For most people, the left side is the best side to sleep on. It benefits digestion, reduces acid reflux, and supports your body’s natural waste-clearing processes. More than 60% of adults already sleep on their side, making it the most common sleep position, and small adjustments to which side you choose can make a meaningful difference for specific health conditions.
That said, the “best” side depends on your body. People with heart failure, shoulder pain, or other conditions may do better on the right. Here’s what the evidence says for each situation.
Why the Left Side Wins for Digestion
The anatomy behind this is straightforward. Your stomach sits slightly to the left of your midline, and when you sleep on your left side, the stomach rests below the esophagus. Gravity keeps stomach acid where it belongs. When you sleep on your right side or your back, acid can travel upward into the esophagus more easily, causing heartburn and reflux symptoms.
Researchers at Amsterdam UMC confirmed this in a study of 100 patients with acid reflux. Patients who slept on their left sides had less stomach acid in their esophagus compared to right-side or back sleeping. Even when acid did reach the esophagus, it drained back into the stomach more quickly in the left-side position. If you deal with GERD or nighttime heartburn, this single change can noticeably reduce symptoms.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that works primarily while you sleep, flushing out metabolic byproducts including the proteins linked to Alzheimer’s disease. A study published in The Journal of Neuroscience found that this system works most efficiently in the lateral (side-lying) position compared to sleeping on the back or stomach. The researchers noted that the popularity of side sleeping across species may have evolved specifically to optimize this waste clearance process.
The study was conducted in rodents, so direct translation to humans requires some caution. But the finding is consistent with what we know about fluid dynamics in the brain, and it adds another point in favor of side sleeping over back or stomach sleeping.
When the Right Side Is Better
People with heart failure often find that sleeping on the left side worsens shortness of breath. The likely reason is that the left-side position shifts the heart slightly, increasing the sensation of pressure on an already struggling organ. According to the American Heart Association, many heart failure patients naturally prefer their right side for this reason. If you have heart failure or feel chest discomfort on the left, switching to the right side is a reasonable choice.
Shoulder pain also dictates which side works. If your left shoulder is injured or inflamed, sleeping on it compresses the joint and makes pain worse. The same applies to the right. The simple rule: sleep on the opposite side of the affected shoulder. Place a pillow between your knees to keep your spine straight, and stack another pillow in front of your chest to support the injured arm, raising it slightly to reduce joint pressure.
Side Sleeping and Snoring
If you snore or have obstructive sleep apnea, side sleeping (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 the airway. Rolling onto your side opens that space up. The American Academy of Sleep Medicine recognizes positional therapy, which essentially means avoiding back sleeping, as a supplemental treatment for sleep apnea in people whose breathing events are worse in the supine position.
Not everyone with sleep apnea benefits equally from positional changes. A sleep study can confirm whether your episodes are position-dependent before you rely on side sleeping as a strategy.
Side Sleeping During Pregnancy
In later pregnancy, side sleeping helps prevent the weight of the uterus from compressing major blood vessels, specifically the aorta and the large vein that returns blood from the lower body to the heart. This compression can reduce blood flow to the placenta. Most pregnancy guidance recommends left-side sleeping in the third trimester for this reason.
An NIH-funded study found that through 30 weeks of pregnancy, sleeping on the back or either side did not appear to increase the risk of stillbirth, reduced birth size, or blood pressure complications. The researchers cautioned, however, that their data did not extend into late pregnancy, when the uterus is heaviest and compression risks are greatest. The takeaway: don’t stress about sleep position in early or mid-pregnancy, but favoring your left side in the final weeks is a sensible precaution.
How to Keep Your Spine Aligned
Side sleeping only helps if your body is properly supported. The goal is a straight line from your head through your spine to your hips. A pillow that’s too flat lets your head drop toward the mattress, straining the neck. A pillow that’s too thick pushes your head upward. Side sleepers generally need a thicker pillow than back sleepers because the gap between the head and the mattress is wider when you’re on your side.
Place a pillow between your knees, running along the full length of your legs. This prevents your top leg from pulling your pelvis forward, which twists the lower back. Keep your arms relaxed in front of you rather than tucked under the pillow or your head, which can compress nerves and cause numbness. If you wake up with lower back pain as a side sleeper, the knee pillow is usually the fix.
Why Stomach Sleeping Is the Worst Option
Stomach sleeping ranks last for spinal health. It arches the lower back into an unnatural curve and forces the neck to twist to one side for hours. This combination stresses the cervical spine and can lead to chronic neck pain, stiffness, and nerve compression over time. If stomach sleeping is the only way you can fall asleep, a very thin pillow (or no pillow) under the head and a flat pillow under the pelvis can reduce some of the strain, but side or back sleeping is better for nearly everyone.
Quick Summary by Condition
- Acid reflux or GERD: Left side
- Heart failure: Right side
- Snoring or sleep apnea: Either side (avoid back sleeping)
- Pregnancy (third trimester): Left side
- Shoulder pain: Opposite side from the affected shoulder
- Back or neck pain: Either side with proper pillow support
- No specific health concerns: Left side is the default best choice