For most people, the left side is the best side to sleep on. It improves digestion, keeps airways more open, and supports better circulation. But the answer shifts depending on your specific health situation, and in some cases, the right side is actually the better choice.
Why Left-Side Sleeping Works for Most People
The left side gets the most universal recommendations because of how your organs are arranged. Your stomach sits slightly to the left of your body’s midline, and when you lie on your left side, the esophagus and its muscular valve sit higher than the stomach. This means gravity helps keep stomach acid where it belongs rather than letting it creep upward into your throat. If you deal with acid reflux or heartburn, especially at night, switching to your left side can make a noticeable difference.
Left-side sleeping also appears to benefit how your brain cleans itself during sleep. A study published in the Journal of Neuroscience found that the brain’s waste-clearance system works most efficiently when sleeping on the side compared to sleeping on the back or stomach. This system flushes out metabolic byproducts, including proteins linked to neurological disease. The research was conducted in animals, but it aligns with the fact that side sleeping is the most common natural sleep position across mammals.
Left Side During Pregnancy
If you’re pregnant, particularly in the second and third trimesters, left-side sleeping is strongly recommended. Your body’s largest vein returning blood to the heart, the inferior vena cava, runs along the right side of your spine. As your uterus grows, it can press against this vein when you lie on your back or right side, reducing blood flow back to your heart and down to the placenta.
Sleeping on your left side keeps your uterus from compressing that vein, which means better oxygen and nutrient delivery to your baby and improved kidney function for you. Stanford Medicine recommends left-side sleeping specifically because it allows the most blood flow to the baby.
That said, the right side is a perfectly acceptable alternative when the left gets uncomfortable. It still keeps major blood vessels relatively uncompressed compared to back sleeping, which becomes increasingly problematic as pregnancy advances. The key thing to avoid is spending long stretches flat on your back, where the full weight of your uterus presses down on those blood vessels.
When the Right Side Is Better
People with heart failure often find that sleeping on the left side makes breathing harder. The heart sits slightly left of center in the chest, and lying on the left puts more of the body’s weight directly over it. For a healthy heart this doesn’t matter, but when the heart isn’t pumping effectively, that extra pressure can worsen shortness of breath. Many heart failure patients instinctively prefer their right side for this reason, and the American Heart Association has noted this pattern.
If you have pain or an injury on your left shoulder or left hip, the right side is also the obvious choice. Side sleeping puts sustained pressure on the shoulder and hip that contact the mattress. Inflamed joints, bursitis, or rotator cuff problems can flare up significantly when you sleep directly on the affected side for hours at a time.
Side Sleeping and Snoring
Regardless of which side you choose, sleeping on either side is substantially better than sleeping on your back if you snore or have sleep apnea. Research consistently shows that the number of breathing interruptions per hour roughly doubles when people sleep on their backs compared to either side. Back sleeping lets gravity pull the tongue and soft tissues backward into the airway, partially blocking it.
One study found a slight advantage to the right side over the left for sleep apnea patients specifically, with fewer breathing events per hour on the right. But both sides represent a major improvement over back sleeping, so the priority is simply getting off your back if nighttime breathing is a concern.
How to Sleep on Your Side Comfortably
Side sleeping only works well if your spine stays in a neutral line from your head to your hips. Without the right support, you can wake up with neck pain, shoulder stiffness, or lower back aches that make you think side sleeping isn’t for you.
Your pillow should be thick enough to fill the gap between your ear and the mattress, keeping your neck straight rather than kinked up or drooping down. A pillow that’s too thin lets your head drop toward the mattress; one that’s too tall pushes your head upward. Either way, your neck muscles spend all night compensating.
Place a pillow between your knees. This prevents your top leg from pulling your pelvis forward and twisting your lower spine. The Ohio State University Wexner Medical Center recommends keeping your hips and knees slightly bent but not pulled up so high that your spine curves into a fetal position. A gentle, relaxed bend is the goal.
If shoulder pressure is an issue, try hugging a pillow against your chest. This props your top arm forward slightly and takes some strain off the bottom shoulder. Rotating between left and right sides throughout the night is also fine and helps distribute pressure more evenly. You don’t need to stay locked on one side all night to get the benefits.
Left vs. Right: A Quick Comparison
- Acid reflux or heartburn: Left side keeps stomach acid below the esophageal valve.
- Pregnancy (second and third trimester): Left side maximizes blood flow to the baby; right side is an acceptable backup.
- Heart failure: Right side reduces pressure on the heart and eases breathing.
- Snoring or sleep apnea: Either side cuts breathing interruptions roughly in half compared to back sleeping.
- Shoulder or hip pain: Sleep on whichever side is pain-free, or alternate with back sleeping.
- General health with no specific conditions: Left side has the most broad-spectrum benefits.
Your body often knows what it needs. If you consistently wake up on one side, that position is likely working for you. The most important factor is getting quality sleep, and no position helps if it’s too uncomfortable to fall asleep in. Start with the left side as a default, adjust based on your health situation, and use pillows to keep everything aligned.