For most people, the left side is the best side to sleep on. It offers the widest range of benefits: better digestion, improved brain waste clearance, and optimal blood flow during pregnancy. That said, the right side has its own advantages in specific situations, and the “best” position ultimately depends on your body and health.
More than 60% of adults already sleep on their side, making it the most popular position by a wide margin. If you’re one of them, here’s what the research says about which side gives you the most benefit.
Why the Left Side Wins for Digestion
The left side is the clear winner if you deal with heartburn or acid reflux at night. The reason is simple anatomy: when you lie on your left side, your esophagus and the muscular ring connecting it to your stomach sit higher than your stomach itself. Gravity helps acid drain back down rather than creeping upward into your throat. Flip to your right side, and that arrangement reverses. Your stomach ends up above the junction point, making it easier for acid to splash into your esophagus.
This doesn’t mean right-side sleeping causes reflux in everyone. But if you already have GERD or occasional nighttime heartburn, switching to your left side can meaningfully reduce how long acid lingers in your esophagus and how uncomfortable your nights feel.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that kicks into high gear 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 when sleeping on your side (lateral position) compared to sleeping on your back or stomach.
In the study, side sleeping produced faster clearance of beta-amyloid, one of the hallmark proteins that accumulates in Alzheimer’s disease, while the prone (stomach-down) position was the least efficient. The researchers noted that side sleeping is the dominant position across most mammals and suggested it may have evolved specifically to optimize this waste-removal process. The research was conducted in rodents, so the exact magnitude of the effect in humans isn’t confirmed yet, but the anatomical principle holds: posture changes how fluid moves through the brain.
Heart Health and the Right Side Exception
For healthy adults without heart conditions, sleeping on either side is fine for cardiovascular health. The American Heart Association notes that the most important thing for most people is simply finding a position where they sleep deeply and wake up least often.
The exception is heart failure. People with heart failure often feel more short of breath when lying on their left side, because the position shifts the heart’s weight slightly and can increase the sensation of pressure. Many naturally gravitate to their right side for comfort. If you have heart failure and notice that one side feels harder to breathe on, that instinct to switch is worth following.
For people with sleep apnea, the key recommendation is to avoid sleeping on your back entirely. Sleep apnea tends to be significantly worse in the supine position because gravity pulls soft tissues in the throat downward, narrowing the airway. Either side is a major improvement.
Pregnancy and Left-Side Sleeping
During pregnancy, left-side sleeping is considered the gold standard. Healthcare providers recommend it because of how your body’s plumbing is arranged: the inferior vena cava, the largest vein returning blood to your heart, runs along the right side of your spine. As your uterus grows, sleeping on your back or right side can compress this vein, reducing blood flow back to your heart and potentially decreasing oxygen delivery to your baby.
Left-side sleeping avoids this compression entirely. The benefits are measurable: improved placental blood flow, better kidney function, reduced swelling, and more efficient elimination of waste products. On the other hand, back sleeping in later pregnancy can cause what’s called supine hypotensive syndrome, where the compressed vein triggers dizziness, nausea, a racing heartbeat, or sudden drops in blood pressure.
That said, don’t panic if you wake up on your right side or your back. Your body will usually signal discomfort before any real harm occurs. The goal is to start on your left and return to it when you notice you’ve shifted.
How to Set Up for Comfortable Side Sleeping
The biggest mistake side sleepers make is using the wrong pillow. When you’re on your side, there’s a gap between your head and the mattress created by the width of your shoulder. Your pillow needs to fill that gap exactly so your neck stays in a neutral, straight line with the rest of your spine. For most side sleepers, that means a pillow between 5 and 7 inches in loft.
If you have broader shoulders or a larger frame, aim for the higher end of that range, closer to 7 inches. If you have a narrower build, 5 to 6 inches is usually enough. A quick way to check: measure the distance from the outside of your shoulder to the base of your neck while lying on your side. That’s roughly the loft you need. If your pillow is too tall, your head tilts upward and compresses the neck. Too flat, and your head drops, creating tension through the upper back and shoulders.
Placing a second pillow between your knees also helps. Without it, your top leg pulls on the pelvis and can rotate the lower spine out of alignment. A firm pillow or folded blanket between the knees keeps your hips stacked and your spine straight from head to tailbone.
When Right-Side or Back Sleeping Makes Sense
Left-side sleeping has the most universal benefits, but it isn’t ideal for everyone. People with heart failure often breathe more comfortably on the right. Some people with shoulder injuries or chronic shoulder pain find that sleeping on the affected side worsens their symptoms, making the opposite side or back sleeping a better choice regardless of the digestive or circulatory tradeoffs.
Back sleeping remains a good option for spinal alignment if you don’t have sleep apnea, don’t snore heavily, and aren’t pregnant. It distributes weight evenly and avoids putting pressure on either shoulder or hip. Stomach sleeping, however, consistently comes out as the least recommended position. It forces the neck into rotation, compresses the lower back, and, based on the brain-clearance research, appears to be the least efficient position for removing metabolic waste during sleep.
If you don’t have reflux, sleep apnea, heart failure, or a pregnancy to consider, the most honest advice is this: sleep in whatever position lets you fall asleep quickly, stay asleep through the night, and wake up without pain. The best sleep position is the one that actually gives you good sleep.