For most people, the left side is the best default sleeping position. It reduces acid reflux, keeps airways open, and supports healthy circulation. That said, the “best” side depends on your specific health situation, and in some cases the right side offers unique advantages the left doesn’t.
Most adults already spend the majority of their sleep on one side or the other. Accelerometer data from free-living sleep studies shows people spend about 54% of their time in bed on their side, 38% on their back, and only 7% on their stomach. So if you’re a side sleeper, you’re in the majority. The question is which side deserves priority.
Left Side for Digestion and Acid Reflux
The strongest evidence for choosing a specific side involves acid reflux. When you lie on your left side, your esophagus sits above your stomach due to the way these organs are positioned in your body. Gravity helps keep stomach acid where it belongs. Flip to the right side and the anatomy reverses: the esophagus drops below the junction with the stomach, making it easier for acid to flow upward and harder for it to clear once it does.
A systematic review in the World Journal of Clinical Cases confirmed that left-side sleeping significantly improves reflux symptoms. In one study within that review, reflux episodes per hour dropped to 1.2 on the left side compared to 2.1 when lying face up and 1.5 on the right. Position-related reflux events were 60% on the left versus 100% on the right. If you deal with heartburn, nighttime reflux, or GERD, sleeping on your left side is one of the simplest changes you can make.
Right Side for Brain Waste Clearance
Your brain has its own waste-removal system that ramps up during sleep, flushing out metabolic byproducts using cerebrospinal fluid. Research published in Brain Sciences found that this system works most efficiently in the right lateral sleeping position, clearing more waste compared to sleeping on your back or stomach.
This matters because the waste being cleared includes proteins linked to neurodegeneration. The same research noted that patients with dementia spend a much larger percentage of sleep time on their backs compared to people without dementia. While this is an association rather than proof that back sleeping causes cognitive decline, it suggests that side sleeping in general, and possibly the right side specifically, supports better long-term brain health. For most people without reflux issues, this is worth considering.
Left Side During Pregnancy
If you’re pregnant, left-side sleeping has been standard medical advice for decades. The reason is straightforward: a growing uterus is heavy enough to compress the large vein (the inferior vena cava) that returns blood from your lower body to your heart. This compression can lower your blood pressure and reduce blood flow to the placenta and baby.
Lying on your left side shifts the uterus off that vein, which runs slightly to the right of your spine. Clinicians routinely move laboring women into the left lateral position at signs of fetal distress, and the same logic applies during sleep in the second and third trimesters. Back sleeping is the position to actively avoid. If you wake up on your right side, don’t panic, but starting on your left gives you the best baseline for circulation.
Right Side With Heart Failure
Heart failure creates an exception to the usual left-side recommendation. People with heart failure often feel short of breath when sleeping on their left side because the heart, already struggling to pump effectively, sits closer to the chest wall in that position. Many naturally shift to their right side for comfort. If you have heart failure, the right side is generally the more comfortable and practical choice. Follow whatever position lets you breathe easiest.
Why Both Sides Beat Sleeping on Your Back
Regardless of which side you choose, side sleeping offers a major advantage over back sleeping for anyone who snores or has obstructive sleep apnea. When you lie on your back, gravity pulls the tongue and soft tissues toward the back of the throat, narrowing the airway. A systematic review of 13 studies found this effect is dramatic: in one study, the average number of breathing disruptions per hour was 34.2 on the back versus 15.1 on the side. Another found apnea events dropped from 33 per hour on the back to just 5 on the side.
Across nearly every study reviewed, 11 out of 13 confirmed that respiratory events were fewer in any lateral position compared to back sleeping. If your partner says you snore, or if you wake up with a dry mouth and daytime fatigue, switching from your back to either side is a high-impact, zero-cost intervention.
Downsides of Side Sleeping
Side sleeping does come with trade-offs, mostly for your skin and your shoulders. When half your face presses into a pillow for hours, the compression, shear, and stress forces create “sleep wrinkles” that differ from expression lines. Over years, this can contribute to facial asymmetry and skin expansion on the side you favor. If this concerns you, alternating sides or using a silk pillowcase can reduce friction.
Shoulder pain is the more common complaint. Sleeping on your side loads your bottom shoulder with your body weight, and if the joint is already irritated, this can worsen impingement symptoms. Placing a pillow under your armpit while side sleeping takes some of the direct pressure off the shoulder joint. A pillow between your knees keeps your hips aligned and prevents your top leg from pulling your spine out of neutral. Both are small adjustments that make a noticeable difference overnight.
Setting Up Your Bed for Side Sleeping
Pillow height matters more for side sleepers than for any other position. The gap between your head and the mattress is wider when you’re on your side because your shoulder creates distance. You want a pillow that fills that space exactly, keeping your spine in a straight line from your neck through your lower back. A simple test: your ear should line up level with your shoulder. If your head tilts down, the pillow is too thin. If your neck bends upward, it’s too thick.
Most side sleepers need a medium-density pillow in the five-to-seven-inch range, which is considered mid to high loft. If you have broad shoulders, go toward the higher end. A second, firmer pillow between the knees completes the setup by preventing your pelvis from rotating forward, which is a common source of lower back pain for side sleepers who skip this step.
Picking Your Best Side
If you have acid reflux or GERD, sleep on your left. If you’re pregnant, sleep on your left. If you have heart failure, sleep on your right. If none of those apply and you want to optimize brain waste clearance, the right side has a slight edge based on current evidence. If you snore or have sleep apnea, either side is a significant improvement over your back.
In practice, most people shift positions multiple times per night, and that’s normal. The position you fall asleep in is the one you spend the most time in, so starting on your chosen side gets you the greatest benefit even if you move later. If you tend to roll onto your back, a body pillow or a pillow placed behind you can keep you lateral through more of the night.