For most people, the left side is the best side to sleep on. It helps with digestion, reduces acid reflux symptoms, and keeps your spine in a comfortable position. But the ideal side depends on your specific health situation. People with heart failure, shoulder pain, or hip problems may do better on their right side or on their back. More than 60% of adults already sleep on their side, making it the most popular sleep position by a wide margin.
Why the Left Side Gets Top Marks
The main advantage of left-side sleeping comes down to anatomy. Your stomach naturally curves to the left, so when you lie on that side, the junction between your esophagus and stomach sits higher than the pool of gastric acid below. Gravity helps keep acid where it belongs. When you sleep on your right side or your back, that acid can creep upward more easily, leading to heartburn and reflux symptoms during the night.
The frequency of reflux episodes doesn’t change dramatically between positions, but the duration does. Acid clears from the esophagus faster when you’re on your left side, which means less irritation and better sleep quality overall. If you deal with GERD or even occasional heartburn after a late meal, switching to your left side is one of the simplest changes you can make.
The Best Position During Pregnancy
Side sleeping becomes especially important during the third trimester. Lying on your back puts pressure on the inferior vena cava, the large vein that returns blood from your lower body to your heart. That pressure can reduce blood flow to the baby and leave you feeling dizzy or lightheaded.
The left side is preferred during pregnancy because it allows the greatest blood flow to the placenta and baby. That said, waking up on your right side isn’t cause for alarm. The goal is to fall asleep on your left and avoid spending long stretches flat on your back.
Snoring and Sleep Apnea
If you snore or have obstructive sleep apnea, side sleeping on either side can help. Apnea episodes tend to cluster when people lie on their backs, because gravity pulls the tongue and soft tissues toward the airway. Rolling to either side opens that space up. A Cochrane review found that people who used positional therapy (staying off their backs) experienced about 7 fewer breathing interruptions per hour compared to those who didn’t. That’s a meaningful improvement, though not quite as effective as a CPAP machine, which reduced events by about 6 more per hour on top of positional therapy alone.
For mild to moderate sleep apnea, side sleeping can be enough to bring symptoms under control. For severe cases, it works best as a complement to other treatments rather than a replacement.
When Right-Side Sleeping Is Better
People with heart failure often find that sleeping on the left side makes them feel short of breath. The likely reason is that left-side positioning shifts the heart slightly, increasing pressure in ways a weakened heart struggles to manage. Many heart failure patients naturally gravitate to their right side for comfort, and that instinct is well supported by clinical observation.
If you have shoulder pain on your left side, sleeping on it will only make things worse. Rotator cuff injuries and shoulder bursitis are aggravated by the sustained compression of side sleeping. In those cases, switching to your right side or sleeping on your back with a pillow supporting the affected arm is a better choice.
Protecting Your Spine and Joints
Side sleeping is generally good for spinal alignment, but only if your setup is right. The gap between your head and the mattress is wider when you’re on your side than when you’re on your back, so you need a thicker pillow to keep your neck straight. A pillow that’s too thin lets your head drop, straining the muscles on one side of your neck. One that’s too thick pushes your head upward, creating the same problem in reverse. The ideal pillow fills the space so your head, neck, and spine form a straight horizontal line.
Your hips and knees matter too. Without support, your top leg tends to fall forward and pull your pelvis out of alignment, which can create lower back pain or irritate the hip you’re lying on. Placing a pillow or rolled towel between your knees keeps your hips stacked and takes pressure off both joints. This is a small adjustment that makes a noticeable difference, especially if you wake up with stiffness in your lower back or outer hip.
Effects on Your Skin
There is one downside to side sleeping that back sleepers avoid entirely: compression wrinkles. When your face presses against a pillow for hours, the mechanical pressure creates creases that, over years, can become permanent lines on the forehead, cheeks, and around the lips. These wrinkles tend to run perpendicular to the expression lines caused by smiling or squinting, and because they aren’t caused by muscle contractions, treatments like Botox don’t help.
Studies estimate that people spend about 65% of the night on their side, which adds up to thousands of hours of facial compression over a lifetime. If you consistently sleep on the same side, you may notice more pronounced lines on that side of your face. Sleeping on your back eliminates this pressure entirely, but as many dermatologists acknowledge, consciously changing a lifelong sleep habit is extremely difficult. Silk pillowcases and specialty pillows that reduce facial contact can help, but they won’t eliminate the effect completely.
Choosing the Right Side for You
There’s no single correct answer that works for everyone. Here’s a quick breakdown based on common health concerns:
- Acid reflux or GERD: Left side, to keep stomach acid below the esophagus.
- Pregnancy (third trimester): Left side, to maximize blood flow to the baby.
- Snoring or sleep apnea: Either side, to keep the airway open.
- Heart failure: Right side, to reduce shortness of breath.
- Shoulder or hip pain: The opposite side from the painful joint, or on your back with pillow support.
- Wrinkle prevention: Back sleeping is the only position that avoids facial compression.
If none of these conditions apply to you, the left side is a solid default. It supports digestion, keeps your airway open, and works well for most body types. The most important thing is that your pillow height matches your sleeping position and that your joints are supported. A comfortable setup you can maintain through the night will always matter more than choosing the theoretically perfect side.