For most people, sleeping on either side is fine, and both are far better than sleeping on your back or stomach. The “best” side depends on your specific health situation. If you’re pregnant past 28 weeks, have heart failure, or deal with acid reflux, the distinction matters. For everyone else, the side that feels comfortable and lets you stay asleep is the right choice.
Why Side Sleeping Beats Back and Stomach
Before worrying about left versus right, the bigger win is simply sleeping on your side at all. Your brain has a waste-clearance system that works best during sleep, flushing out harmful proteins linked to cognitive decline. A study published in the Journal of Neuroscience found that this system operates most efficiently in the lateral (side-lying) position compared to sleeping on your back or stomach. The researchers observed significantly better clearance of amyloid-beta, one of the proteins that accumulates in Alzheimer’s disease, when subjects were lying on their side.
Side sleeping also makes a dramatic difference for people with obstructive sleep apnea. In position-dependent cases, the number of breathing interruptions per hour drops from roughly 38 events on your back to about 10 events on either side. That’s nearly a fourfold improvement, simply from rolling over. Both the left and right sides produced essentially identical results.
When the Right Side Is Better
If you have heart failure, you may already notice that sleeping on your right side feels more comfortable. Research confirms this isn’t imagined. In one echocardiographic study, 54% of heart failure patients naturally preferred the right side, while 40% actively avoided the left. The reason: when you lie on your left, your heart shifts closer to the chest wall due to gravity, which can reduce how effectively both ventricles pump blood. On the right side, measurements of left and right ventricular function were significantly higher compared to lying on the left or on your back.
People with acid reflux sometimes get conflicting advice here. Sleeping on the right side can worsen reflux symptoms because of how the stomach and esophagus are positioned. If you have both heart failure and reflux, that’s a conversation worth having with your cardiologist.
When the Left Side Is Better
The left side has long been recommended during pregnancy, and there’s solid physiology behind it. As the uterus grows, it can compress the large vein (the inferior vena cava) that returns blood from your lower body to your heart. Lying on your back makes this compression worse, reducing cardiac output and blood flow to the placenta. Lying on your left side takes the weight off that vein entirely.
That said, the guidance has loosened in recent years. Current evidence shows that before 28 weeks, your sleeping position doesn’t affect pregnancy outcomes at all. After 28 weeks, the key recommendation is to avoid falling asleep on your back. Going to sleep on your right side appears to be equally safe as the left. So if you’re pregnant and can’t get comfortable on your left, the right side is a perfectly good alternative. The important thing is to not sleep flat on your back in the third trimester.
If you deal with acid reflux or GERD, the left side is generally the better option. Your stomach sits slightly to the left of your midline, and the junction between your esophagus and stomach is positioned so that left-side sleeping keeps stomach acid below that junction. On the right side, acid can pool closer to the opening and travel upward more easily.
Quick Guide by Condition
- Heart failure: Right side typically allows better cardiac output
- Pregnancy (after 28 weeks): Either side works; avoid sleeping on your back
- Acid reflux or GERD: Left side keeps stomach acid lower
- Sleep apnea: Either side, both reduce breathing interruptions equally
- No specific health concerns: Whichever side you find comfortable
Protecting Your Shoulder and Hip
The most common complaint from side sleepers isn’t about organs. It’s about waking up with a sore shoulder or aching hip. When you sleep on one side, your full body weight presses into a relatively small area of your shoulder joint, and your top leg can pull your pelvis out of alignment.
A pillow between your knees keeps your spine straight and takes strain off both your hip and lower back. If you have shoulder pain, sleep with the painful shoulder facing the ceiling rather than the mattress. Build a “pillow wall” in front of your chest and rest your upper arm on it, keeping the arm at roughly the same height as your body. This prevents the shoulder from rolling forward and compressing the joint. If you must sleep on the affected side, try placing a flat pillow at waist height with a gap between it and your head pillow, creating a channel for your arm so it isn’t trapped under your weight.
Getting Your Pillow Height Right
Side sleepers need a thicker pillow than back sleepers because of the gap between your head and the mattress created by your shoulder width. If your pillow is too thin, your head tilts downward and your neck bends. Too thick, and it pushes your head upward. Either way, you wake up stiff.
Most side sleepers do best with a medium or high loft pillow, roughly 5.5 to 7.5 inches thick. The goal is to fill the triangular space between your head, neck, and the mattress so your spine stays in a straight horizontal line. People with broader shoulders generally need the higher end of that range. If you’re not sure, lie on your side and have someone check whether your nose lines up with the center of your chest. If your head is tilting in either direction, adjust your pillow.
Your mattress matters too. A surface that’s too firm won’t let your shoulder and hip sink in enough, creating pressure points. One that’s too soft lets your midsection sag. Medium-firm mattresses tend to work best for side sleepers because they cushion the pressure points while still supporting the curve of your waist.