For most people, the left side is the best side to sleep on. It reduces acid reflux, improves circulation, and may even help your brain clear waste more efficiently during sleep. That said, certain health conditions can make the right side a better choice, and how you position your body matters just as much as which side you pick.
Why the Left Side Is Generally Best
The left side gets the most consistent recommendation across sleep research, and the reasons come down to anatomy. Your stomach curves to the left, so when you sleep on that side, the stomach sits below the esophagus. Gravity keeps stomach acid where it belongs rather than letting it creep upward. This is why left-side sleeping is one of the simplest ways to reduce nighttime acid reflux, especially if you deal with heartburn regularly.
The left side also supports better blood flow. Your body’s largest vein, which returns blood from your lower body to your heart, runs along the right side of your spine. Sleeping on your left keeps your body weight off that vein, allowing blood to circulate more freely. This is why the left side is specifically recommended during pregnancy: it allows maximum blood flow to the baby and improves kidney function. Sleeping on the back during pregnancy puts pressure on that major vein and can reduce circulation to both mother and baby.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that’s most active while you sleep. It flushes out metabolic byproducts, including proteins associated with Alzheimer’s disease. A study published in the Journal of Neuroscience found that this system works most efficiently when sleeping on your side compared to sleeping on your back or stomach. Side sleeping appears to be the position humans (and most mammals) naturally evolved to optimize this overnight brain cleaning process. The research was conducted in animals, so the exact benefit in humans is still being confirmed, but it adds another point in favor of lateral sleeping in general.
When the Right Side Is Better
People with heart failure often find left-side sleeping uncomfortable. Lying on the left can worsen shortness of breath in people whose hearts don’t pump effectively, so many naturally shift to their right side. If you have a heart condition and notice discomfort or breathlessness on your left, sleeping on your right is a reasonable alternative. The circulation benefits of left-side sleeping matter less than being able to breathe comfortably through the night.
Shoulder pain is another common reason to switch sides. If your left shoulder is injured or inflamed, sleeping on it will compress the joint and make things worse. The simplest fix is to sleep on your non-painful side and use pillows to keep yourself from rolling over during the night.
How to Set Up Your Pillows
Choosing the right side is only half the equation. Poor pillow placement can leave you with neck pain, back stiffness, or shoulder problems regardless of which side you sleep on.
Your head pillow has one job: filling the gap between your ear and the mattress so your neck stays in a straight line with your spine. If you imagine a line running from your forehead to your chin, it should be parallel to the bed. A pillow that’s too thin lets your head droop, stretching the muscles along your neck and top shoulder. A pillow that’s too thick pushes your head upward, compressing the shoulder joint underneath you.
A pillow between your knees is equally important. Without one, your top leg drops and pulls your pelvis out of alignment, creating a twist that runs from your lower back up to your neck. A firm pillow between the knees keeps your hips, pelvis, and spine stacked in a neutral position. A full-length body pillow works well if you tend to shift around at night.
If you deal with shoulder tension, try hugging a medium-firm pillow against your chest. Resting your top arm on it prevents the shoulder from rolling forward and keeps the joint in a more open position. This lets the muscles around your shoulder blade and rotator cuff actually relax instead of working all night to stabilize the arm.
The Tradeoff: Side Sleeping and Wrinkles
There is one cosmetic downside to side sleeping. When your face presses against a pillow for hours, the repeated compression creates sleep wrinkles over time, most visibly on the forehead, cheeks, and lips. About 65% of people sleep on their side, so this affects most of us to some degree. These wrinkles get worse with age as skin thins and loses elasticity. Unlike expression lines, sleep wrinkles can’t be treated with Botox because they aren’t caused by muscle contractions.
Back sleeping avoids facial compression entirely, which is why dermatologists often call it the ideal position for skin. But for most people, the digestive, circulatory, and spinal benefits of side sleeping outweigh the cosmetic concern. If wrinkles are a priority, silk or satin pillowcases create less friction than cotton, though they won’t eliminate the compression effect completely.
Mattress Firmness for Side Sleepers
Side sleeping concentrates your body weight on a smaller surface area than back sleeping, so your mattress needs to accommodate that pressure. A medium-firm mattress with a contouring top layer, like memory foam or a pillow top, tends to work best. The softer surface lets your shoulder and hip sink in enough to relieve pressure, while the firmer base keeps your spine from sagging. A mattress that’s too firm will create pressure points at the shoulder and hip. One that’s too soft lets your midsection dip, pulling your spine out of alignment.