For most people, the left side is the best side to sleep on. It helps with digestion, reduces acid reflux, and may even support your brain’s ability to clear waste while you sleep. That said, the “best” side depends on your specific situation, because certain health conditions make the right side a better choice.
Why the Left Side Wins for Most Sleepers
The left side gets the most universal recommendation, and the reasons come down to anatomy. Your stomach sits slightly to the left of your midline, and when you lie on your left side, the junction between your esophagus and stomach sits higher than the stomach itself. This means gravity keeps stomach acid where it belongs rather than letting it creep upward into your throat. A 2022 study in The American Journal of Gastroenterology confirmed that left-side sleeping reduces nighttime acid reflux more effectively than sleeping on your back or right side.
This matters more than you might think. About 80% of people with GERD experience symptoms at night, and simply switching to the left side can make a noticeable difference in how often acid wakes you up.
Your digestive tract also benefits from the left-side position further downstream. The large intestine follows a specific path: it climbs up the right side of your abdomen (ascending colon), crosses to the left (transverse colon), then drops down the left side (descending colon) toward the rectum. When you sleep on your left side, gravity helps move waste along this natural route, which is why many people find that left-side sleeping encourages a more regular morning bathroom trip.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that works primarily while you sleep. It flushes out metabolic byproducts, including proteins linked to Alzheimer’s disease, using cerebrospinal fluid. Research published in The Journal of Neuroscience found that this cleanup system works most efficiently when sleeping on the side (lateral position) compared to sleeping on the back or stomach. Stomach sleeping was the least efficient position, with slower clearance and more retention of waste products in the brain. The study was conducted in rodents, so direct translation to humans requires some caution, but side sleeping is already the most common human sleep position, which researchers noted may have evolved for exactly this reason.
Pregnancy: Left Side Is Standard Advice, but Context Matters
If you’re pregnant, you’ve probably heard that you should sleep on your left side, especially in the third trimester. The reasoning is that lying flat on your back can compress the large vein (vena cava) that returns blood from your lower body to your heart, potentially reducing blood flow to both you and the baby. Some women do feel lightheaded or faint when lying on their backs during late pregnancy.
However, the actual risk is smaller than the anxiety around it suggests. Only about 2% to 4% of pregnant women who experience symptoms from back-sleeping have significant compression of that blood vessel, and even in that small group, there is no evidence of harm to the baby. Researchers at the University of Utah Health have noted that the medical community does not have enough data to say that back sleeping causes stillbirth. The link, if any, may relate more to sleep apnea than to the sleeping position itself.
So left-side sleeping during pregnancy is a reasonable default, particularly if back sleeping makes you feel dizzy or short of breath. But if you wake up on your back, there’s no need to panic. Your body will typically prompt you to shift positions before any real problem develops.
When the Right Side Is Better
People with heart failure often find that sleeping on the left side makes their shortness of breath worse. The American Heart Association has noted that many heart failure patients naturally prefer the right side for this reason. If you have heart failure and notice that one side feels more comfortable than the other, that instinct is worth following.
The right side can also be the better choice if you have pain or an injury on your left shoulder or hip. Sleeping directly on an irritated joint compresses it for hours, which tends to increase inflammation and stiffness by morning.
Making Side Sleeping More Comfortable
Side sleeping puts more pressure on your shoulder and hip than back sleeping does, so getting the setup right matters. The most common mistake is using a pillow that’s too flat. When you lie on your side, there’s a gap between your head and the mattress roughly equal to the width of your shoulder. Your pillow needs to fill that space to keep your neck aligned with your spine. A pillow that’s too thin lets your head drop toward the mattress, straining your neck. One that’s too thick pushes your head upward.
If you have shoulder pain, propping your top arm on a folded blanket or small pillow takes pressure off the shoulder joint and prevents it from rolling forward. A pillow between your knees serves a similar purpose for your hips and lower back, keeping your pelvis level so your spine doesn’t twist. These are small adjustments, but they’re often the difference between waking up comfortable and waking up stiff.
Your mattress also plays a role. Side sleepers generally do better on a medium or medium-soft mattress that allows the shoulder and hip to sink in slightly. A very firm mattress creates pressure points at those two contact areas, which can cause numbness or aching overnight.
What if You Can’t Stay on One Side?
Most people shift positions multiple times per night, and that’s completely normal. You don’t need to stay locked on your left side for eight hours to get the benefits. If you tend to roll onto your back, placing a body pillow behind you can discourage the shift without waking you up. Some people also find that a slight wedge under the mattress on one side creates just enough of a slope to make the preferred side feel more natural.
If you’re a lifelong back sleeper and find side sleeping genuinely uncomfortable, back sleeping is still a perfectly healthy position for most people. It distributes weight evenly across your spine and avoids shoulder compression entirely. The main situations where back sleeping becomes a concern are significant acid reflux, sleep apnea (which worsens on the back), and late pregnancy with symptoms of dizziness.