Thanks for this interesting question. Sleep disorders are quite common in various forms. You seem to be describing a subtle tremor in your movement rather than full body shaking, but it is not clear from your question. Full body shaking would possibly be related to some sort of difficulty in regulating your muscle control as you transition between sleep cycles, but it is not clear to me exactly what you are describing. A fine tremor on the other hand could have more to do with the levels of fine motor control that the various areas of your brain are exerting at the specific time, and could indicate a different problem. Either way, neither of these issues are normal and are something that should be discussed with a doctor
in the near futures. He or she may want to do a sleep study, which may require some monitoring with both video and some equipment that can sometimes be used to trace your brain activity while you sleep. All of this is something that can be decided after you speak with your doctor. A sleep medicine doctor
is most likely the one who will be able to help you most, and so please speak with your doctor about this question.