For most people, sleeping on the left side offers more benefits, particularly for digestion and acid reflux. But the right side has its own advantages, especially for brain health. The “best” side depends on your body and any conditions you’re managing. Adults already spend about 54% of their sleep time on their side, so chances are you’re a side sleeper already. Here’s what the evidence says about which side to choose.
Left Side Sleeping and Digestion
The strongest evidence for choosing a side comes from acid reflux research. A systematic review and meta-analysis published in 2023 found that sleeping on the left side significantly decreases nighttime acid exposure compared to both right-side and back sleeping. The difference isn’t subtle: acid exposure time dropped by roughly 2 percentage points compared to the right side and nearly 3 percentage points compared to lying on your back. That translates to meaningfully less time with stomach acid irritating the esophagus overnight.
The reason is anatomy. Your stomach curves to the left, and when you lie on that side, the junction between your esophagus and stomach sits above the level of gastric acid. Gravity keeps acid pooled in the stomach rather than letting it creep upward. Flip to your right side, and that junction dips below the acid line, making reflux far more likely. If you deal with heartburn, GERD, or even occasional nighttime indigestion, left-side sleeping is the clearer choice.
Right Side Sleeping and Brain Waste Clearance
Your brain has its own waste-removal system that kicks into high gear during sleep, flushing out metabolic byproducts linked to neurodegeneration. Research published in Brain Sciences found that this cleaning process is most efficient in the right lateral sleeping position, with greater fluid clearance occurring compared to sleeping on your back or stomach. The same research noted that people with neurodegenerative conditions tend to spend a much larger percentage of their sleep time on their backs, establishing an association between back sleeping and dementia.
This is a newer area of science, and researchers haven’t yet pinpointed exactly why the right side appears to outperform the left for brain clearance. But it’s a noteworthy counterpoint: if digestive issues aren’t your primary concern, the right side may carry a distinct neurological benefit.
Side Sleeping and Sleep Apnea
If you snore or have obstructive sleep apnea, either side is dramatically better than your back. When you lie face-up, gravity pulls the tongue and soft tissues backward, narrowing the airway. A Japanese retrospective study found that in “positional” sleep apnea patients (those whose breathing events are position-dependent), the average number of breathing disruptions per hour dropped from about 38 on their back to roughly 10 on either side. That’s nearly a fourfold reduction.
Both the left and right sides performed almost identically for airway improvement, so the specific side matters less here than simply avoiding back sleeping. If you’ve been told you snore heavily or stop breathing during sleep, switching to side sleeping is one of the simplest changes you can make.
Pregnancy and the Left Side
During pregnancy, especially in the second and third trimesters, left-side sleeping is the standard clinical recommendation. The reason is the inferior vena cava, a large vein that carries blood back to the heart from the lower body. It runs along the right side of the spine, and as the uterus grows heavier, lying on the back compresses this vein. That compression can cause a drop in the mother’s blood pressure and reduce blood flow to the baby.
Lying on the left side shifts the uterus off the vena cava entirely. Clinicians routinely move laboring women to this position if there are signs of fetal distress, and pregnant women are generally advised to avoid sleeping flat on their backs during later pregnancy. If you wake up on your back or right side, there’s no need to panic. The goal is to start on the left and spend as much of the night there as your body allows.
Protecting Your Shoulders and Spine
Side sleeping does put sustained pressure on whichever shoulder is beneath you, and poor alignment can leave you with a stiff neck or aching hip by morning. A few adjustments make a big difference. Your pillow should be thick enough to fill the gap between your ear and the mattress, keeping your neck in a straight line with your spine rather than tilting up or down. A pillow that’s too flat lets your head drop, straining the neck and compressing the lower shoulder.
Placing a pillow between your knees is one of the most effective tweaks for side sleepers. It aligns your hips and takes pressure off the pelvis, which is especially helpful if you have sciatica or lower back pain. If your shoulder tends to ache, try propping your top arm on a folded blanket or small pillow so it doesn’t pull forward across your chest. Alternating sides throughout the night, when possible, helps distribute pressure more evenly and prevents one shoulder from bearing the full load every night.
Skin and Wrinkles
One common concern about side sleeping is that pressing your face into a pillow accelerates wrinkles. It’s a reasonable theory, since repeated compression could crease the skin over time. However, a study that compared wrinkle patterns in dedicated right-side and left-side sleepers found no significant correlation between sleep side and the side with more wrinkles or facial sagging. Whatever role pillow contact plays in skin aging appears to be minor compared to sun exposure, genetics, and natural loss of skin elasticity.
Which Side Is Right for You
There’s no single answer that works for everyone, but you can match your sleep position to whatever your body needs most. If acid reflux, heartburn, or digestive discomfort disrupts your nights, the left side is the better choice by a meaningful margin. If you’re pregnant, left-side sleeping protects blood flow to the baby. If neither of those applies and you’re more interested in long-term brain health, the right side may offer a slight edge in metabolic waste clearance during sleep.
For snoring and sleep apnea, both sides work equally well. And for general comfort, the best approach is often to alternate between sides while using proper pillow support to keep your spine aligned. Most people shift positions dozens of times per night without realizing it, so the side you fall asleep on is a starting point, not a rigid rule. Pick the side that addresses your most pressing concern, set yourself up with good support, and let your body do the rest.