I believe the missing component to be the pressure of one's body weight, downward facing, along with the pressure and possible constriction of blood flow from that "artery" under our collar bone that supplies our arms when above or perpendicular to our body.
Sleep also as you stated because of the pressure and angle during an extended period of time.
All due to "more than nessesary" muscle mass on our upper bodies, particularly in our upper chest and shoulder girdle (sp?!) , AAS or not (my case).
Very interesting that alot of people suffer from this, thankfully it's not life threatening and we'll all live after we experience the slight scare in the morning and flopping around of "dead" appendages for a few momments.