I woke up with a stiff neck three mornings in a row last winter. I blamed my desk setup, my phone scrolling habit, the fact that I am not twenty-five anymore. Then my physical therapist asked one question: “How old is your pillow?”

Three years. Maybe four. I had not thought about it once in all that time. She nodded like she heard this every day. “Pillows have a shelf life,” she said, “and yours expired two years ago.” Six weeks and a new pillow later, the neck pain was gone. Here is everything she told me about pillows and neck pain that I wish I had known sooner.

The height thing nobody talks about

Your pillow’s job is to fill the gap between your ear and your shoulder so your neck stays in a straight line with your spine. Too high = your neck is cranked upward all night. Too low = your neck is bent downward. Both create muscle strain that you feel the next morning.

The right height depends on how you sleep:

• Back sleepers: medium loft (4-5 inches). Your head should not be pushed forward or tilted back. Your chin should be level, not pointing up or tucked down.
• Side sleepers: higher loft (5-7 inches) to fill the wider gap between shoulder and ear. Your nose should be aligned with your sternum — draw an imaginary line, it should be straight.
• Stomach sleepers: very low loft (under 3 inches), or no pillow at all. Stomach sleeping puts the most strain on your neck regardless — it forces you to twist your head to one side for hours. If you can train yourself out of this position, your neck will thank you.

How to test your current pillow

Fold your pillow in half and squeeze the air out. Let go. If it does not spring back to its original shape within a few seconds, the fill is broken down and it is not supporting you anymore. A flat, lumpy pillow is basically a neck cramp delivery device.

Also: lay the pillow across your forearm and press down. If your arm sinks through and touches the bed with almost no resistance, the pillow has collapsed. You are basically sleeping on the mattress.

When to replace pillows

Down and feather pillows: every 1-2 years. The quills break down into dust that you are breathing all night. Synthetic fill: every 1-2 years. Memory foam: 2-3 years. Latex: 3-4 years.

If you have allergies or wake up with a stuffy nose every morning, your pillow might be full of dust mites even if it still looks fine. Pillow protectors help, but they only delay the inevitable.

Write the date you bought it on the tag with a Sharpie. Six months from now you will not remember, and that is how pillows get to be four years old.

The material difference

Memory foam conforms to your head but sleeps hot. Latex is cooler and more supportive but heavier and more expensive. Down is soft and moldable but needs frequent fluffing and does not last as long. My PT recommended shredded memory foam — it is adjustable (you can remove or add filling to get the exact height you need), sleeps cooler than a solid block, and costs about forty dollars.