· Léa Vasseur
Sleeping on your side
Why it demands more than the other positions
Lying on your side, your shoulder becomes a column holding your entire upper body off the mattress. That puts your ear four to six inches (10–15 cm) in the air with nothing underneath it, and it concentrates a large share of your body weight onto one joint.
On your back, the gap to fill is an inch or two and the load spreads across your whole back. On your side, the gap doubles or triples and the load funnels into one shoulder and one hip. Nothing about the position is wrong — it is simply less forgiving of a bed that is set up carelessly.
1. The head has to stay level
This is the whole of the pillow question. If the pillow is lower than your shoulder width, your head tips down toward the mattress and your neck bends sideways for seven hours — the direction it has the least range in. If it is higher, the head is levered up and the bend goes the other way.
The check takes thirty seconds: lie on your side, have someone photograph you head-on at bed level, and look at whether nose, chin and the notch between your collarbones fall on a straight line. The full method and what to do with the result.
Firmness matters here only after height. A firm pillow that is two inches too low holds your head rigidly in the wrong place, which is worse than a soft one that sags into roughly the right one.
2. Something between the knees
The most useful thing on this page, and it costs nothing to try tonight.
With your legs stacked, the upper leg slides forward and down, which rotates your pelvis toward the mattress. Your shoulders stay where they are, so your lower back twists between the two — all night. That twist is behind a lot of morning back stiffness that people attribute to the mattress.
A pillow between the knees keeps the upper leg level with the lower one and the pelvis square. It also stops your knees pressing on each other, which is its own source of waking up. A folded blanket does the job; a dedicated knee pillow does it without migrating.
3. The arm underneath
Everyone does something with the lower arm and none of the options are obviously right.
- Under the pillow. Adds an inch or two of height you did not choose, and changes it again when the arm goes numb and you move.
- Under the head. Your arm becomes part of the pillow, and the shoulder underneath takes the load of your skull as well as your torso.
- Stretched out in front. The most stable, and the one a shaped pillow can accommodate — a notch between the raised side and the middle gives the arm room without moving the surface your head is on.
If your hand goes numb every night, that is usually the arm being compressed rather than anything neurological, and moving it in front of you is the first thing to try. Numbness that persists after you get up is a different matter and worth mentioning to a doctor.
4. The shoulder underneath
Side sleepers who wake with a sore shoulder often blame the mattress. The pillow is worth checking first, because it is cheaper to change and it acts on the same joint.
A pillow that is too low lets the head drop, which shifts more of the upper-body load down through the shoulder that is already carrying you. Raising the head to level redistributes it. A knee pillow helps here too, by stopping the torso rotating forward and dragging weight onto the leading shoulder.
If the mattress is very firm, the shoulder cannot sink into it at all and takes the pressure directly. A topper is the usual answer to that, and it is a mattress problem rather than a pillow one.
Which side, and does it matter
People ask, and for most healthy adults it makes little practical difference to how you sleep. Left-side sleeping is often suggested for reflux and during pregnancy; that is a conversation with a doctor or a midwife rather than a bedding decision, and it is outside what a pillow page should be advising on.
What does matter: if you always sleep on the same side and always wake stiff on that side, alternate for a week and see whether it follows you. If it does, the cause is the setup. If it stays on the original side, something else is going on.
If you change position during the night
Almost everyone does, and it is the reason a pillow that is perfect on your side leaves you with a stiff neck on the nights you spend on your back. The side height is roughly double the back height, and one flat object cannot be both.
A pillow with two sides of different heights settles it without asking you to decide: taller side for side sleeping, lower for your back, and you turn it over. Which side to start on, and how long to give each. If you mostly sleep on your back, that guide is here.