Stay with me, because this is the whole thing.
Your head weighs about 10 to 12 pounds — roughly a bowling ball. When you're standing with good posture, your neck's natural curve carries that weight effortlessly. The muscles are relaxed. Everything's in line.
Now you lie down for the night. Here's what almost every pillow does wrong: a soft or flattened pillow lets your head sink. A too-tall or too-firm one props your head up at an angle. Either way, your neck can spend the next six to eight hours bent out of its natural line — held in a position it was never built to hold that long.
Your neck muscles don't get to clock out. All night, they're working to compensate for the bad angle your pillow put them in. You're asleep, but your neck isn't resting.
That's the stiffness before you've done anything. The ache at the base of your skull. The "I slept eight hours and feel like I didn't sleep at all." Your body spent the night compensating instead of recovering.
And here's the cruel twist: the fluffier and more "comfortable" a pillow feels in the store, the worse it often is for this. Comfort at the surface, collapse underneath. Every one of those pillows was designed to cushion your head — not to support your neck. They were all solving the wrong problem, which is why trying a tenth version of the wrong solution was never going to work.