Side sleeping is the most common position and the most demanding on a mattress. Lying on your back spreads your weight across a large area. Lying on your side puts nearly all of it through two narrow points — the shoulder and the hip.
That changes what the mattress has to do. It needs to let those two points sink enough to keep your spine straight, without letting your pelvis drop below the line. This covers the firmness and thickness that requires, how your weight shifts the answer, and a simple test for whether your current mattress is getting it wrong.
The Short Answer
Medium-soft to medium, in a mattress 25 to 30 cm thick, with a comfort layer deep enough for your shoulder to sink into without your hips following it down.
Side sleepers are the one group for whom firmer is usually wrong. If you take one thing from this: your shoulder needs somewhere to go.
"Lying on your side, the mattress has two jobs at once — let the shoulder and hip in, keep everything between them level. Firmness that gets one right often gets the other wrong."
Why Side Sleepers Need Something Different
On your back, weight distributes across shoulders, ribcage, pelvis and legs. On your side, the same weight passes through a shoulder joint and a hip joint, each only a few centimetres across.
Pressure concentrates there, and if the surface will not yield, blood flow reduces and the joint aches. That is where numb arms, sore shoulders and hip discomfort come from — not from weakness, but from a surface that refuses to accommodate the two hardest points on your body.
Alignment is the second requirement, and it pulls in the opposite direction. From neck to tailbone your spine should stay roughly straight when viewed from behind. If the mattress is too soft, your hips sink further than your shoulders and your spine bends at the waist. Muscles then work all night to stabilise it, which is why you can sleep eight hours and wake up tired. More in why side sleeping can lead to pain over time.
Firmness: Where to Start
Most side sleepers land between medium-soft and medium. Firmer than that and the shoulder cannot sink; softer and the pelvis drops.
| Your weight | Start at | Why |
|---|---|---|
| Under 60 kg | Soft to medium-soft | You compress less, so you need more give |
| 60–90 kg | Medium-soft to medium | Standard range |
| Over 90 kg | Medium to medium-firm | Needs a stronger core to stop the hips dropping |
Heavier side sleepers need a firmer support core but still a soft comfort layer — those are different layers doing different jobs.
That last point is the one most people miss. "Firmer" for a heavier side sleeper means a denser core underneath, not a harder surface on top. Get it wrong and you solve the sinking problem while creating a shoulder problem. Background in how firmness affects sleep comfort.
Thickness: 25 to 30 cm
Thickness matters more for side sleepers than for anyone else, because you need physical depth for the shoulder to travel into before it meets the support core.
Below about 20 cm there is usually not enough comfort layer, and you feel the firm core through it. Between 25 and 30 cm you get a comfort layer deep enough to accept the shoulder plus a support layer thick enough to hold the pelvis level. In Indonesian sizing, mattresses are commonly sold in 20, 25 and 30 cm depths — for side sleeping, 25 cm is a sensible minimum and 30 cm suits heavier sleepers.
Which Material
Memory foam contours most closely and gives the best raw pressure relief, which is why it is so often recommended for side sleepers. The trade-offs are heat and slow response — it releases you gradually, which is noticeable if you turn over often.
Latex adapts more gently and stays responsive, so it is easier to move on and far cooler. A softer latex comfort layer gives most side sleepers the relief they need while keeping airflow — which in this climate matters. Comparison in latex vs memory foam.
Hybrids put foam or latex over a coil base, combining surface cushioning with airflow and edge support.
The best construction for side sleeping is usually a soft comfort layer over a firm core — the two requirements handled separately rather than compromised into one firmness. Categories in the guide to mattress types.
The Shoulder Test
A two-minute check on the mattress you already own.
Lie on your side and check
- Does your shoulder feel compressed or supported?
- Is there a gap under your waist?
- Is your neck level with your spine?
- Do your hips sit lower than your shoulders?
- Does your arm go numb within minutes?
- Do you keep rolling onto your back?
Have someone look at you from behind. Your spine should run roughly parallel to the mattress. Curving upward at the waist means too firm. Sagging downward at the hips means too soft.
Waking with shoulder pain, hip soreness or a numb arm, or finding you cannot stay on your side, all point the same way — alongside the broader signals in 8 signs it is time to replace your mattress.
Two Fixes That Are Not the Mattress
A pillow between your knees. The cheapest meaningful improvement available to a side sleeper. It stops the upper leg dragging the pelvis forward and rotating the lower spine, which relieves a surprising amount of lower back and hip discomfort. A bolster does this job well.
Pillow height under your head. Side sleepers need a higher loft than back sleepers, because there is a wider gap between the mattress and your head — the width of your shoulder. Too flat and your neck bends downward all night. If your pain is in the neck and upper shoulder rather than the hip, test the pillow before blaming the mattress.
If back pain is the main symptom, choosing a mattress for back pain covers how the two interact.
Frequently Asked Questions
What firmness is best for side sleepers?
Medium-soft to medium for most people. Adjust softer under about 60 kg and firmer over about 90 kg — but for heavier sleepers, firmer should mean a denser support core, not a harder surface.
Do side sleepers need a firm mattress?
Generally no, and this is the most common mistake. A firm surface prevents the shoulder and hip from sinking, which concentrates pressure exactly where side sleepers already carry the most of it.
Why does my arm go numb when I sleep on my side?
Usually because the mattress will not let your shoulder sink, so your body weight compresses the joint and restricts circulation. A deeper, softer comfort layer normally resolves it.
How thick should a mattress be for a side sleeper?
Around 25 to 30 cm. You need enough comfort layer for the shoulder to travel into, plus enough support underneath to keep the hips level. Below about 20 cm there is rarely enough of either.
Does a pillow between the knees actually help?
Yes, and it is the cheapest fix on this list. It keeps the pelvis from rotating forward, which reduces twist through the lower spine and eases both hip and lower back discomfort.
Soft Where Your Shoulder Is, Firm Underneath
Rearrangeable layers let you set a softer comfort layer over a firm core — and set it differently on your partner's side if they sleep another way.
Explore the Original Mattress →Give the Shoulder Somewhere to Go
Side sleeping asks two opposite things of a mattress: yield at the shoulder and hip, hold firm everywhere between them. A single firmness rarely does both well, which is why layer construction matters more here than in any other position.
Start medium-soft, aim for 25 to 30 cm of depth, and adjust for your weight — remembering that heavier means a stronger core, not a harder top.
Then add a pillow between your knees and check your pillow height. Both cost almost nothing and both change how the mattress performs.