For most people, the left side is the best side to sleep on. It reduces acid reflux, helps digestion move in the right direction, and allows better blood flow during pregnancy. More than 60% of adults already sleep on their side naturally, but which side you choose makes a meaningful difference for several aspects of your health.
Why the Left Side Wins for Digestion
The anatomy of your digestive system favors the left side. Your stomach curves in a way that, when you lie on your left, keeps it positioned below your esophagus. This means gravity works against stomach acid traveling back up, which is why left-side sleeping reduces acid reflux. If you lie on your right side, your stomach sits above your esophagus, making it easier for acid to escape upward and cause heartburn.
Beyond reflux, left-side sleeping helps move waste through your intestines more efficiently. Your small intestine empties into your large intestine through a valve in your lower right abdomen. When you sleep on your left side, gravity guides waste upward through the ascending colon, across the transverse colon, and down into the descending colon on your left side. This natural path encourages a bowel movement in the morning. Sleeping on your right side works against this flow, potentially slowing things down.
Left-Side Sleeping During Pregnancy
The left side is especially important during the third trimester of pregnancy. A large vein called the inferior vena cava runs along the right side of your spine, carrying blood from your lower body back to your heart. When you lie flat on your back late in pregnancy, the weight of the uterus compresses this vein, reducing blood flow to both you and the baby. This can cause dizziness, shortness of breath, and lower the amount of oxygen reaching the placenta.
Sleeping on your left side takes pressure off this vein entirely, allowing the most blood flow to reach the baby. It also improves kidney function, which helps reduce swelling in your feet and ankles. While the right side is still far better than sleeping on your back, the left side provides the greatest circulatory benefit.
Side Sleeping and Snoring
If you snore or have sleep apnea, sleeping on either side is generally better than sleeping on your back. When you lie on your back, gravity pulls the tongue and soft tissues in the throat backward, partially blocking the airway. Rolling to the side keeps the airway more open. That said, the difference between the left and right side can vary depending on individual anatomy. In some people, one side produces significantly fewer breathing interruptions than the other, so it’s worth paying attention to which side leaves you feeling more rested.
How Side Sleeping Affects Your Brain
Your brain has a waste-clearance system that works primarily during sleep, flushing out metabolic byproducts including proteins linked to Alzheimer’s disease. Research published in The Journal of Neuroscience found that this system works most efficiently when sleeping on your side compared to sleeping on your back or stomach. In the study, waste removal was slowest in the stomach-down position, where fluid clearance was characterized by retention and slower drainage. Side sleeping, which mirrors the natural resting position of most mammals, allowed the fastest transport of waste out of the brain.
When the Right Side Might Be Better
The left side isn’t ideal for everyone. If you have heart failure, some cardiologists suggest the right side may feel more comfortable because left-side sleeping can make you more aware of your heartbeat due to the heart’s proximity to the chest wall. People with certain heart conditions sometimes report discomfort or palpitations when lying on the left. If you don’t have a heart condition, this isn’t a concern.
If you have pain or an injury on one shoulder, you’ll naturally want to sleep on the opposite side to avoid putting pressure on it. In that case, protecting the injured shoulder takes priority over the digestive benefits of any particular side.
Protecting Your Shoulders and Spine
The main downside of side sleeping is the pressure it puts on your shoulder and hip. Over time, compressing one shoulder against the mattress night after night can lead to stiffness and pain. A few adjustments help prevent this.
- Pillow height matters. Side sleepers need a thicker pillow than back sleepers, typically 4 to 6 inches, to fill the gap between the mattress and the side of the head. A pillow that’s too flat lets your head drop, bending your neck out of alignment. A pillow that’s too thick pushes your head upward, creating the same problem in the opposite direction.
- Arm position matters. Avoid sleeping with your arm tucked under your pillow or with your elbow above your head. Both positions add pressure to the tendons in your shoulder and can cause impingement over time.
- A pillow between the knees helps. Placing a pillow between your knees keeps your hips level and takes strain off your lower back. Without it, the top leg pulls the pelvis forward, twisting the spine slightly throughout the night.
If you’re waking up with shoulder pain, a mattress topper can reduce pressure on the joint, and a contoured cervical pillow can help keep your neck properly supported without requiring you to stack multiple pillows.
How to Train Yourself to Sleep on Your Side
If you’re a back or stomach sleeper trying to switch, the transition takes a bit of patience. One common technique is placing a firm pillow or rolled towel behind your back after you lie on your side, which prevents you from unconsciously rolling onto your back during the night. Some people sew a tennis ball into the back of an old shirt for the same effect.
Hugging a body pillow can also help. It gives your top arm somewhere to rest, preventing you from rolling forward onto your stomach, and keeps your spine in a more neutral position. Most people adjust within one to three weeks. You don’t need to stay perfectly still on one side all night. Naturally shifting between left and right throughout the night is normal and healthy, as it prevents too much sustained pressure on one shoulder or hip.