For most people, sleeping on the left side offers more health benefits than the right. Left-side sleeping reduces acid reflux, supports digestion, and improves blood flow during pregnancy. That said, the “better” side depends on your specific health situation, and in some cases, the right side is the smarter choice.
Acid Reflux Favors the Left Side
If you deal with heartburn or gastroesophageal reflux, sleeping on your left side makes a measurable difference. In studies comparing sleep positions, acid exposure time in the esophagus was significantly lower on the left side compared to the right side or lying on your back. The median acid exposure was 0.0% of the night on the left, versus 1.2% on the right and 0.6% on the back.
The reason is anatomy. Your stomach curves to the left, and the junction where your esophagus meets your stomach sits above the pool of stomach acid when you’re on your left side. Roll to the right, and that junction dips below the acid level, making it easier for acid to creep upward. If nighttime heartburn disrupts your sleep, switching to your left side is one of the simplest fixes available.
Digestion and Gut Transit
Your digestive tract also benefits from left-side sleeping. The valve connecting your small intestine to your large intestine (the ileocecal valve) sits in your lower right abdomen. When you lie on your left side, gravity helps move waste from that valve 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 the right side doesn’t offer the same gravitational advantage, since waste would need to move against gravity for part of that journey.
Pregnancy: Left Side Is Recommended
During pregnancy, the left side is widely recommended by obstetricians, particularly in the second and third trimesters. The American College of Obstetricians and Gynecologists advises sleeping on your side with one or both knees bent, noting that a pillow between the knees and another under the belly can help with comfort.
The left side is preferred because it allows maximum blood flow to the fetus and improves kidney function. As the uterus grows, lying on your back puts pressure on the inferior vena cava, the large vein that returns blood from your lower body to your heart. This pressure can reduce blood flow to both you and the baby. While the right side is still far better than lying flat on your back, the left side keeps the heavy uterus from compressing that vein as much and optimizes circulation.
If you wake up on your back, there’s no need to panic. Simply roll to your side and use pillows to help you stay there. A full-length body pillow can make side sleeping more sustainable throughout the night.
Snoring and Sleep Apnea
For snoring and obstructive sleep apnea, either side is better than sleeping on your back. When you lie face-up, gravity pulls your tongue and soft tissues toward the back of your throat, narrowing or blocking the airway. Side sleeping keeps the airway more open. Studies on positional therapy found that switching from back sleeping to a side position reduced breathing interruptions by roughly 7 events per hour on average. The American Academy of Sleep Medicine recognizes side sleeping as a supplemental therapy for people whose apnea is worse when lying on their back, which applies to a large percentage of cases. Left versus right doesn’t appear to make a significant difference here; the key is simply getting off your back.
Heart Health Considerations
This is one area where the right side may have a slight edge, particularly if you have a heart condition. When you sleep on your left side, the heart shifts and rotates slightly within the chest due to gravity. Researchers using imaging techniques have confirmed this physical repositioning, and it shows up as changes on electrocardiogram readings. On the right side, the heart stays more firmly in place, held by the tissue layer between the lungs.
For healthy people, this shift has no functional impact on heart rhythm or performance. The electrical changes that appear on monitoring equipment reflect the heart’s new position, not an actual change in how it’s working. However, some people with congestive heart failure report more discomfort when lying on their left side, likely because the repositioned heart creates a sensation of pressure. If you have heart failure or another cardiac condition and find left-side sleeping uncomfortable, the right side is a reasonable alternative.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that’s most active during sleep, flushing out metabolic byproducts including proteins linked to Alzheimer’s disease. A study published in the Journal of Neuroscience found that this clearance system worked most efficiently in the lateral (side-lying) position compared to sleeping on the back or stomach in animal models. The researchers noted that side sleeping mimics the natural resting posture most mammals adopt, suggesting an evolutionary advantage to this position. The study did not find a clear difference between left and right sides for brain clearance, so either side appears beneficial compared to back or stomach sleeping.
Which Side Is Best for You
If you’re generally healthy with no specific conditions, the left side is the stronger default choice. It supports digestion, minimizes reflux, and offers the same brain-clearance benefits as the right side. During pregnancy, the left side is the clear winner for blood flow.
Choose the right side if you have heart failure or find that left-side sleeping causes chest discomfort. For sleep apnea and snoring, either side works, as long as you’re not on your back.
In practice, most people shift positions multiple times per night, and that’s perfectly normal. If you want to train yourself to stay on one side, placing a pillow behind your back can prevent you from rolling over. Starting the night on your preferred side also increases the total time you spend in that position, even if you move later.