For most people, sleeping on the left side offers the widest range of health benefits. It reduces acid reflux, supports the brain’s natural waste-clearing process, and is the position most adults already gravitate toward. More than 60% of adults sleep on their side, making it the most common sleep position by a wide margin. But the “best” side depends on your specific situation, whether that’s pregnancy, sleep apnea, shoulder pain, or concerns about skin aging.
Why Left-Side Sleeping Wins for Most People
The left side gets the edge over the right because of simple anatomy. Your stomach sits slightly to the left of your body’s midline, and its opening to the esophagus sits at the top. When you lie on your left side, your stomach hangs below that opening, so gravity keeps acid pooled at the bottom of the stomach and away from the esophagus. Roll to the right, and the stomach sits above the esophagus opening, making it easier for acid to travel upward. If you deal with heartburn or gastroesophageal reflux, this one change can make a noticeable difference at night.
Left-side sleeping also appears to help the brain clean itself more effectively. During sleep, cerebrospinal fluid flushes through brain tissue and carries away metabolic waste, including beta-amyloid, the protein linked to Alzheimer’s disease. A study published in the Journal of Neuroscience found that this waste-removal system, called the glymphatic system, worked most efficiently in the lateral (side) position compared to sleeping on the back or stomach. The researchers proposed that side sleeping may have evolved specifically to optimize this brain-cleaning process during rest.
Acid Reflux and Digestion
If nighttime heartburn is your main complaint, sleep position matters more than you might expect. Lying flat on your back allows stomach acid to sit right at the junction between the stomach and esophagus. Lying on your right side positions the stomach above that junction, essentially pouring acid toward your throat. The left side is the clear winner here: the stomach hangs below the esophageal opening, and gravity does the work of keeping acid where it belongs. Researchers at Amsterdam UMC confirmed that this anatomical explanation holds up clinically, with left-side sleeping consistently reducing reflux episodes.
Pairing left-side sleeping with a slight elevation of your upper body (using a wedge pillow or raising the head of your bed a few inches) can compound the benefit.
Sleep Apnea and Snoring
If you snore or have obstructive sleep apnea, getting off your back is the single most important positional change you can make. When you lie face-up, gravity pulls the tongue and soft tissues of the throat backward, narrowing or blocking the airway. Side sleeping keeps the airway more open.
For most people with positional sleep apnea, either side helps. However, the specific side can matter in individual cases. One case documented by the American Thoracic Society showed dramatic differences: a patient recorded 87 breathing interruptions per hour on the right side, compared to just 17 per hour on the left. That kind of variation is unusual and likely related to the individual’s anatomy, but it illustrates why a sleep study that tracks position can be valuable. If you’ve been diagnosed with sleep apnea and still feel unrested despite treatment, ask whether your sleep position data shows a pattern.
Sleep Position During Pregnancy
Pregnant women are often told to sleep on their left side, particularly in the third trimester, to avoid compressing the large vein that returns blood from the lower body to the heart. This advice is widespread, but the evidence behind it is less alarming than many expectant parents fear.
The Division of Maternal-Fetal Medicine at the University of Utah reviewed the available data and concluded that sleeping on your back during pregnancy has not been shown to cause stillbirth. Their position: “Good sleep in pregnancy is important for a healthy mom and a healthy baby. In our opinion, worrying about sleep position is more harmful than helpful.” They emphasized focusing on well-established risk factors like smoking and obesity rather than losing sleep over sleep position.
That said, many women in late pregnancy find back sleeping uncomfortable on its own. The weight of the uterus pressing on blood vessels can cause dizziness, shortness of breath, or lower back pain. Side sleeping, on either side, tends to feel better naturally. If you wake up on your back, simply roll over. There’s no need to set alarms or use elaborate pillow systems unless your doctor has specifically recommended them.
Shoulder and Hip Pain
Side sleeping does have a downside: it concentrates your body weight on one shoulder and one hip. If you always sleep on the same side, you may develop aching in the shoulder pressed against the mattress or soreness in the hip bearing the load.
Pillow placement makes a big difference. Side sleepers need a thicker, firmer pillow under the head than back sleepers do, because the pillow has to fill the gap between the ear and the outer shoulder to keep the neck aligned with the spine. A pillow that’s too thin lets the head droop, straining the neck. A pillow that’s too thick pushes the head upward, creating the same problem in the opposite direction. Look for pillows designed with a shoulder notch at the bottom edge so the shoulder can rest comfortably while the head and neck stay supported.
Placing a second pillow between the knees keeps the hips, pelvis, and lower spine in a more neutral position. Without it, the top leg drops forward and rotates the pelvis, which can aggravate lower back and hip pain over time. If you’re waking up with shoulder pain on your sleeping side, try alternating sides throughout the night or switching to the side that doesn’t hurt for a few weeks to let the irritated shoulder recover.
Skin Aging and Wrinkles
Here’s the one area where side sleeping loses to back sleeping. When you sleep on your side or stomach, gravity presses your face into the pillow for hours, compressing and stretching the skin. Over time, this mechanical stress creates “sleep wrinkles,” lines that form in patterns distinct from the expression wrinkles caused by smiling or squinting. Young skin bounces back from this nightly compression, but as skin loses elasticity with age, these creases start to stick around.
The amount of time spent in each position, the pressure applied, and how much surface area of the face contacts the pillow all affect how quickly these lines develop. If this concerns you, a few practical steps can help without requiring you to retrain yourself to sleep on your back. Silk pillowcases reduce friction and let skin glide rather than bunch against the fabric. Specialty pillows designed to cradle the head while minimizing face contact can also slow wrinkle formation. Moisturizing before bed keeps skin more supple overnight.
Back sleeping eliminates face-to-pillow contact entirely, making it the best position for skin. But for most people, the digestive, respiratory, and neurological benefits of side sleeping outweigh cosmetic concerns.
How to Make Side Sleeping More Comfortable
If you’re already a side sleeper, small adjustments can improve your alignment and reduce morning stiffness. Keep your legs slightly bent rather than pulled tightly toward your chest, which rounds the lower back. Place a pillow between your knees. Choose a head pillow thick enough to keep your ear, shoulder, and hip in a straight line when viewed from behind.
If you’re trying to switch from back or stomach sleeping to your side, a body pillow can help. Hugging a full-length pillow gives your top arm something to rest on (preventing the shoulder from rolling forward) and keeps you from unconsciously flipping onto your stomach. Some people also place a pillow or rolled towel behind their back to prevent rolling over during the night.
Mattress choice plays a role too. Side sleepers generally do better on a slightly softer surface than back sleepers, because the mattress needs to contour around the shoulder and hip rather than simply supporting a flat back. If your mattress is very firm and you’re waking with pressure-point pain, a mattress topper can bridge the gap without replacing the whole bed.