Side sleeping is the most common sleeping position among UK adults. It is also the one most poorly served by the average pillow sitting on a supermarket shelf. The geometry of this position makes specific demands that a soft, mid-height, one-size-for-everyone pillow will not reliably meet. So here is what side sleepers actually need, answered directly.
What is a pillow supposed to do when you sleep on your side?
Fill the gap. That is the whole job. When you lie on your side, there is a lateral space between your shoulder and the side of your head. Your spine needs to run in a neutral horizontal line from your lower back through your neck. The pillow's only task is to bridge that shoulder-to-head gap so your neck is neither tilted downward nor cranked upward.
If the pillow does not fill that gap fully, your neck drops toward the mattress. If it overfills it, your neck bends the other way. Either direction creates tension that builds over a full night.
How do I know if my current pillow is the right height?
Have someone look at you from behind while you lie on your side in your usual sleeping position. Your ears, shoulders, and hips should form a straight line. If your head tilts down toward the mattress, the pillow is too low. If your head tips away from the mattress, it is too high.
Waking with a stiff neck, tension across the top of one shoulder, or a dull ache through the base of the skull on one side are the usual signs of a height mismatch. They are easy to normalise over time, which is exactly why most people tolerate the wrong pillow for years without connecting the discomfort to the cause.
What happens when the pillow is too low?
Your neck drops into lateral flexion, meaning the muscles on one side are compressed and those on the other are held under stretch for hours at a time. The short result is a stiff neck in the morning. The longer result is disrupted sleep, because the body registers discomfort even when the conscious mind does not, and it shifts position in response. You may not remember waking, but you wake.
What about too high?
Different muscles, same principle. A pillow that stacks too thick pushes the head away from the shoulder, compressing the structures on the upper side of the neck. Side sleepers with broad shoulders who use a pillow designed for someone narrower are particularly vulnerable to this.
A pillow that is too high is the less obvious problem of the two, because it often feels comfortable at first. The discomfort tends to arrive later in the night or into the following day rather than immediately on waking.
Should a side sleeper use a firm or soft pillow?
Firmer, in most cases. A soft pillow compresses under the weight of the head and ends up sitting lower than it should, which puts you back in the too-low scenario. A pillow that holds its structure through the night is more likely to maintain the loft you need.
Shredded memory foam is well suited to this position because it provides structure without being rigid. The Martian Made StarFirm is built specifically for side sleepers, using high-density 50D shredded memory foam with cooling PE gel infusion for firm, consistent support throughout the night. The LunaCore Hybrid sits just below it on the firmness ladder, with 50% blue gel-infused shredded memory foam blended with hollow microfibre, and suits sleepers who want some structure with a little more give.
Does pillow width matter in this position?
More than most people realise. A standard UK pillow is 50x75cm. A super king pillow runs 50x90cm. The extra length makes no difference to support, but it does affect how the pillow interacts with your shoulder. If you are tall or broad-shouldered, you may find a shorter pillow rides up away from your shoulder through the night, because your shoulder is pushing against the edge of it rather than underneath it.
This is one of those things that sounds like a minor detail until you have experienced it, and then it is not minor at all.
What if I start on my side but wake up on my back?
A significant number of people do this, and it need not be a problem. If your pillow is set to the right height for your shoulder gap as a side sleeper, it will likely be a fraction too high when you roll onto your back, since back sleeping requires less loft. But most people who shift mid-sleep do so because they are comfortable, not because they are seeking a different support profile. The neck muscles relax with the change in position and most back sleepers need only a couple of centimetres less height than they do on their side.
The practical solution, if you sleep in both positions regularly, is an adjustable pillow set slightly toward the side-sleeping height, and used with a lower, flatter secondary pillow if you tend to stay on your back for long stretches.
What does adjustable fill have to do with this?
Quite a lot, and it is more relevant to side sleepers than to any other position, because shoulder width varies enormously between individuals. A pillow manufactured to a single loft is calibrated to one body type. Anyone whose shoulder gap is wider or narrower than the manufacturer's assumption is getting the wrong support, and there is no way to know which camp you are in until you sleep on it.
Every Martian Made pillow has removable and adjustable fill. This means the loft is not a fixed specification you either suit or do not. You remove fill until the pillow sits at the height that puts your neck in line with your spine. Or you add it back if you need more.
Most people do not know that adjustable fill is possible in a non-specialist pillow. It is not a feature marketed prominently because the category tends to sell on softness or material rather than precision, but for a side sleeper it is arguably the most useful thing a pillow can offer.
If you have been waking with neck stiffness for years and assumed it was just how things were, the first adjustment worth making is almost certainly the height of your pillow.
