For most people, sleeping on your left side offers the widest range of health benefits. It improves digestion, clears acid reflux faster, and helps your brain flush out waste more efficiently during sleep. That said, the “best” side depends on your specific health situation. People with heart failure, shoulder injuries, or pregnancy each have different considerations worth knowing about.
Why the Left Side Wins for Most People
Your digestive system is asymmetrical, and left-side sleeping works with that anatomy rather than against it. Your small intestine empties waste into the large intestine through a valve in your lower right abdomen. When you sleep on your left side, gravity helps move that waste through the ascending colon, across the transverse colon, and down into the descending colon on your left side. The result is a more natural path toward a bowel movement in the morning.
If you deal with heartburn or acid reflux, the left side is especially helpful. A Harvard Health review of the research found that while sleep position didn’t change how often acid backed up into the esophagus, acid cleared significantly faster when people slept on their left side compared to their back or right side. The difference comes down to the angle between your esophagus and stomach: on your left side, your stomach sits below the junction where acid would escape, making it harder for acid to creep upward.
Brain Waste Clearance During Sleep
Your brain has its own cleanup system that works primarily while you sleep, flushing out metabolic waste products that accumulate during waking hours. A study published in The Journal of Neuroscience found that this waste-removal process was most efficient in the lateral (side-lying) position compared to sleeping on the back or stomach. Sleeping on the stomach was the least efficient position, characterized by slower clearance and more waste retention.
The researchers noted that side sleeping is the most common sleep position across mammals and proposed that this preference may have evolved specifically to optimize brain waste removal. Both left and right side positions outperformed stomach sleeping, though the lateral position overall showed the strongest results.
When the 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 when the heart isn’t pumping effectively. Many people with heart failure naturally gravitate toward their right side for this reason, and that instinct is well-supported.
If you don’t have heart failure, sleeping on the right side doesn’t offer significant advantages over the left for heart health. But it’s not harmful either. Healthy sleepers who prefer the right side aren’t putting themselves at risk.
Pregnancy and Sleep Position
During pregnancy, left-side sleeping is the standard recommendation. As the uterus grows, lying on your back puts pressure on the inferior vena cava, a major vein that carries blood from your lower body back to your heart. This can reduce blood flow to both you and the baby. Sleeping on the left side allows the most blood flow to the baby and improves kidney function, which helps reduce swelling in the legs and feet. The right side is also acceptable, but the left is preferred as pregnancy progresses and the uterus gets larger.
Protecting Your Shoulder and Hip
The biggest downside to side sleeping is the pressure it puts on the shoulder and hip you’re lying on. If you already have shoulder pain, avoid sleeping on the affected side. When lying on your good side, stack two pillows in front of your chest so the top pillow supports your injured arm. This keeps the sore shoulder in a more natural position and reduces strain on the joint.
For your hips, placing a pillow between your knees keeps your pelvis aligned and prevents your top leg from pulling your spine out of position. This is one of the simplest changes you can make, and it often reduces morning stiffness in the lower back and hips.
That pins-and-needles feeling you sometimes wake up with happens because lying on a limb compresses the nerves running through it, temporarily disrupting their function. It’s not dangerous, but if you notice it frequently, switching sides during the night or adjusting your arm position before falling asleep can help.
Getting Your Pillow Height Right
Side sleepers need a taller pillow than back sleepers because of the gap between the shoulder and head. The goal is to keep your neck and spine in a straight line, not kinked up or drooping down. A pillow height of about 4 inches tends to provide the best spinal alignment, the least muscle strain, and the most comfort, based on research comparing different foam pillow heights. The general recommended range is 4 to 6 inches, with larger-framed people typically needing a pillow closer to the higher end.
If you wake up with neck pain or headaches, your pillow is likely too high or too low. A quick test: have someone look at you from behind while you’re lying on your side. Your spine from your lower back through your neck should form a roughly straight line.
What Side Sleeping Does to Your Skin
There is one area where side sleeping has a clear cosmetic downside. When you sleep on your side or stomach, gravity presses your face into the pillow, compressing and stretching the skin in ways that back sleeping doesn’t. In younger skin, these “sleep wrinkles” disappear within minutes of waking. But as skin loses elasticity with age, those creases become more persistent and eventually permanent.
People who consistently sleep on one side tend to develop a flatter face and more visible lines on their sleeping side. If this concerns you, sleeping on your back eliminates these compression forces entirely. Changing positions frequently throughout the night or using a silk pillowcase can also reduce friction and skin distortion, though neither eliminates the effect completely.
Choosing Your Best Position
If you’re healthy and don’t have a specific condition driving the decision, the left side is the strongest all-around choice. It supports digestion, reduces acid reflux symptoms, and works with your body’s natural waste-clearance systems. The right side is a close second and the better option if you have heart failure or simply find it more comfortable. Alternating between sides during the night is perfectly fine and can help distribute pressure more evenly across your joints.
Whatever side you choose, the pillow setup matters more than most people realize. A properly sized pillow under your head, a second one between your knees, and a supportive mattress that doesn’t let your hip sink too deeply will make any side-sleeping position significantly more comfortable and easier on your body long-term.