While sleeping left arm goes to sleep and fingers are somewhat numb what is the cause?

Vague: There can be many reasons, but i would not hesitate to get into a doctor and be examined. It could be a pinched nerve, sleep position, or other nerve/circulation issues. If this continues, do not hesitate.

Common: If the symptoms abate in a few minutes, relax. Due to sleeping with neck or arm twisted, sleeping on arm. If severe pain, if symptoms persist, you need a DR.