I woke up unable to turn my head to the left three times last year. Each time it lasted about two days. I would shuffle around like a robot, turning my whole torso to look at things instead of just rotating my neck. I blamed it on “sleeping wrong” and moved on.

Then my physical therapist friend asked what kind of pillow I used. I showed her. She winced. Apparently my pillow — a cheap polyester-filled one I had owned since college — was actively working against my spine every night.

Your pillow is either too high or too flat

When you lie on your back, your spine should be straight from your neck to your tailbone. If your pillow is too thick, your chin tilts toward your chest and your neck muscles are stretched all night. If it is too thin, your head tilts back and the muscles on the front of your neck are strained. Either way, you wake up stiff because your muscles have been held in an unnatural position for eight hours.

The right pillow height: your ear should be aligned with your shoulder when viewed from the side. Not above it, not below it.

Side sleepers need more height

If you sleep on your side — which most people do — you need a thicker pillow than a back sleeper. The goal is to fill the gap between your ear and the mattress so your neck does not bend sideways. Measure it: lie on your side and have someone check if your spine looks straight. If your head dips down toward the mattress, your pillow is too thin. If it angles up, too thick.

Stomach sleeping is the worst

Sleeping on your stomach forces you to turn your head to one side for hours. Try it right now: turn your head all the way to the left and hold it. After about sixty seconds you will feel the strain. Now imagine doing that for an entire night. Stomach sleepers are the most likely to have chronic neck pain. If you cannot break the habit, use the thinnest pillow possible — almost flat — or no pillow at all, and put a pillow under your hips to reduce the arch in your lower back.

When to replace your pillow

Fold your pillow in half and let go. If it springs back, it still has life. If it stays folded or feels lumpy, the filling has broken down and it is no longer supporting anything. Most pillows last 18 to 24 months. I replaced mine with a memory foam contoured pillow that cost about forty dollars. I have not woken up with a stiff neck since.