Weird Covulsions In the First Stages of Sleep???

Male - 25 years old

for about the last 6 years I have been experiencing what I would describe as strange convulsions while I fall to sleep. They dont happen everyday maybe 3 or 4 times a week. But they always seem to occur in the very first stage of sleep, right as feel myself falling into sleep. When they occur I am in a semi-conscious state but am unable to control any of my body as it shakes uncontrollably. The shaking is not violent and not even firghtnening. The strange part about them is I can snap out of them and wake up at will. Initially I have to admit they were frightnening, but over time ive learned that they are harmless and sometimes even challenge myself to see how long I could last in them. Usually after I wake from the convulsions I fall asleep uneventfully. The only other symptoms I can describe after waking up from these convulsion fits is a rapid heart beat. If anybody knows what this might be or what is causing this please respond. Like I said ive never experienced any negative effects from these convulsions but I would like to know if they can be dangerous. Thanks!
-ray