Most probably, you have experienced a so-called “sleep start” at least once in your life. It reminds of the shocking spasm or cramp when getting ready for sleep or, vice versa, trying to wake up. It is one of the numerous forms of myoclonus. Even people without any problems suffer from this problem sometimes. When thinking about other examples, the first thing to come to mind is an abnormal startle. The worst type of the phenomena is the pathological myoclonus as it may disable the whole body for a while.