There is no single “bad” side to sleep on for everyone. The answer depends on your body and what conditions you’re dealing with. For most healthy adults, sleeping on either side is fine and actually better than sleeping on your back or stomach. But if you have acid reflux, shoulder pain, or are pregnant, one side can be noticeably worse than the other.
Right-Side Sleeping and Acid Reflux
If you deal with heartburn or gastroesophageal reflux disease (GERD), the right side is the worse side to sleep on. When you lie on your right side, the muscular valve between your stomach and esophagus relaxes under pressure, making it easier for stomach acid to flow upward into your throat. Sleeping on your stomach causes the same problem.
Left-side sleeping keeps your stomach positioned below that valve, so gravity works in your favor. This is one of the more consistent findings in sleep research, and it’s why gastroenterologists commonly recommend the left side for people with nighttime reflux. If you wake up with a sour taste, chest discomfort, or a morning cough, switching to your left side is worth trying before anything else.
Shoulder Pain Gets Worse on the Affected Side
Sleeping directly on a painful or injured shoulder compresses the fluid-filled sac (bursa) that cushions the joint. That compression increases inflammation and can turn mild daytime soreness into sharp nighttime pain that wakes you up. This is especially true if you have a rotator cuff issue, bursitis, or impingement.
The fix is straightforward: sleep on the opposite side, or switch to your back. If both shoulders bother you, back sleeping with a small pillow under each arm can reduce the gravitational pull on your tendons. Side sleepers who don’t have shoulder problems can reduce their risk by using a pillow thick enough to keep the head and neck aligned, so the top shoulder doesn’t roll forward and pinch the joint.
Pregnancy and Sleeping Position
During the third trimester, back sleeping is the position to avoid. The weight of the uterus compresses the large vein (inferior vena cava) that returns blood from your lower body to your heart. This can lower blood pressure and reduce blood flow to the baby. Research has linked back sleeping in late pregnancy to an increased risk of stillbirth.
Left-side sleeping is generally recommended for pregnant women because it keeps pressure off that vein and promotes better circulation to the placenta. The right side is also acceptable, though. The key distinction during pregnancy isn’t left versus right. It’s side versus back. If you wake up on your back, simply roll to either side. Most people shift positions dozens of times per night, and briefly ending up on your back is not cause for alarm.
Sleep Apnea Improves on Your Side
For people with obstructive sleep apnea, back sleeping is the worst position by a wide margin. When you lie face-up, gravity pulls the tongue and soft tissues toward the back of the throat, partially or fully blocking the airway. In studies of people whose apnea worsens with position, the number of breathing interruptions per hour dropped from around 43 on their backs to about 8 when sleeping on their sides. That’s roughly an 80% reduction.
More than half of people with obstructive sleep apnea are “positional,” meaning their condition is significantly worse on their backs. For these individuals, either side works. Some people use positional therapy devices, essentially wearable bumpers that prevent rolling onto the back, as a supplement or alternative to a CPAP machine.
Side Sleeping and Brain Waste Clearance
Your brain has a self-cleaning system that flushes out metabolic waste, including proteins linked to Alzheimer’s and Parkinson’s disease. Researchers at Stony Brook University used MRI imaging in animal models to measure how efficiently this system works in different sleeping positions. The lateral (side) position cleared waste most efficiently compared to both back and stomach sleeping.
It’s worth noting that side sleeping is already the most common position in humans and across most animal species. As one of the researchers put it, we seem to have adapted to the position that best clears the brain of byproducts that accumulate during waking hours. This research is still in early stages with animals, but it adds another point in favor of side sleeping over back or stomach sleeping for long-term health.
Skin Aging and Facial Wrinkles
One genuine downside of side sleeping applies to both sides equally. When your face presses into a pillow night after night, the repeated compression and shearing forces create wrinkles that differ from expression lines. Researchers using transparent air-filled pillows documented how side sleeping causes crow’s feet, lines around the mouth, flattening of the forehead, and deepening of the folds between the nose and cheeks.
Over years, these “sleep wrinkles” can become permanent because the skin loses elasticity with age and can no longer bounce back from nightly compression. If this concerns you, sleeping on your back eliminates the pressure entirely. Specialty pillows designed to cradle the head while keeping the face suspended can also reduce contact, though they take some getting used to.
Which Side Is Best for Most People
For the general population, left-side sleeping has the most advantages and the fewest drawbacks. It keeps acid reflux in check, supports circulation during pregnancy, and offers the same brain-clearance and airway benefits as the right side. The right side is only clearly “bad” if you have reflux or GERD.
Back sleeping is the position with the most condition-specific risks: it worsens sleep apnea, raises concerns in late pregnancy, and increases acid reflux. Stomach sleeping compresses the spine and neck, worsens reflux, and was the least efficient position for brain waste clearance in animal research. If you’re a healthy person with no reflux, shoulder problems, or sleep apnea, sleeping on either side is a solid choice. Your body naturally shifts throughout the night, and the position you fall asleep in isn’t necessarily the one you stay in. The goal isn’t perfection. It’s avoiding the positions that clearly make an existing problem worse.