Is it forbidden to sleep on one’s back? If one sees their spouse sleeping on the back, should they wake them to tell them to roll over?
It is forbidden for a man to sleep on his back (or stomach), rather he should go to sleep on his side. If you see someone sleeping on his back, etc. you don’t have to wake him up, however if you know that he is a type of person that won’t mind than you can.
Bracho 13b, E:H 23-3, M:B 239-6, Oz NIdberu 12- 52 (1), Olas Yitzchok 1-48, 2-90, Ase Licha Rav 8 pg. 369, Lev Simcha pg. 213, Poskim.