Does a Pillow Between Your Knees Help Back Pain?

Your top leg is the reason it helps

On your side, your top leg has nowhere to rest except forwards and down. Dr Ambreen Chohan, a senior research fellow at the University of Lancashire, answered our questions on side sleeping and back pain in October 2026, and this is how she explains it:

With nothing between your knees, your top knee slides forward until it lands on the mattress in front of you, and your pelvis rolls with it while your shoulders stay put. Your lower back is the bit in the middle that turns, and you hold that twist for hours without knowing. A pillow between your knees keeps the top leg up and level, so your hips stay stacked. Dr Chohan adds that research has long recommended a curled, foetal position for the same reason: to keep the lower back neutral. I think of the pillow as the thing that keeps the curl in place once you're asleep. My partner sleeps on their back and has never once had to think about where their knees go, which I find faintly unfair.

The research tested a purpose-made cushion, and it held up in a trial

A better angle at the pelvis is one thing, and a back that feels better is the part you care about. So Dr Chohan's group went on to put the cushion through a trial with people who had back pain, and she summed up the result:

I want you to know exactly where that comes from. Both studies were run by Dr Chohan's own research group, and both tested one purpose-made alignment cushion in adults with what doctors call mechanical low back pain. In the trial, 71 adults who'd had back pain for at least three months were split into two groups for four weeks: one group carried on with standard care, and the other had the same care plus the cushion. That's why I take the tip seriously. Research on knees and cushions will never make the evening news, and I'm glad somebody did it. It also means the pillow goes on top of whatever your GP or physio has already told you, and the NHS advice still stands: stay active, carry on with your daily activities, and do not stay in bed for long periods.

Start with an old thin pillow tonight

You can find out whether this suits you without buying a thing. Dr Chohan's advice for side sleepers is practical, and cheap:

The old thin pillow is her practical suggestion; the trial was about the purpose-made cushion. I'd still start with the pillow, because it costs nothing and you've almost certainly got one: the flat one at the back of the airing cupboard that you keep meaning to throw out. Thin is the point. You want your knees roughly hip-width apart with the top leg resting level, so leave the plump pillow from your bed where it is, since a fat one props the top knee higher than your hip. An NHS trust's lower back pain advice lists this among its comfortable positions: "Lie on your side with a pillow between your knees".¹ To set it up:

1. Lie on your usual side with your knees bent a little. A relaxed half-curl, the semi-foetal position Dr Chohan describes, suits this better than legs held dead straight.
2. Slide the thin pillow between your knees. Your usual one stays under your head, and the thin one is only for your legs.
3. Check your top knee. It should sit roughly level with your hip and rest on the pillow, with your knees about hip-width apart, instead of sliding forward onto the mattress.
4. Take it with you when you turn. If you roll onto your other side in the night, move the pillow across too.

Fair warning: it will escape. Some mornings you'll find it on the floor or squashed down by your ankles, and that's no reason to give up on it. Put it back and settle the way you started; keeping a pillow between two knees all night is a faff for the first few nights.

Give it a fair run of nights before you judge it

Judging the pillow on night one is like judging new shoes in the shop. You're changing how you lie for hours at a time, so I'd give it at least a week, with the same setup every night so you're comparing like with like. These checks are about whether the pillow is doing its job, so tick them as you go:

• The pillow was still between your knees when you woke, or you put it back each time it went
• Your top knee rests on the pillow, not on the mattress in front of you
• The pillow is thin enough that your knees sit about hip-width apart
• You moved it across when you turned onto your other side
• You've given it at least seven nights in a row
• You've carried on with whatever your GP or physio already advised

The mattress matters too, for a different reason. A mattress can't hold your top leg up; what it changes is how far your shoulder and hip sink in when you lie on your side. My rule is a notch softer for a side sleeper than for a back sleeper, with a softer comfort layer over a supportive core so the hip sinks in a little. If your hip feels like it's resting on the floor, our guide to mattresses for side sleepers is the next place to look, once the pillow has had its week.

When to see a GP about back pain at night

A pillow changes how you lie, and that's all it's for. The NHS says back pain usually improves within a few weeks, and one of the reasons it gives to see a GP is pain that "does not improve after resting or is worse at night".² Night is exactly when you'll be trying the pillow, so if the pain wakes you or gets worse when you lie down, book the appointment instead of spending weeks rearranging pillows. In many areas you may be able to get help, such as physiotherapy, from NHS community musculoskeletal (MSK) services without needing a referral from a GP.

Questions people ask about a pillow between the knees