Hey there, welcome aboard!

There can be several reasons why you would have more nightmares when sleeping in a certain position. I don't think there is a direct link between sleeping position and nightmares, but we do know that bodily sensations can influence your dreams. For instance having to go to the toilet could make you dream about having to go to the toilent, being too cold could make you dream about being on the northpole, etc etc...

So with that in mind, there's several possibilities. Just three from the top of my head:

- You're not really so comfortable for one reason or another lying on your back, or perhaps your body begins to get less comfortable after a while (usually when you're already asleep). This feeling of 'uncomfortableness' can translate itself into your dreams, possibly into nightmares.

- You can't breathe so well lying on your back. There's a fairly common sleep disorder called sleep apnea, where people begin to have trouble breathing while sleeping. Not suggesting you have this, but lesser degrees of it are even more common then the disorder itself and happen with a lot of people. Erratic breathing or a slight difficulty in breathing could create a feeling of 'oppressiveness' which again translates into your dream as a nightmare.

- Associations. Our mind works very strongly on the concept of association, and this is even more so when it comes to dreams and sleep. If you believe yourself to have a nightmare when lying on your back (even subconsciously), the chances are that your believe will make it come true. Suppose that you have a string of nightmares for one reason or another, one each night for a few days in a row, and you were lying on your back for each one. The association 'sleeping on back' - 'nightmare' is very quickly made, and can turn into a self-fulfilling prophecy.

Anyways, hope that helps a bit,

-Redrivertears-