funny thing is – i always thought it was from lying awkwardly but i'm now fairly sure it also happens when i lie on my back, with my arms either at my side or loosely crossed over, or if i lie on my side, it's just as likely to be the arm that's on top as the one the one i'm lying on
as a side note – i used to get ulnar nerve pain (outside edge of palms) during cycling when i had narrow bars but wide bars stopped this pain occuring (been back on wide for 2.5yrs)