What is the correct position to sleep in

The whole aim is to keep your spine in a neutral position, not too arched, not too flat... somewhere in between. for reference here's a picture. 

 

For me the problem is sleeping on my stomach. when I do this there's no way that my spine is neutral, It has a backward bend because the stomach sinks down. The other problem is with my neck, I have to breath... so I would naturally turn my head sideways at a weird angle, causing pain in the morning. Fun Fact sleeping on your stomach is the worst way to sleep. 

Sleeping on the side is okay as long as you have the proper support to keep that neutral spine position. There are three points of support, One: the head has to again stay level... more now cause The shoulders have more height sideways. If the pillow is too thin than fold it and use it.  Two: just above your waist would also be bent so a pillow there would also be great. Three: The top leg would also drop down pulling the spine a bit, so stick a pillow between the knees and it should be levelled.

Sleeping on the back, you're gonna need only two places of support. One: the head again keep it levelled. Two: The legs are flat a bit too low, but keeping a pillow there is gonna fix it. The other one is the lower back, If there was no pillow at the legs the back would be flat (You need a slight bend at the lower back). Putting that pillow under your legs automatically arches the lower back slightly.

That's it 

Here is a link to a video that explains it better: https://www.youtube.com/watch?v=-Qxmc4E7IwA