For most people, the left side is the best side to sleep on. It reduces acid reflux, supports digestion, and may help your brain clear waste more efficiently during sleep. That said, the “best” side depends on your body and your health. People with heart failure, for example, often do better on the right. Here’s what the evidence says for each situation.
Why the Left Side Wins for Most Sleepers
Your stomach sits on the left side of your body, and the junction where your esophagus connects to your stomach is also on the left. When you sleep on your left side, gravity keeps stomach acid pooled away from that junction. Roll to your right, and the acid can more easily creep upward into your esophagus. The American Gastroenterological Association specifically recommends left-side sleeping to reduce nighttime acid exposure, and this is one of the most well-supported sleep position recommendations in medicine.
Beyond reflux, sleeping on your left side helps waste move naturally from the small intestine into the large intestine, following the path gravity would take given how your digestive tract is arranged. If you frequently wake up with heartburn or a sour taste in your mouth, switching to your left side is one of the simplest changes you can try.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that kicks into high gear while you sleep. During deep sleep, the spaces between brain cells expand, allowing cerebrospinal fluid to flush out metabolic byproducts, including the proteins linked to Alzheimer’s disease. A study published in the Journal of Neuroscience found that this cleaning system, called the glymphatic pathway, worked most efficiently in the lateral (side) position compared to sleeping on the back or stomach.
That research was conducted in animals, and the authors noted it still needs to be confirmed in humans. But given that roughly 65% of people naturally sleep on their side, it’s possible we’ve evolved toward the position that best supports brain health during rest.
Sleep Apnea and Snoring
If you snore 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 your tongue and soft palate toward the back of your throat, narrowing your airway. A meta-analysis of positional therapy research found that switching from back sleeping to side sleeping reduced the severity of obstructive sleep apnea by about 54%. That’s a substantial improvement from simply changing position, and many sleep specialists recommend it as a first-line strategy alongside other treatments.
Pregnancy: Left Is Preferred, but Either Side Works
Pregnant women have long been told to sleep exclusively on the left side. The reasoning is sound: the left side allows maximum blood flow to the baby and improves kidney function. Sleeping on your back in later pregnancy puts pressure on the inferior vena cava, the large vein that returns blood from your lower body to your heart, which can reduce circulation to the uterus.
But the guidance has softened in recent years. The American College of Obstetricians and Gynecologists notes that sleeping on either side is fine. You don’t need to stay locked on your left throughout the entire pregnancy. The key is to avoid prolonged back sleeping as your belly grows, particularly in the second and third trimesters. If you wake up on your back, simply roll to whichever side is comfortable.
When the Right Side Is Better
People with heart failure often feel more short of breath when they sleep on the left side, likely because the position shifts the heart slightly and increases pressure on it. Many heart failure patients naturally gravitate toward their right side for this reason, and it’s a reasonable choice. If you have a heart condition and find that one side feels noticeably more comfortable than the other, that preference is worth listening to.
The Downsides of Side Sleeping
Shoulder and Hip Pressure
Side sleeping concentrates your body weight on a relatively small area. Over time, this can aggravate shoulder bursitis or hip pain, especially if your mattress doesn’t provide enough cushioning. If you already have shoulder inflammation, sleeping on that side will almost certainly make the pain worse. Alternating sides throughout the night helps distribute the load, and placing a pillow between your knees takes stress off your hips and keeps your spine better aligned.
Sleep Wrinkles
When your face is pressed into a pillow for hours, the mechanical compression creates wrinkles that are distinct from the expression lines caused by smiling or squinting. These sleep wrinkles typically appear on the forehead, lips, and cheeks, and they tend to run perpendicular to expression lines. Unlike expression wrinkles, they can’t be treated with Botox because they aren’t caused by muscle contractions. Sleeping on your back is the only way to fully avoid them, but since most people can’t sustain that position all night, it’s a tradeoff most side sleepers accept.
How to Set Up Your Bed for Side Sleeping
Pillow height matters more for side sleepers than for any other position. When you’re on your side, there’s a wider gap between your head and the mattress that needs to be filled to keep your neck in a neutral line with your spine. A pillow that’s too thin lets your head tilt downward, straining your neck. One that’s too thick pushes it upward. Side sleepers generally need a pillow with a loft of 4 to 6 inches, which is noticeably thicker than what back sleepers use.
A second pillow between your knees prevents your top leg from pulling your spine out of alignment. This is especially helpful if you have lower back pain. For pregnant women, a full-length body pillow can support both the belly and the knees simultaneously, making side sleeping more sustainable through the night.
Quick Comparison: Left vs. Right vs. Back
- Left side: Best for acid reflux, digestion, and pregnancy. The default recommendation for most healthy adults.
- Right side: Better for people with heart failure. Comparable to the left for sleep apnea and snoring reduction.
- Back: Best for avoiding shoulder pressure and facial wrinkles. Worst for snoring, sleep apnea, acid reflux, and late pregnancy.
Most people shift positions multiple times per night, and that’s normal. If you’re trying to stay on one side, placing a body pillow behind your back can prevent you from rolling over. But for the average sleeper without a specific medical concern, starting the night on your left side and letting your body do what it does from there is a solid approach.