The proximal cause is a nerve getting pinched or a lack of blood flow making nerves anoxic. So either your circulation is getting easier to cut off or your nerves are getting pinched more easily.

It's probably circulation and it's impacted by a lot of things. Getting good exercise tends to help, lol. Also being hydrated. You also might just need to change your sleeping posture or get a different pillow to avoid squishing just the right part of your arm.