nervous system winds down and muscles relax. Hypnic jerks usually affect one side of the body and are painless (Getty/iStock) Sometimes, the brain misreads this relaxation as a sensation of falling or tripping. The brain sends a quick but powerful signal to the body. The hypnic jerk is the result.…