I woke up with lower back pain every morning for about a year. Not the kind that sends you to the ER — just a dull ache that took an hour of moving around to go away. I blamed the mattress. I blamed getting older. I never once blamed my sleep position until a physical therapist asked me one question: “Do you sleep on your side with your top leg falling forward?”

Yes. Yes I did. And that was the entire problem. When you sleep on your side, your top leg drops forward and rotates your pelvis, torquing your lower spine for eight hours. A pillow between your knees fixes this instantly.

Why the pillow works

Your hips are wider than your knees. When you lie on your side without support, your top leg falls forward to rest on the mattress. This twists your pelvis and creates a sideways curve in your lumbar spine. Your lower back muscles spend the entire night trying to pull your spine back into alignment. You wake up sore because your muscles have been fighting gravity for eight hours.

A pillow between your knees keeps your top leg parallel to the bottom leg. Your pelvis stays neutral. Your spine stays straight. Your back muscles relax because there’s nothing to fight. It is the cheapest, simplest fix for morning back pain and I am still annoyed that no one told me about it sooner.

What kind of pillow (it matters less than you think)

A standard bed pillow works fine. Fold it in half if one pillow is too thick — you want your knees about hip-width apart, not spread wide. There are specialty knee pillows shaped like an hourglass with a strap, and they’re nice because they stay put when you roll over. But you don’t need one. A regular pillow between your knees has been fixing back pain since before memory foam existed.

If you sleep on your back: put the pillow under your knees instead of between them. This flattens your lower back against the mattress by slightly tilting your pelvis. Same principle, different placement.

If you sleep on your stomach: honestly, just try to stop. Stomach sleeping torques your neck and flattens your lumbar curve no matter what you do. It’s the worst sleep position for your spine. I was a stomach sleeper for years and switching to side-sleeping sucked for about two weeks, then it became normal.

My first night with the pillow

I wedged an old pillow between my knees, half-expecting it to be uncomfortable or to wake up with it on the floor. Neither happened. I woke up, stood up, and realized my back did not hurt. For the first time in months. I stood in the bathroom just kind of processing that.

It’s been over a year and I still use a pillow every night. When I travel and the hotel only has two pillows (one for my head, one for between my knees), I will literally use a rolled-up towel or a sweatshirt. I will not sleep without knee support anymore. The difference is that noticeable.

• Side sleepers: Pillow between knees — keeps pelvis neutral, spine straight
• Back sleepers: Pillow under knees — flattens lower back against mattress
• Pillow thickness: Knees roughly hip-width apart — fold if too thick
• Travel: Rolled towel or sweatshirt works in a pinch