Why About Everything?

What are those twitches and jerks that occasionally wake us just as we are falling asleep? 

It has probably happened to you. You are nestled snugly under the covers. You aren't quite asleep but you're not quite awake. Just as your brain waves start to slow, and as you fantasize about something, you are jolted awake by an unaccountable spasm, usually in a leg. This is called "hypnic jerk". It occurs when nerve fibres leading to the leg, in a bundle nearly as thick as a pencil, suddenly fire in a unison. Each tiny nerve in the bundle produces a harsh tightening of a tiny portion of muscle fibre that is linked to it down in the leg, and when they all fire together the leg twitches as a whole. 

        Sleep specialists haven't exactly pinned down what causes hypnic jerks or why they occur only at the onset of sleep. Although some theories exist, like the one which says that they are a natural part of body's transition from alertness to sleep.Another interesting hypothesis links hypnic jerks to evolution-that it evolved when humans used to slept in trees. That is why brain essentially misinterprets relaxation as a sign that a individual is falling from a tree and this cause muscles to react quickly.