For most people, the left side is the better side to sleep on. Left-side sleeping benefits digestion, reduces acid reflux, and supports better blood flow. That said, certain conditions like heart failure can make right-side sleeping the smarter choice. More than 60% of adults already sleep on their side, so the real question for most people isn’t whether to side-sleep, but which side to favor.
Why Left-Side Sleeping Wins for Digestion
The biggest advantage of left-side sleeping comes down to anatomy. Your stomach naturally curves to the left, so when you lie on that side, gravity keeps stomach acid pooled away from the opening to your esophagus. The American Gastroenterological Association specifically recommends the left-side position for people who experience nighttime acid reflux, because it measurably reduces the amount of acid that reaches the esophagus.
Sleeping on your right side does the opposite. It positions the stomach in a way that makes it easier for acid to flow upward, increasing reflux episodes. If you deal with heartburn at night, even occasionally, switching to your left side is one of the simplest changes you can make.
Left Side Is Best During Pregnancy
Pregnant women, especially in the second and third trimesters, benefit most from left-side sleeping. This position allows maximum blood flow to the baby and improves kidney function, which helps reduce swelling in the legs and feet. Back sleeping becomes a problem during pregnancy because the weight of the uterus presses on the inferior vena cava, the large vein that returns blood from your lower body to your heart. That compression can reduce blood flow to both you and the fetus.
The left side is preferred over the right because it keeps the uterus from pressing on the liver (which sits on your right side) and optimizes circulation through the placenta. If you find yourself waking up on your back, don’t panic. Just roll back to your left side and use a pillow between your knees for support.
When Right-Side Sleeping Is Better
People with heart failure are the main exception to the left-side rule. Sleeping on the left side often worsens shortness of breath in people with this condition, because the heart shifts slightly under gravity and creates a sensation of pressure or difficulty breathing. Many people with heart failure naturally prefer their right side for this reason.
If you have no heart conditions, left-side sleeping places no meaningful extra strain on your heart. The preference for right-side sleeping applies specifically to people managing heart failure symptoms.
Side Sleeping and Brain Waste Clearance
Your brain has its own waste-removal system that’s most active during sleep. A study published in the Journal of Neuroscience found that this system, which clears proteins associated with Alzheimer’s disease, works most efficiently when the body is in a lateral (side) position compared to sleeping on the back or stomach. The researchers used imaging and radioactive tracers in animal models to measure how quickly waste products moved through brain tissue, and lateral positioning consistently showed the best transport.
This research doesn’t distinguish between left and right sides for brain health purposes. The key takeaway is that side sleeping in general appears to give your brain the best chance to clean house overnight.
Side Sleeping Helps With Sleep Apnea
If you snore heavily or have obstructive sleep apnea, side sleeping on either side is significantly better than sleeping on your back. When you lie face-up, gravity pulls the tongue and soft tissues toward the back of the throat, narrowing or blocking the airway. Rolling onto your side opens that space back up. A meta-analysis found that switching from back sleeping to side sleeping reduced the number of breathing interruptions per hour by about 54%. That’s a substantial improvement, sometimes enough to bring mild cases into a normal range without any other treatment.
Protecting Your Shoulder and Hip
The downside of side sleeping is pressure. Your full body weight concentrates on one shoulder and one hip, and over time this can aggravate joint pain or bursitis. If you already have shoulder bursitis, sleeping on that shoulder will reliably make it worse.
A few adjustments make a big difference. A pillow height of about 4 inches provides the best spinal alignment for most side sleepers, keeping your head level with your spine rather than cranked up or drooping down. One small study found that this height minimized muscle activity in the neck, meaning your muscles aren’t working overtime to compensate for poor positioning. Placing a pillow between your knees keeps your hips stacked and prevents your top leg from pulling your spine out of alignment. If you consistently wake up with shoulder pain on the side you sleep on, alternating sides through the night or using a softer mattress can help distribute the pressure.
Quick Comparison: Left vs. Right vs. Back
- Left side: Best for acid reflux, digestion, pregnancy, and general circulation. The default recommendation for most people.
- Right side: Preferred for people with heart failure. Still good for sleep apnea and brain health. Slightly worse for reflux.
- Back: Good for spinal alignment if you don’t snore, but worst for sleep apnea, pregnancy, and reflux.
How to Train Yourself to Stay on One Side
Most people shift positions dozens of times per night, so “sleeping on your left side” really means starting on your left and spending the majority of the night there. A body pillow running the length of your torso gives you something to lean into, making it harder to unconsciously roll onto your back. Some people place a tennis ball in a pocket sewn to the back of a sleep shirt to make back sleeping uncomfortable enough that they naturally stay on their side.
If you’re switching sides for a specific reason, like managing reflux or a new pregnancy, give yourself a couple of weeks. The first few nights in an unfamiliar position can feel awkward enough to disrupt sleep, but most people adjust within one to two weeks and stop noticing the difference.