For most people, sleeping on your left side is the better choice. It improves digestion, reduces acid reflux, and helps your body clear waste more efficiently overnight. That said, the right side is genuinely better in certain situations, particularly for people with heart failure. The best side depends on your specific health needs.
Why the Left Side Wins for Most People
The left side gets the strongest recommendation because of simple anatomy. Your stomach sits slightly to the left of your midline, and when you lie on your left side, the opening between your esophagus and stomach sits higher than the stomach itself. Gravity keeps stomach acid pooled below that opening, where it belongs. Flip to your right side, and that geometry reverses: acid can slosh up toward the esophagus more easily, which is why right-side sleeping tends to worsen heartburn.
Your digestive tract also benefits from left-side sleeping in a less obvious way. Waste moves from the small intestine into the large intestine through a valve in your lower right abdomen. From there, it travels up the right side of your abdomen (the ascending colon), across the top (the transverse colon), and down the left side (the descending colon) toward your rectum. When you sleep on your left side, gravity assists that entire journey, encouraging a bowel movement in the morning.
Acid Reflux and GERD
If you deal with heartburn or GERD, side choice matters more for you than for the average sleeper. According to Johns Hopkins Medicine, sleeping on your right side can actively make reflux symptoms worse, whether the heartburn comes from GERD, pregnancy, or other causes. Switching to your left side is one of the simplest non-medication strategies for reducing nighttime acid exposure. Some people notice the difference within the first night.
When the Right Side Is Better
Heart failure is the clearest case for choosing the right side. People with heart failure often experience worsened shortness of breath when lying on their left side, because the heart shifts slightly and presses against the chest wall in that position. Many naturally gravitate toward sleeping on their right. If you have heart failure and find left-side sleeping uncomfortable, that instinct is well-founded.
People without heart conditions sometimes worry that left-side sleeping “puts pressure on the heart.” For a healthy heart, this isn’t a meaningful concern. The heart is well-protected by the rib cage and surrounding tissue, and the slight positional shift doesn’t affect function in someone with normal cardiac health.
Side Sleeping and Sleep Apnea
If you snore heavily or have obstructive sleep apnea, either side is a significant upgrade over sleeping on your back. When you lie face-up, gravity pulls your tongue and soft tissues backward, narrowing or blocking the airway. Rolling to either side keeps the airway more open. A Cochrane review found that positional therapy (staying off your back) reduced breathing disruptions by roughly 7 events per hour compared to sleeping supine. That’s a meaningful improvement, though it’s worth noting that CPAP machines still outperform positional changes alone for moderate to severe cases.
There’s no strong evidence that the left side is better than the right for apnea specifically. The key benefit comes from getting off your back, not from choosing a particular side.
Pregnancy and Sleep Position
During the second and third trimesters, side sleeping is recommended over back sleeping. As the uterus grows, lying on your back can compress major blood vessels, reducing blood flow to both you and the baby. The American College of Obstetricians and Gynecologists advises sleeping on your side with one or both knees bent.
The left side gets a slight edge during pregnancy because it keeps the uterus off the liver (which sits on your right side) and may optimize blood flow through the placenta. It also helps with the heartburn that’s common in later pregnancy. That said, if you wake up on your right side, there’s no need to panic. The most important thing is avoiding prolonged back sleeping in the third trimester, not rigidly staying on one side all night. Placing a pillow between your knees and another under your belly can make side sleeping more comfortable as your body changes.
Protecting Your Shoulders and Hips
Side sleeping does come with a tradeoff: more pressure on the shoulder and hip you’re lying on. Over time, this can cause stiffness or pain, particularly if your mattress is too firm or you stay locked in one position all night. A pillow between your knees helps keep your hips aligned and takes stress off your lower back. If you have a shoulder injury, sleep on the opposite side or switch to your back until it heals.
Alternating sides throughout the night is natural and healthy. Most people shift positions dozens of times while asleep. If you’re trying to favor your left side, start there when you fall asleep, but don’t stress about where you end up at 3 a.m. Your body will adjust on its own, and the position you fall asleep in is the one you spend the most time in.
Quick Reference by Condition
- Acid reflux or GERD: Left side
- General digestion: Left side
- Pregnancy (second and third trimester): Left side preferred, either side acceptable
- Heart failure: Right side
- Snoring or sleep apnea: Either side (just avoid your back)
- Shoulder or hip pain: Whichever side doesn’t aggravate the injury, with a pillow between the knees