平躺仰臥睡對身體有問題嗎?

其實平臥或側臥睡是沒多大分別的,因為入睡以後,你的身體會不自主地轉動多次,唯一分別是你入睡前(還有知覺時),覺得舒服與否.

輕微巻曲是嬰兒在母體內的自然姿態,所以一般人以這睡姿睡覺都有一種安全感;而輕微巻曲只可在側臥時做到,故這睡姿較多人用.

試買一個承托力較好的枕頭去改善腰酸情況.

I am not professional in this field so my answer is only based on my own opinion only. .
I used to 右側臥睡 before but I always twist my waist when sleeping thus I suffered back pain. The proper way of 側臥 is that one should keep the body in a line (not 100% straight but at least not to twist your waist (the left leg crossing the right )while sleeping.
平躺仰臥睡 is OK for one can let one's back lay on the mattress and gain full support from it. If you are not more tired when get up/have more serious back pain/neck pain, your present sleeping posture is right for you. No matter 平躺仰臥睡 or 臥睡 , you will change your posture when you are sleeping. Therefore it doesn't matter.
The only posture is not quite ideal is left 臥睡 for giving extra pressure to our heart.