If you are sure that you do not have any known diseases or health issues, then probably you slept on your Left Arm, causing a compression of the supplying Left Axillary or/& Brachial artery
, because of which the blood supply to the arm reduced (or even may have stopped), causing loss of sensation and function in turn.
Try sleeping straight for a few days, rather than on a side, and try to notice if there is any improvement.
However, if you are still doubtful, you can always consult your physician, who can the advice you to take a sleep cycle
study, &/or Cardiac investigations, if neccessary.