I learned about this in psychology class. Essentially, your body paralyzes itself when it enters REM (vivid dream phase) so you do not act out your dreams. If you somehow wake up during this time, or during the preparation for REM, you can feel your body vibrating since your brain is no longer sleeping. It's slowly waking up and getting ready to "move around". To jerk in your sleep, from what I understand, happens during other phases of sleep not characterized by paralysis. Perhaps this is happening to you frequently during the sleep stage just before REM, you can still move, you do move, but other parts of your body was still slightly paralyzed.
I found for myself, if I am stressed, both of these things will happen more frequently.. Try meditating before you go to sleep, as this will calm you before you embark on your slumber and could result in a more restful sleep.
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