For most people, the left side is the better side to sleep on. It reduces acid reflux, supports smoother digestion, and keeps your airway open to minimize snoring. That said, the best side depends on your specific health situation. People with heart failure, for example, often do better on their right side. Here’s what the evidence shows for each position and how to choose the one that fits you.
Why the Left Side Wins for Digestion
The anatomy of your stomach and esophagus favors the left side. When you lie on your left, the junction between your esophagus and stomach sits higher than the pool of stomach acid below it. This means acid drains away from the esophagus more quickly, reducing heartburn. Up to 80% of people with gastroesophageal reflux disease (GERD) experience symptoms at night, and simply switching to the left side can make a noticeable difference.
Left-side sleeping also helps waste move through your intestines. Your small intestine empties into the large intestine through a valve in your lower right abdomen. When you sleep on your left side, gravity helps pull waste upward through the ascending colon, across the transverse colon, and down into the descending colon on the left. The practical result: more regular morning bowel movements.
Side Sleeping and Breathing
Side sleeping in general, whether left or right, is considered the best position for keeping your airway open at night. When you sleep on your back, your tongue and the soft tissues in your throat can collapse backward under gravity, partially blocking airflow. This is the primary cause of snoring and a major contributor to obstructive sleep apnea. On your side, those tissues stay out of the way.
If snoring is your main concern, either side works. The key is simply staying off your back.
Heart Health and Sleep Side
This is where the answer gets more individual. For people with heart failure, sleeping on the left side often worsens shortness of breath. The heart sits slightly left of center in the chest, and the left-side position may increase pressure on it. Many people with heart failure naturally prefer their right side for this reason.
If you have no heart condition, left-side sleeping is unlikely to cause problems. But if you experience chest discomfort or breathlessness on your left side, switching to the right is a reasonable move.
Left Side During Pregnancy
Healthcare providers worldwide recommend left-side sleeping during pregnancy, especially in the second and third trimesters. The reason is a large vein called the inferior vena cava, which runs along the right side of your spine and carries blood back to your heart from your lower body. As the uterus grows, it can compress this vein when you lie on your back or right side.
Sleeping on the left keeps the uterus off the vena cava, allowing unrestricted blood flow back to the heart and better circulation to the placenta. This means more oxygen and nutrients reaching the baby. If you find it uncomfortable to stay on your left all night, don’t stress. Briefly ending up on your right side or back isn’t dangerous, but starting on your left and returning to it when you wake up is the general goal.
Your Brain Cleans Itself Better on Your Side
During sleep, your brain flushes out metabolic waste through a system sometimes called the glymphatic system. A study published in The Journal of Neuroscience found that this waste clearance process, including the removal of proteins linked to Alzheimer’s disease, was most efficient in the lateral (side) position compared to sleeping on the back or stomach. Sleeping face-down was the worst, characterized by slower clearance and more retention of waste products.
The researchers proposed that side sleeping may have evolved specifically to optimize this brain-cleaning process. The study was conducted in rodents, so direct translation to humans requires some caution, but the finding aligns with the fact that roughly 65% of people naturally sleep on their side.
The Trade-Off: Wrinkles and Shoulder Pressure
Side sleeping does have downsides. The mechanical compression of your face against a pillow night after night contributes to sleep wrinkles, particularly on the forehead, cheeks, and lips. These wrinkles tend to run perpendicular to normal expression lines and, unlike expression wrinkles, can’t be treated with Botox because they aren’t caused by muscle contractions. Sleeping on your back is the only position that eliminates facial compression entirely, though most people find it difficult to maintain.
Shoulder and neck pain are the other common complaints. Side sleeping requires your pillow to fill the gap between the point of your shoulder and your head. A pillow that’s too thin lets your head drop, straining your neck. Most side sleepers need a full-size pillow or even two to keep the spine aligned. If you wake up with a numb arm, your pillow is probably too low, and you’re unconsciously tucking your arm under your head to compensate.
Protecting Your Shoulders
If you have shoulder pain on the side you sleep on, try placing a pillow in front of your torso to rest your upper arm on, preventing it from hanging forward and pulling on the joint. Keep the affected arm below a 75-degree angle from your body when possible. If the painful shoulder is the one pressing into the mattress, a thin pillow tucked under that shoulder blade can reduce direct compression. Avoid tucking your hand under your pillow or head, as this creates sustained pressure on the shoulder and wrist.
Putting a Pillow Between Your Knees
One simple addition that improves side sleeping for almost everyone: a pillow between or slightly below your knees. Without it, your top leg drops forward and rotates your pelvis, pulling your lower spine out of alignment. A knee pillow keeps your hips stacked and reduces strain on your lower back and hip joints. This is especially helpful if you wake up with hip soreness or low back stiffness.
How to Choose Your Side
For the general population, the left side offers the most combined benefits: less acid reflux, smoother digestion, open airways, and efficient brain waste clearance. Choose the right side if you have heart failure or if left-side sleeping causes chest discomfort. During pregnancy, default to the left.
As a Harvard sleep specialist put it, there isn’t one universally “healthiest” sleep position. Your body often tells you what works. If you sleep well, wake without pain, and don’t have reflux or breathing issues, the side you naturally prefer is probably fine. If you do have one of those specific concerns, the evidence points clearly toward one side over the other.