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Simple numbness from blood flow being cut. Nothing particular here, happens if you sit on it, or if you keep it in awkward positions for long (IE train naps).
This has happened to me once, it was pretty funny atually. I rolled over and tried to lift my arm and put it down on the other side as I rolled over. What happened was it was completely numb so it didn’t lift up and ended up slapping me in the face! 
But yeah, I only remember it happening once. I guess every now and then we just fall asleep in a strange position…
This happens to me quite often.
It’s when during night you lay on your arm, the blood flows slower inside and the arm becomes feelingless and numb. 
Once the blood can go through your arm again it gets back to normal.
I somehow fall asleep with my arm above my head sometimes. When i wake up the entire arm is numb and i have to use my other arm to move it in position so it gets blood 
I’ll never forget the first time it happened to me a couple of years ago. 
I woke up in the morning, laying on my left side, feeling something on my right shoulder. I decided to check it out with my left hand. “Okay, that’s something big…”. “O.o it’s a hand”. At that point, I was struck with absolute terror. I was thinking some sicko maniac had killed my family, then delicately laid a severed hand on me, so my plan was to throw it away and run out of the apartment. I only got to step one, which resulted in my right arm flying through the air and reaching the floor, still attached to my body though. I was feeling both confused and relieved.
One time when I was about to sleep, I noticed that while I could move my arm it felt really really heavy. Maybe not numb. I got the impression that if I kept it still, the rest of my body would follow into SP, which I really hated, so I just kept flopping it about. (Like I said, my arm felt really heavy, so it had to flop rather than wriggle.) Maybe yours was a partial SP like that?
It is called obdormition, and it is not because of a blockage of bloodvessels, but pressure on nerves. Cutting off bloodflow would cause tremendous damage to the arm.
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afaik you don’t have to sleep on it, just have your nerves pressured through a twist or anything for some time is enough
Last week a nasty little incident happened to me where I woke up and my right arm was numb. Nothing new there, but as the feeling started to return to it I discovered that I’d slept on my hand while it had been clenched into a fist and my fingernails had digged into the flesh on my palm and left these little crescent imprints that stung quite a bit. Thankfully they faded within an hour and the nails hadn’t actually pierced the skin or anything. Still, not a nice thing to find when you wake up.