Though side sleeping is the most common sleeping posture people use and it is unintentional as well. People sleep in a way that feels comfortable to them and they always try to keep their body in a relaxed manner that lets them sleep while resting the whole body in the desired position. But when you sleep, it is somewhat unintentional to move your body or relax in a certain way during sleep as you are not consciously moving during sleep. Because you just want to relax and whichever posture suits you, you will go for it and get a cozy and relaxed sleep.
But still, you may intentionally select your sleep posture when going to bed whether you feel relaxed on your back or lying on one side of your body.
So if you choose to lie on one side of your body you are said to be a side sleeper. And side sleepers enjoy sleeping and relaxing on one side of their body. But sometimes when you don’t turn after some time and keep the same posture for the whole night, you must be developing a certain level of pressure on the muscles on your backs, neck, and the shoulders as well.