For most people, the left side is the best side to sleep on. Left-side sleeping reduces acid reflux, supports digestion, and may help your brain clear waste more efficiently during the night. That said, the “best” side depends on your body and any health conditions you’re dealing with. More than 60% of adults already sleep on their side naturally, so if you’re one of them, a small adjustment in direction could make a real difference.
Why Left-Side Sleeping Gets the Top Recommendation
The American Gastroenterological Association recommends sleeping on the left side, and the reason comes down to anatomy. Your stomach sits slightly to the left of your abdomen, and when you lie on your left side, gravity keeps stomach acid pooled away from the opening to your esophagus. Roll to the right, and that acid can flow more easily upward, triggering reflux episodes.
This matters even if you haven’t been diagnosed with GERD or chronic heartburn. Anyone who occasionally eats late or has a heavy meal before bed will notice the difference. If you do have reflux, right-side sleeping is actively associated with more frequent acid exposure overnight.
How Side Sleeping Affects Your Brain
Your brain has its own waste-removal system that kicks into high gear while you sleep. A study published in the Journal of Neuroscience found that this cleaning process, which flushes out proteins linked to neurological disease, works most efficiently when the body is in a lateral (side) position compared to sleeping on the back or stomach. In the stomach-down position, waste clearance was slowest, with more fluid retention and reduced flow. Side sleeping appears to mimic the natural resting posture that mammals evolved for, and your brain’s plumbing seems optimized for it.
When Right-Side Sleeping Is Better
People with heart failure often find that sleeping on the left side worsens shortness of breath. The likely reason is that left-side positioning shifts the heart slightly and changes how blood returns to it, increasing the sensation of breathlessness. Many heart failure patients naturally gravitate to their right side for comfort, and the American Heart Association has noted this preference. If you have a heart condition and feel more comfortable on your right, that’s a valid reason to stay there.
Side Sleeping During Pregnancy
From 28 weeks of pregnancy onward, going to sleep on your side can halve the risk of stillbirth compared to falling asleep on your back. The reason is straightforward: after 28 weeks, lying on your back presses the weight of the uterus onto major blood vessels, reducing blood flow to the womb and oxygen supply to the baby.
Either the left or right side works. The key is the position you fall asleep in, since that’s the one you hold longest during the night. If you wake up on your back, just roll over. There’s no need to worry about positions you shift into unconsciously.
Side Sleeping for Snoring and Sleep Apnea
If you snore or have obstructive sleep apnea, side sleeping is one of the most effective non-medical interventions available. A meta-analysis of positional therapy studies found that switching from back sleeping to side sleeping reduced the number of breathing interruptions per hour by about 54%. Back sleeping lets gravity pull the tongue and soft tissues backward, partially blocking the airway. Side sleeping keeps the airway more open simply by changing the direction gravity acts on those tissues.
For people whose apnea is significantly worse on their back (called positional sleep apnea), this shift alone can sometimes bring symptoms into a manageable range.
The Downside: Shoulder and Hip Pressure
Side sleeping isn’t without tradeoffs. When you lie on your side, the full weight of your torso presses down through your shoulder joint for hours at a time. The collarbone-to-shoulder-blade joint at the top of your shoulder gets compressed, and the tendons of the rotator cuff can get pinched in the narrow space they pass through. Over time, this can cause inflammation, tendonitis, or worsen existing shoulder problems.
Your hips face similar pressure. If your mattress is too firm, the bony point of your hip bears a disproportionate load, leading to bursitis or soreness. People who already have shoulder or hip injuries often need to sleep on the unaffected side or alternate sides throughout the night.
How to Set Up for Comfortable Side Sleeping
Pillow height matters more than most people realize. Research on spinal alignment suggests a pillow around 4 inches tall offers the best balance of comfort and neutral neck positioning for side sleepers. Too thin, and your head drops toward the mattress, bending your neck. Too thick, and your head gets propped up at an angle that strains muscles all night. A good test: your nose should be roughly in line with the center of your chest, not tilted up or down.
Placing a second pillow between your knees keeps your hips stacked and prevents your top leg from pulling your spine into a twist. For shoulder protection, try hugging a pillow against your chest with your top arm. This keeps the upper shoulder from rolling forward and reduces the rotational stress on both the shoulder joint and your thoracic spine. These are small adjustments, but they’re the difference between waking up rested and waking up stiff.
Your mattress plays a role too. Side sleepers generally need something soft enough to let the shoulder and hip sink in slightly, so the spine stays level rather than bowing. If you’re waking up with numbness in your arm or aching hips, the surface is probably too firm for your body weight.
What If You Can’t Stay on Your Side?
Most people shift positions 10 to 30 times per night, so staying locked on one side all night isn’t realistic. The position you fall asleep in is the one you spend the most time in, so focus on that. If you tend to roll onto your back, a body pillow running the length of your torso can act as a gentle barrier. Some people place a tennis ball in a pocket sewn onto the back of a sleep shirt, which creates just enough discomfort to nudge them back to their side without fully waking them.
Ultimately, the “right” side to sleep on is the one that addresses your most pressing health concern. Left is the default recommendation for digestion and general health. Right works better for certain heart conditions. Either side beats back sleeping for snoring, apnea, and late pregnancy. And whichever side you choose, the pillow setup matters just as much as the direction you’re facing.