The pillow is a spine alignment tool
A pillow has one structural job: fill the gap between your head and the mattress so your cervical spine holds the same neutral curve it holds when you are standing. Too much loft and your neck bends upward; too little and it sags. Both produce the same morning stiffness people usually blame on the mattress.
The gap you need to fill depends almost entirely on sleep position. Side sleepers have the widest gap — roughly the distance from the outside of the shoulder to the ear — and need the most loft. Back sleepers need a medium loft that supports the natural cervical curve without pushing the head forward. Stomach sleepers need almost nothing, and are usually better off with the thinnest pillow they can tolerate.
