I notice that people in some cultures sleep with their arm as a pillow when they're on their side but I'm wondering how their arms don't go numb, is there a trick to it?
Post moved to "Side Stretchlying" section.
I have not studied this phenomenon particularly, but I do know that people who actively use their bodies in a healthy way actually have much more space in many of their joints. In our culture, peoples hips, shoulders, and neck tend to be quite compressed. There is less space in the joints themselves, and the muscles around them tend to be tighter and less elastic.
This excessive tightness is going to lead to compression on the nerves and blood vessels, leading to numbness and tingling, especially in positions where the joint is extended in a way that it isn't often used.
If you want to be able to use your arm as a pillow without it going to sleep, start with some shoulder rolls. Then work on raising your arms above your head without your shoulder going up--let the movement come from the rotation of the shoulder blade. When you can easily do that, try hanging from a bar, or even doing some pull ups. Then try sleeping on your arm again, and let me know how it works!
Gokhale Method Instructor