“How Should I Sleep With A Herniated Disc In My Neck?”
by Dr. Scott Hodges

Best to have your neck in a neutral alignment. That is, you don't want your neck to be forward, flexed too much, with a big thick pillow. You don't want a real flat pillow in which you extend your neck too much. If you lie on your side, you can, but you need something between your shoulder and your neck so that you're not lying with your neck side-bent for a long period of time. So you just want a neutral alignment.