What Causes Numbness In The Arms?

Hi, I have been getting dead arms in my sleep for a couple of years now, it's started to get worse recently. I'm 33, otherwise healthy, and do personal training twice a week. I'm sure I simply sleep on my arms to cut the blood flow but I have read a lot about carpal tunnel and issues with C5/6/7 in the neck. I was wondering if the personal training could be putting pressure on my neck, my boyfreind says I have bad posture and will end up kyphotic when I'm older if I don't correct it! What I really want to know now though, is whether there could be long term effects of this nightly numbness? Is there a chance the arm tissue could die over time? Last night was the worst it has ever been, it was numb from the shoulder down and it took about 3 minutes to get any feeling back at all (usually I can slightly move my middle finger after about 30 seconds of moving the arm with the other hand). Thank you!