The Side Sleeper
Knee pillow guides · Start here

Why sleep with a pillow between your knees?

Nell, the Side Sleeper
Updated · Data collected · How we research

When you lie on your side, your top leg slides forward and down, and it pulls your hip, pelvis and lower back round with it. A pillow between your knees stops that slide, so you stay lined up from your shoulders to your knees all night.

It's cheap, it's what the NHS suggests for some kinds of hip and pelvic pain, and you can try it tonight with a pillow you already own.

Bare legs stretched out on rumpled white sheets.
Photo: rawpixel

What is a knee pillow?

A small, firm pillow, usually memory foam, made to sit between your knees while you sleep on your side. The typical one is about 10 by 7.9 inches, roughly the size of a paperback, and 5 to 7.2 inches thick. Most cost around $31.

The shapes differ in how they cradle your knees and how much of your leg they hold. Hourglass and heart shapes have a dip on each side for your knees. Long and full-leg pillows run down the thigh or to the ankle. Which shape suits you explains the differences, and the comparison table has every one.

What happens without one

Your hips are wider than your knees. Lying on your side with nothing between them, your top knee has nowhere to rest but on the bottom one, so your top thigh slopes down. That turns your pelvis forward and puts a slight twist in your lower back, which you then hold for hours.

Without a pillow: The top knee drops onto the bottom one. The top thigh angles down, the pelvis rolls forward with it and the lower back twists.

✗ Without a pillow
The top knee drops onto the bottom one. The top thigh angles down, the pelvis rolls forward with it and the lower back twists.

With a knee pillow: The knees are held a hip-width apart. The top thigh stays level, so the pelvis stays square and the spine stays straight.

✓ With a knee pillow
The knees are held a hip-width apart. The top thigh stays level, so the pelvis stays square and the spine stays straight.

Lying on your side with knees bent, top leg drawn above the bottom one. The dashed line is the height of your top hip; the line through the pelvis shows whether it stays square. Schematic, not to scale.

A pillow between the knees fills that gap. Your top thigh stays level with your hip, and nothing is pulling your pelvis round.

Who it helps

Achy top hip

For pain on the outside of the hip, the NHS suggests lying on your good side "with a pillow between your legs to keep them in line with your hip joints".

Lower back

A specialist at the Hospital for Special Surgery in New York suggests side sleepers with low back pain try "a small pillow between your knees to help even out your pelvis".

Pregnancy

For pelvic pain in pregnancy, the NHS lists "put a pillow between your legs for extra support in bed" among the things to try.

Bony knees

Even without any aches, it stops your knees and ankles pressing bone on bone, which is reason enough for plenty of side sleepers.

What the research says

Less than you'd think. A 2019 review in BMJ Open looked for studies linking sleep position and back or neck pain and found only four. Side-lying was generally linked to fewer symptoms on waking, but the authors concluded there weren't enough high-quality studies to say much more. One small pilot study in 20 women in their sixties found that four weeks of advice on sleeping position, including where to put pillows, left them with less back pain than a group who got no advice. It tested the advice as a whole, not the knee pillow on its own.

So the case for a knee pillow rests on simple mechanics and on mainstream advice rather than on trials. That's no reason to skip it. It's cheap and low-risk, and your own week of trying it tells you more than a study could.

Try it tonight, for free

  1. Fold a regular pillow in half, or roll up a bath towel, and put it between your knees as you lie on your usual side.
  2. Bend your knees a little, the way you'd naturally curl up.
  3. Check your top thigh. It should run about level with the mattress, not slope down to your other knee or get propped up above your hip.
  4. Give it a week. Notice how your hip and lower back feel in the morning.

If it helps, but the pillow is flat or on the floor by morning, that's when a proper knee pillow earns its money: it holds its height all night, and some come with a strap. Start with how thick it should be, then compare the best sellers.

When it's not the answer

Likely to suit you

  • You sleep mostly on your side
  • Your top hip or lower back aches in the morning
  • Your knees knock together or feel bony

Look elsewhere if

  • It's the hip you lie on that hurts. That's mattress pressure, not alignment
  • You sleep mostly on your back or front
  • Pain wakes you most nights, whatever you try. See a doctor or physio

Questions people ask

Is it good to sleep with a pillow between your knees?

For most side sleepers it is a cheap, low-risk way to keep the top leg from dragging the hips and lower back out of line. The NHS suggests it for outer-hip pain and for pelvic pain in pregnancy. The research on it is thin, so the best test is a week of trying it with a pillow you already own.

Can a pillow between your knees make things worse?

Yes, if it is the wrong height. Too thick and it props the top leg up; too thin and the knee drops anyway. If your hip aches more with the pillow than without, the height is the first thing to check.

Do you need a knee pillow if you sleep on your back?

Not between the knees. On your back, a pillow goes under your knees instead, which the NHS also suggests for outer-hip pain. Any regular pillow does that job.

How long does it take to get used to sleeping with a knee pillow?

Any change of position feels odd for a few nights. If the same ache is still there after a week, the pillow is probably the wrong height or shape for you.

Sources