Side-sleeping: The way we slept as we evolved
During the Miocene epoch (23 to 5 million years ago), apes began growing bigger. Around 14 to 18 million years ago, apes started building sleeping platforms in trees to support their increasing size and weight. This is when they switched from sleeping sitting up in trees to lying on their sides. Not only did the sleeping platforms support the apes more securely, but they also allowed the apes to get more deep, non-REM sleep—which scientists believe improved cognitive ability and contributed to their evolution.
Large primates and people in tribal cultures sleep on the ground in various side-sleeping positions with no pillow, as illustrated in this article by physiotherapist Michael Tetley. If you sleep on your side, you’re among the majority of civilized people as well—surveys estimate that around 70% of us sleep on our sides. Even though side-sleeping is instinctive to us, we need to be aware of how it can contribute to poor posture, tension, and pain.
The main thing to consider is the alignment of your spine while you’re sleeping. You spend around 8 hours every night in this position, so imagine standing in that same position all day. When you’re sleeping, your spine should be aligned from your tailbone all the way up to the top of your head.
If you sleep on your side, use a pillow that allows your neck to be straight. It shouldn’t be too full or too flat—it needs to be just right. Formed pillows that provide shaped support for your neck and head can be very comfortable, but whether or not you find one necessary is a personal choice.
Mattress firmness is very important for side sleepers. Your hips and shoulders must be able to sink down a bit so that your spine remains straight, as shown in the illustration below. Appropriate mattress firmness depends on body weight; the heavier you are, the firmer your mattress should be to provide the right amount of support. Side sleepers may find that putting a small pillow between their knees improves overall alignment and reduces lower back pain.