For most people, the left side is the better side to sleep on. It reduces acid reflux, supports digestion, and may even help your brain clear waste more efficiently during sleep. That said, the “best” side depends on your body and your health. Side sleeping in general, whether left or right, beats sleeping on your back or stomach for issues like snoring and sleep apnea. Here’s what the evidence says for each concern.
Left-Side Sleeping and Acid Reflux
The anatomy here is straightforward. Your stomach sits slightly to the left side of your body, and the junction where your esophagus meets your stomach is positioned in a way that, when you lie on your left side, your stomach hangs below that junction. Gravity keeps stomach acid pooled away from the opening to your esophagus. Roll to your right side, and that relationship flips: acid can more easily creep upward into the esophagus, causing heartburn.
If you deal with gastroesophageal reflux, especially at night, left-side sleeping is one of the simplest lifestyle changes you can make. It won’t replace other treatments, but it can meaningfully reduce how often acid reaches your throat while you sleep.
Digestion and Bowel Regularity
Your digestive tract has a specific layout that left-side sleeping works with, not against. Waste moves from the small intestine into the large intestine through a valve in your lower right abdomen. From there, it travels up the right side of your abdomen (the ascending colon), across the top (the transverse colon), and down the left side (the descending colon) toward the rectum.
When you sleep on your left side, gravity assists this entire journey. Waste moves more naturally through each section of the colon, which may encourage a bowel movement in the morning. It’s a subtle effect, but people who struggle with sluggish digestion or bloating sometimes notice a difference.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that works primarily while you sleep. It flushes cerebrospinal fluid through brain tissue to carry away metabolic byproducts, including the proteins associated with Alzheimer’s disease. A study published in The Journal of Neuroscience found that this flushing system worked most efficiently in a lateral (side-lying) position compared to sleeping on the stomach, where the head is more upright.
The research was conducted in rodents, so the findings haven’t been confirmed in humans yet. But the researchers noted that the lateral position closely mimics the natural sleeping posture most mammals adopt, which may hint at an evolutionary advantage to side sleeping for brain health.
Snoring and Sleep Apnea
If you snore or have obstructive sleep apnea, side sleeping on either side is significantly better than sleeping on your back. When you lie face up, gravity pulls your tongue and the soft tissues in your throat backward, partially blocking your airway. This is why snoring tends to be louder and apnea events more frequent in the supine position.
For many people with positional sleep apnea, the number of breathing interruptions per hour can drop by half or more simply by switching to a side position. Positional therapy, which just means training yourself to stay off your back, is sometimes recommended alongside other treatments. Tennis balls taped to the back of a sleep shirt, wedge pillows, or wearable position trainers can all help if you tend to roll onto your back during the night.
Pregnancy: Side Sleeping From 28 Weeks
During the third trimester, sleeping on your side is the safest option. Research shows that going to sleep on your side from 28 weeks of pregnancy can halve the risk of stillbirth compared to sleeping on your back. The reason is mechanical: lying on your back in late pregnancy puts the full weight of your uterus on major blood vessels, reducing blood flow to both you and your baby.
Either side is acceptable, though many practitioners suggest the left side to optimize blood flow. The most important thing is your starting position. Your body naturally shifts during the night, and waking up on your back doesn’t mean you’ve done something wrong. Just roll back to your side when you notice. This applies to naps, too, not only nighttime sleep.
Skin and Sleep Wrinkles
There is one downside to side sleeping. When your face presses against a pillow for hours, the skin gets compressed, stretched, and pulled in ways that don’t happen when you sleep on your back. In younger skin, these compression lines disappear within minutes of waking. But over time, as skin loses its elasticity, those temporary creases can become permanent wrinkles.
People who consistently sleep on the same side tend to develop a flatter facial profile on that side, with more visible lines. If this concerns you, switching sides throughout the night, using a silk or satin pillowcase (which creates less friction), or sleeping on your back can reduce the effect. For most people, the cardiovascular and digestive benefits of side sleeping outweigh the cosmetic trade-off.
How to Make Side Sleeping More Comfortable
Side sleeping puts specific demands on your body that back sleeping doesn’t. Your pillow needs to be tall enough to fill the gap between your shoulder and your ear, keeping your neck in a neutral line with your spine. Research suggests a pillow height of roughly 4 inches offers the best spinal alignment and comfort for most people, though larger frames may need closer to 6 inches. If you wake up with neck stiffness, your pillow is likely too high or too low.
Placing a pillow between your knees is one of the most effective tweaks for side sleepers. Without it, your top leg drops forward and pulls your pelvis out of alignment, creating a subtle twist in your lower back that compounds over hours. A knee pillow, or a full-length body pillow, keeps your hips stacked and your spine neutral. This is especially helpful if you have lower back pain or hip pain.
Your mattress matters, too. Side sleepers put more concentrated pressure on their shoulders and hips than back sleepers do. A mattress that’s too firm won’t let those areas sink in enough, leading to pressure pain and numbness. Medium to medium-firm surfaces generally work best, allowing your joints to settle while still supporting your midsection.
When Right-Side Sleeping Might Be Better
Left-side sleeping wins for most health concerns, but it’s not universally ideal. Some people with heart conditions feel uncomfortable on their left side because the heart sits slightly left of center in the chest, and lying on that side can create a sensation of pressure or make them more aware of their heartbeat. If left-side sleeping causes discomfort or palpitations, the right side still gives you the airway and brain-clearance benefits of lateral sleeping without the cardiac discomfort.
Comfort also matters more than optimization. The best sleep position is ultimately the one that lets you fall asleep, stay asleep, and wake up without pain. If you sleep well on your right side and have no reflux or digestive issues, there’s no strong reason to force a switch. But if you’re dealing with heartburn, sluggish digestion, or pregnancy, the left side offers clear, specific advantages worth trying.