For most people, the left side is the best side to sleep on. It reduces acid reflux, supports healthy circulation, and may even help your brain clear waste more efficiently. That said, the “best” side depends on your body and any health conditions you’re managing. More than 60% of adults already sleep on their side, making it the most common position by a wide margin.
Why the Left Side Wins for Most People
The left side gets the edge primarily because of anatomy. Your stomach sits slightly to the left of your midline, and when you sleep on your left, the stomach rests below the esophagus. That means gravity works in your favor, making it harder for stomach acid to travel upward. Research from Amsterdam UMC also found that when acid does reach the esophagus during left-side sleep, it drains back into the stomach more quickly. If you deal with heartburn or acid reflux, especially at night, the left side can make a noticeable difference.
Left-side sleeping also benefits circulation during pregnancy. In later trimesters, the growing uterus can press against the inferior vena cava, a large vein that runs along the right side of the spine and returns blood to the heart. Sleeping on the left keeps the uterus from compressing this vein, allowing unrestricted blood flow back to the heart and better oxygen and nutrient delivery to the placenta. This is why healthcare providers worldwide recommend it as the go-to position for pregnant people.
When the Right Side Is Better
The right side is the better choice if you have heart failure. People with this condition often experience worsened shortness of breath when lying on the left side, likely because the position shifts the heart slightly and increases the sensation of pressure in the chest. The American Heart Association notes that many heart failure patients naturally gravitate toward sleeping on their right for this reason.
If you don’t have heart failure or acid reflux, sleeping on either side offers similar benefits for breathing and spinal alignment. The distinction between left and right matters most when a specific condition tips the balance.
Side Sleeping and Your Brain
Your brain has its own waste-clearance system that ramps up during sleep, flushing out proteins and metabolic byproducts that accumulate throughout the day. A study published in the Journal of Neuroscience found that this system worked most efficiently in the lateral (side) position compared to sleeping on the back or stomach, at least in animal models. The researchers noted that side sleeping is also the natural resting posture for the rodents they studied and speculated that this position may have evolved partly to optimize brain waste removal, including clearance of amyloid-beta, a protein linked to Alzheimer’s disease. This hasn’t been confirmed in human trials yet, but it adds to the case for side sleeping in general.
Side Sleeping and Sleep Apnea
If you snore heavily or have obstructive sleep apnea, switching from your back to either side can be significant. Back sleeping allows the tongue and soft tissues to collapse into the airway more easily. A meta-analysis found that positional therapy, primarily shifting from back to side sleeping, reduced the number of breathing interruptions per hour by about 54%. For mild to moderate sleep apnea, simply staying off your back may be one of the most effective non-device interventions available.
The Downsides of Side Sleeping
Side sleeping isn’t without trade-offs. The position concentrates your body weight onto one shoulder and one hip, which can aggravate existing joint problems. Pressure on the outer hip compresses the bursa, tendons, and muscles around the joint, sometimes leading to pain that wakes you up at night. If you already have inflammation or an injury in the hip, side sleeping can make it worse over time.
There’s also a cosmetic consideration. When your face presses into a pillow for hours, the repeated compression creates “sleep wrinkles” on the forehead, cheeks, and lips. These are distinct from expression lines and tend to worsen with age as skin loses elasticity. Unlike wrinkles caused by muscle movement, sleep wrinkles can’t be treated with Botox because they’re purely mechanical. Back sleeping avoids them entirely, but as researchers have noted, consciously changing your sleep position is extremely difficult to maintain.
How to Make Side Sleeping More Comfortable
Pillow choice matters more for side sleepers than for any other position. Because your shoulder raises your head further from the mattress, you need a thicker pillow than a back sleeper would use. The goal is to fill the gap between your head and the bed so your neck stays on the same plane as your spine. A pillow that’s too thin lets your head tilt downward, straining the neck. One that’s too thick pushes it upward, creating the same problem in reverse.
Placing a small pillow or rolled-up towel between your knees is one of the simplest upgrades you can make. It keeps your hips aligned, prevents your top knee from collapsing onto the bottom one, and reduces pressure on both the hip joint and lower back. This is especially helpful if you’ve noticed hip or lower back stiffness in the morning.
If you tend to roll onto your back during the night, a body pillow along your back can help you stay in position. Some people also use a tennis ball sewn into the back of a sleep shirt as a low-tech way to discourage back sleeping, a technique originally popularized for sleep apnea management.
Choosing the Right Side for You
For the general population, left-side sleeping offers the most benefits: less acid reflux, better circulation, and no known cardiovascular downsides for healthy hearts. If you’re pregnant, the left side is strongly preferred starting in the second trimester. If you have heart failure, switch to the right. If you have shoulder or hip pain on one side, sleep on the opposite side and use a knee pillow to keep pressure off your joints.
The most important thing is that you’re sleeping on a side at all rather than on your back or stomach, especially if you snore, have reflux, or want to support your brain’s nightly cleanup process. Between left and right, the differences are real but condition-specific. Pick the side that matches your body’s needs, set up your pillows to keep your spine neutral, and your sleep position is working for you rather than against you.