I’ve been conducting an experiment for the past month.
For years I slept on all four sides. I slept on my back, right side, left side, and front. Sometimes for a time, I would have a preference for one of the four sides. Sometimes I would rotate through all of them in the course of one night.
I decided for the past month to sleep only on my back or on my front.
What I noticed from day one is that my shoulders feel much better each morning when I wake up. They don’t feel the least bit stiff, which is how they normally feel. That in itself is enough for me to continue with the experiment.
What has been most interesting though to observe is the strong desire in me to roll onto my side–even though lying on my side has never been completely pain free for me. Even if I have enough pillows to support my head and neck the shoulder toward the bed is always compressed somewhat and just a little bit painful. But the slightly curled-up fetal position I adopt when on my side is very familiar and the comfort that familiarity provided would win out over the slight discomfort I felt in the shoulder.
Your posture is to a great extent habit. Habits are familiar. When you are familiar with something it is comfortable. But not necessarily comfortable as we traditionally define it: affording physical ease or relaxation. Comfortable because it is known and not something new. An important distinction.
For me, it is more comfortable to get together with old friends than to go to a party where I don’t know many people. Humans tend to resist change. What’s known (the old friends) is familiar and therefore comfortable. Something (or somebody) new is unfamiliar and therefore uncomfortable (at least until you get to know them).
Often a student experimenting with a new way of balancing her weight in sitting or standing will comment that her old way is more comfortable than the new way—even though the old way is compressing her low back and causing pain. This is just one more example of the same phenomenon of describing something familiar as comfortable.
Try this experiment: cross your arms in front of you. Notice how this feels. Cross your arms the opposite way (so if your right arm was on top originally, put your left arm on top and vice versa). Notice how this feels.
Typically the way you cross your arms first will be your habitual way and it will feel comfortable (because it is familiar). The second way will not be your habitual way and it will feel uncomfortable (because it is unfamiliar). You can do this same experiment with crossing your legs as well.
One of the reasons that making changes to your posture is challenging is that we feel this strong pull back towards the familiar. New ways of doing things at first feel uncomfortable (meaning unfamiliar).
Understanding this fact is important. And also realize that the more you practice the more familiar new habits will become and consequently the more comfortable.