I used to always sleep on my back. About 6 years ago I tried to switch to sleeping on my stomach, but I'd wake up with a very stiff neck.

Last year I saw some research that suggests side-sleeping helps improve the circulation in the head. It's been easier to switch to falling asleep on my side, than my stomach-sleeping experiment...

The Effect of Body Posture on Brain Glymphatic Transport - http://www.jneurosci.org/content/35/31/11034.short

Random report in the media: http://www.dailymail.co.uk/news/article-3191826/Lying-sleepi...

I slept on my stomach in a hammock once. My back hurt more than the hangover. As for good stomach sleep, I pull the top mattress* about 18" from the headboard and fold my arms & pillows into the trough to prop my head straight w/ my spine. After decades of neck issues I can say I finally get great sleep w/out neck pain, numb arms & aching shoulder joints. I had tried a massage table w/ face cut-out but it was awkward & I flopped off the side several times.

*I use 2 mattresses on top of box spring, I prefer dropping out of bed to 'getting up' in the morning.

When in bed, I prefer to sleep on the stomach. But I have had great results sleeping on my back in a hammock. Just one data point; however (also anecdotally) I have had several friends sleeping in my hammock and reportedly enjoying it, whatever the normal preferred sleeping position.

Most? It's about 7% of people.