What Causes Quick To Moderate Shocks On Body Of Child While Sleeping?

Recently, i noticed that my 7 years old child, when a sleep his body shakes suddenly. I would not notice this quick shakes which happens all of a sudden in less than a second, if i was not sleeping beside him in the same bed. I felt twice ( about 5 minutes between the two shakes) that his body was just like reacting to a quick and moderate shock. these shock are quick and barely noticed but certanly unusual for him. I have been always sleeping beside him but these shocks did not take place.