Side sleeping is the most common position and the one with the widest gap to fill, which makes it the position where pillow height matters most.
The Gap
Lying on your side, the distance from the mattress to the outside of your shoulder is what a pillow has to fill.
That gap is considerably larger than in any other position, and it varies with shoulder width and with how far the shoulder sinks into the mattress.
A pillow too low lets the head drop toward the shoulder, holding the neck bent sideways all night. That is the position that produces one-sided stiffness.
What It Needs
- The highest loft of the three positions
- Enough firmness that it does not collapse under the head
- Support right into the gap between neck and shoulder
- More again for broad shoulders
- Less on a soft mattress, where the shoulder sinks
Adjustable pillows run at their fullest setting for side sleeping, of the kind described at nuzzle-pillows.com, which is why the same product can suit three positions.
Firmness Matters Here
More than in other positions. A soft high pillow compresses under the weight of a head and becomes a low pillow within minutes.
What matters is the height it holds while you are lying on it, which is why side sleepers frequently do better with a firmer fill even if a softer one feels nicer to the hand.
A Pillow Between the Knees
The other half that nobody mentions. Side sleeping lets the upper leg drop forward, which rotates the pelvis and twists the lower back.
A pillow between the knees keeps the hips stacked and the spine in line. For anyone with hip or lower back discomfort, it frequently helps more than changing the head pillow.
The Shoulder Underneath
The shoulder you lie on takes the body’s weight, and a mattress too firm concentrates that pressure.
Shoulder pain on the side you sleep on is more often a mattress issue than a pillow one, and rotating which side you start on helps.
Which Side
Guidance on reflux commonly suggests the left side. There is no strong general reason to prefer one over the other otherwise.
Pregnancy has its own guidance, and a midwife is the right source for it.
Check From the Front
Have somebody look at you lying down. The head should be level, with the neck continuing the straight line of the spine rather than tilting.
Do Not Tuck an Arm Under
Sleeping with an arm under the pillow is usually compensating for a pillow that is too low.
It also compresses the shoulder and can cause numbness.
Give a Change Several Nights
A correct new height feels wrong at first.