For most people, sleeping on the left side offers the greatest overall health benefits. It improves digestion, keeps airways more open, and supports the body’s natural waste-clearing processes during sleep. But the best side depends on your specific health situation, since certain conditions like heart failure can make the right side a better choice.
Why the Left Side Wins for Digestion
Your stomach sits slightly to the left of your abdomen, and its opening to the esophagus sits at the top. When you sleep on your left side, gravity keeps stomach acid pooled at the bottom of the stomach, well below that opening. Roll to the right, and the acid sloshes closer to the esophagus, making reflux far more likely.
Research from Amsterdam UMC confirmed this isn’t just theoretical. Sleeping on the left side not only reduces the frequency of acid reflux episodes but also allows any acid that does reach the esophagus to drain back into the stomach more quickly. If you regularly experience heartburn at night, switching to your left side is one of the simplest changes you can make.
Heart Health Changes the Equation
For people with heart failure, the left side can actually feel worse. Lying on the left compresses the heart slightly against the chest wall, and according to the American Heart Association, people with heart failure often experience worsened shortness of breath in this position. Many naturally shift to the right side for comfort.
If you have a healthy heart, this compression is negligible and not a concern. But if you’ve been diagnosed with heart failure and notice breathing difficulties on one side, sleeping on your right side or slightly elevated is a reasonable adjustment.
Side Sleeping During Pregnancy
Side sleeping becomes increasingly important as pregnancy progresses. During the second and third trimesters, the growing uterus is heavy enough to compress the inferior vena cava, the body’s largest vein carrying blood back to the heart, when you lie on your back. This compression reduces blood flow to the placenta, which means less oxygen and fewer nutrients reaching the baby.
OB/GYN specialists typically recommend establishing a left-side sleeping habit early in pregnancy so it feels natural by the time it matters most. The left side is preferred because it keeps the uterus off the vena cava and maximizes blood flow through the uterine arteries. That said, sleeping on your right side is also fine. The key distinction is side versus back, not strictly left versus right.
How Your Brain Cleans Itself at Night
Your brain has its own waste-removal system that kicks into high gear during sleep, flushing out metabolic byproducts including proteins associated with Alzheimer’s disease. A study published in the Journal of Neuroscience found that this cleaning system works most efficiently when sleeping on your side compared to sleeping on your back or stomach. The lateral position appears to optimize the flow of cerebrospinal fluid through channels that carry waste out of the brain. While this research was conducted in animal models, the finding is notable given that side sleeping is already the most common position in humans.
Better Breathing for Snorers
Sleeping on your back lets gravity pull the tongue and soft tissues of the throat backward, narrowing the airway. This is why snoring and sleep apnea episodes are almost always worse in the supine position. A Cochrane review found that switching from back sleeping to side sleeping reduced breathing interruptions by about 7 events per hour on average. For people with mild to moderate obstructive sleep apnea, that reduction can be clinically meaningful. Either side works here, as the benefit comes from keeping the airway open rather than any organ-specific positioning.
Keeping Your Spine Aligned
Side sleeping puts more distance between your head and the mattress than back sleeping does, which means your pillow needs to fill a larger gap. A pillow height of roughly 5 to 7 inches works for most side sleepers, compared to 4 to 5 inches for back sleepers. Too thin a pillow lets your head drop toward the mattress, crimping your neck. Too thick forces your head upward, creating strain in the opposite direction.
Placing a pillow between your knees prevents your top leg from pulling your pelvis forward, which twists the lower spine. This is especially helpful if you wake up with hip or lower back stiffness. The goal is a straight line from your head through your spine to your hips, as if you were standing upright but lying down.
The Trade-Off: Sleep Wrinkles
The one genuine downside of side sleeping is cosmetic. Pressing your face into a pillow for hours creates mechanical compression that, over years, forms permanent creases. These “sleep wrinkles” most commonly appear on the forehead, cheeks, and lips. Unlike expression lines caused by muscle movement, sleep wrinkles can’t be treated with Botox because they’re structural, caused by repeated physical pressure rather than muscle contractions. They also worsen with age as skin thins and loses elasticity.
If this concerns you, silk or satin pillowcases reduce friction, and specialty pillows with cutouts minimize facial contact. But for most people, the cardiovascular, digestive, and neurological benefits of side sleeping outweigh the cosmetic effects.
Choosing Your Side: A Quick Guide
- Left side: Best for acid reflux, pregnancy, and general digestive comfort. The default recommendation for most people.
- Right side: Better for people with heart failure who experience shortness of breath on the left. Also perfectly fine for snoring, sleep apnea, and brain waste clearance.
- Either side: Superior to back sleeping for airway openness and brain waste removal. Superior to stomach sleeping for spinal alignment and facial skin health.
Roughly half of men and nearly three-quarters of women already prefer side sleeping, so if you’re naturally a side sleeper, you’re starting from a good place. The most practical approach is to default to the left side unless a specific condition like heart failure gives you a reason to choose the right.