For most people, sleeping on your left side offers the broadest range of health benefits, from better circulation to more efficient brain waste clearance. But the best side depends on your specific situation. People with heart failure, acid reflux, or shoulder pain may do better on their right side, and certain sleep conditions improve with either lateral position. Here’s what the evidence says for each scenario.
Why Side Sleeping Beats Back or Stomach
Before choosing a side, it helps to know that sleeping laterally (on either side) has clear advantages over sleeping on your back or stomach. Your brain has a waste-removal system that flushes out harmful proteins while you sleep, including the kind linked to Alzheimer’s disease. Research published in the Journal of Neuroscience found that this system works most efficiently in a lateral position compared to sleeping on your back or stomach. Stomach sleeping performed worst, with slower clearance and more retention of waste products.
Side sleeping also keeps your airway more open. A Cochrane review found that people with obstructive sleep apnea who switched from back sleeping to side sleeping reduced their breathing interruptions by about 7 events per hour. If you snore or have mild sleep apnea, simply getting off your back can make a meaningful difference, regardless of which side you choose.
The Case for Your Left Side
Left-side sleeping is the most broadly recommended position, and the reasons come down to anatomy. Your body’s largest vein, the inferior vena cava, runs along the right side of your spine. This vein carries blood back to your heart from your lower body. When you sleep on your left side, gravity helps blood flow through this vein without compression. On your right side or back, body weight can press against it, slightly reducing circulation.
For most healthy adults, this difference is subtle. But during late pregnancy, it becomes significant. As the uterus grows, it can compress the inferior vena cava when you lie on your back or right side, restricting blood flow to the placenta. Sleeping on the left side keeps the uterus off this vein, allowing better oxygen and nutrient delivery to the baby. Healthcare providers worldwide consider it the gold standard sleeping position during the second and third trimesters.
Left-side sleeping also helps with acid reflux. Your stomach sits on the left side of your body, and its natural curve means that when you lie on your left, the junction between your esophagus and stomach stays above the level of stomach acid. Flip to your right, and gravity works against you, making it easier for acid to creep upward.
When Your 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 that side can create a sensation of pressure or shortness of breath in people whose hearts are already strained. Many heart failure patients naturally gravitate toward the right side for comfort, and that instinct is well supported.
If you have pain or an injury in your left shoulder, sleeping on your right side is also the obvious choice. Consistently sleeping on a painful shoulder compresses the joint, irritates the rotator cuff, and delays healing. The same applies in reverse: right shoulder pain means sleeping on your left.
How to Set Up for Side Sleeping
Choosing the right side is only half the equation. Poor alignment while side sleeping can leave you with neck pain, hip stiffness, or a numb arm by morning. A few adjustments make a big difference.
Pillow Height
Your pillow’s job is to fill the gap between your ear and the outside of your shoulder, keeping your head in a straight line with your spine. Too thin, and your head droops down, stretching the muscles on the top side of your neck. Too thick, and your head gets pushed upward, compressing the shoulder joint. A quick test: when you lie down, imagine a line from the center of your forehead to your chin. That line should be parallel to the mattress, not tilted in either direction.
Knee Pillow
Placing a pillow between your knees prevents your top leg from pulling your pelvis out of alignment. Keep your knees slightly bent rather than straight, which reduces strain on your lower back. The pillow should be thick enough to keep your knees roughly hip-width apart, but not so thick that it forces your legs apart unnaturally. If you shift positions a lot during the night, a full-length body pillow works better than a standard pillow because it stays in place as you move.
Arm Position
The arm you’re lying on should extend slightly forward rather than being pinned directly under your body. For your top arm, hugging a medium-firm pillow in front of your chest prevents the shoulder from rolling forward and compressing the joint. This keeps the muscles around your shoulder blade and rotator cuff relaxed, which is especially important if you’re prone to shoulder pain. Without that support, your top arm drops across your body, pulling on the joint for hours at a time.
Switching Sides During the Night
Most people don’t stay perfectly still all night, and that’s fine. Rolling between sides is normal and may even be beneficial, since it distributes pressure more evenly and prevents sustained compression on one shoulder or hip. If you’re trying to train yourself to stay on a particular side, a body pillow behind your back can act as a gentle barrier against rolling onto your back. Some people also use a tennis ball sewn into the back of a sleep shirt to make back sleeping uncomfortable enough to avoid.
For pregnant women in the third trimester, the priority is avoiding prolonged time flat on the back rather than maintaining a perfect left-side position all night. Briefly rolling to your right side or shifting positions is not harmful. Your body will typically signal discomfort (dizziness, nausea) before any real circulation issue develops from back sleeping, prompting you to move.