> During sleep, which can take something between five and eight hours per night, the body usually doesn't change position too much, so imagine staying for eight hours with your neck, your spine and your legs bend in all sort of weird positions, keeping them under tension.

I do not have to imagine. I already know it. Sometimes my legs and/or arms hurts or numb after I wake up from sleeping in a bad position. That is obviously not pleasant at all. We indeed not often, but sometimes physically move in the bed during our sleeps, and we may turn our bodies into bad positions without even knowing it. We just realize it by feeling pain or numb (or even the both in different parts of the body) after we wake up. Even when I start sleeping on my back, I may turn my body to the left and/or to the right side during sleeping. Without even knowing it. By the way, I am already using two pillows in my bed.