For most people, the right side is the side to avoid. Right-side sleeping worsens acid reflux, reduces blood flow during pregnancy, and can aggravate existing shoulder pain. Left-side sleeping is generally the better choice, though the “wrong” side ultimately depends on your specific health situation.
Why Right-Side Sleeping Worsens Acid Reflux
If you deal with heartburn or gastroesophageal reflux disease (GERD), sleeping on your right side is the position most likely to cause problems at night. Right-side sleeping promotes acid flow from your stomach back into your esophagus, leading to more reflux episodes compared to other positions.
The reason comes down to anatomy. Your stomach curves in a way that, when you lie on your left side, gravity helps keep acid pooled away from the opening to your esophagus. Roll to your right, and that same gravity works against you, letting acid creep upward more easily. The American Gastroenterological Association specifically recommends left-side sleeping to reduce nighttime acid exposure. If you regularly wake up with a burning sensation in your chest or throat, switching to your left side is one of the simplest changes you can make.
Right-Side Sleeping During Pregnancy
Pregnant women hear a lot about sleep position, and the core advice is consistent: left-side sleeping allows for maximum blood flow to the baby and improves kidney function. While sleeping on either side is generally acceptable, the left side gets the stronger recommendation because of how the body’s major blood vessels are positioned.
The bigger concern during pregnancy is actually back sleeping. Lying flat on your back puts the weight of the uterus directly on the inferior vena cava, the large vein that returns blood from your lower body to your heart. This compression can reduce blood flow and leave you feeling dizzy or lightheaded. As pregnancy progresses and the uterus grows heavier, this effect becomes more pronounced. Side sleeping, particularly on the left, avoids this pressure entirely.
How Side Sleeping Affects Your Brain
Your brain has its own waste-removal system that operates primarily during sleep, flushing out metabolic byproducts including proteins linked to Alzheimer’s disease. A study published in the Journal of Neuroscience found that this cleanup process works most efficiently when sleeping on your side compared to sleeping on your back or stomach. In the study, side sleeping outperformed other positions for clearing harmful waste like amyloid beta, a protein that accumulates in Alzheimer’s patients.
Interestingly, the research did not find a significant difference between left and right lateral positions for brain waste clearance. The key distinction was lateral (side) versus non-lateral. Stomach sleeping, where the head is in a more upright posture, showed the slowest clearance and the most retention of waste products. The researchers proposed that side sleeping may have evolved specifically because it optimizes this brain-cleaning process during rest.
Shoulder Pain Changes the Equation
When you have a sore or injured shoulder, the side to avoid is whichever side hurts. Sleeping directly on a painful shoulder compresses the joint for hours at a time, which can worsen inflammation and slow recovery. The straightforward fix is to sleep on your opposite side.
If you find yourself rolling onto the sore side during the night, there’s a practical trick: place a rolled-up pair of socks in the pajama pocket on your painful side. The slight discomfort encourages your body to naturally shift away from that position while you sleep. When sleeping on the non-painful side, hugging a pillow in front of you can support the injured shoulder and keep it from pulling forward uncomfortably. Back sleeping with a small pillow tucked under each arm is another option that keeps pressure off both shoulders entirely.
Side Sleeping and Wrinkles
One downside of side sleeping that applies to both sides equally is its effect on your skin over time. Research published in the Aesthetic Surgery Journal found that sleeping on your side creates compression, tension, and shear forces against the face that lead to wrinkles. These “sleep wrinkles” typically appear on the forehead, lips, and cheeks, and they worsen over the years as skin thins and loses elasticity.
Unlike expression lines caused by muscle movement (the kind that respond to Botox), sleep wrinkles are purely mechanical. They form from repeated pressure against a pillow surface night after night. The only sleeping position that avoids facial compression entirely is on your back. For people who prioritize skin health but can’t comfortably sleep on their backs, alternating sides throughout the night can at least distribute the pressure more evenly rather than concentrating it on one side of the face.
Which Position Most People Actually Use
Adults spend about 54% of their sleep time on their side, 38% on their back, and just 7% on their stomach. Most people shift positions throughout the night rather than staying locked in one posture, which means your “preferred” position is really just the one you spend the most time in.
If you’re trying to train yourself to sleep on your left side instead of your right, a body pillow placed behind your back can act as a physical barrier that discourages rolling over. It takes time to adjust, and you may not stay on your left side all night, but even increasing the proportion of time spent in a better position can make a meaningful difference for conditions like acid reflux. For most healthy adults without reflux, pregnancy, or shoulder issues, either side is perfectly fine.