There’s no single “wrong” side to sleep on for everyone. The side you should avoid depends on your specific health situation. For acid reflux, the right side is worse. For shoulder pain, avoid the injured side. During late pregnancy, the main concern is avoiding your back, not a particular side. Here’s what the evidence says for each scenario.
Acid Reflux: Avoid the Right Side
If you deal with heartburn or GERD, sleeping on your right side is the position most likely to cause problems. The American Gastroenterological Association recommends left-side sleeping because of how your stomach connects to your esophagus. When you lie on your left side, gravity keeps stomach acid pooled away from the opening to your esophagus. Roll to your right, and that acid flows more easily upward, increasing both the number and duration of reflux episodes overnight.
This isn’t a subtle difference. Right-side sleeping is consistently associated with more acid exposure in the esophagus during sleep. If nighttime heartburn regularly wakes you up or leaves you with a sore throat in the morning, switching to your left side is one of the simplest changes you can make. Elevating the head of your bed by a few inches adds another layer of gravity working in your favor.
Pregnancy: Back Sleeping Is the Real Risk
You’ve probably read that you must sleep on your left side throughout pregnancy. That advice is overstated. The American College of Obstetricians and Gynecologists says you can sleep on either side, left or right. The position to actually avoid in the second and third trimesters is flat on your back, because your growing uterus presses on the large blood vessels that supply it with blood.
Left-side sleeping does offer slightly better circulation, but right-side sleeping still keeps those major vessels relatively uncompressed. It’s a fine alternative when your left hip gets sore. If you wake up on your back, there’s no need to panic. Just roll to whichever side is comfortable. Placing a pillow under your belly, between your knees, and behind your lower back can help you stay on your side through the night and take pressure off your hips.
Heart Conditions: Left Side May Feel Uncomfortable
For people with certain heart conditions, the left side can feel wrong. When you lie on your left, gravity pulls the heart slightly toward the chest wall, changing its electrical activity enough to show up on an electrocardiogram. On the right side, tissues between the lungs hold the heart more firmly in place, and readings stay normal.
For most healthy people, this shift is harmless. But if you have heart failure, arrhythmias, or other cardiac concerns and notice palpitations or discomfort when lying on your left side, sleeping on your right may feel better and reduce that sensation of your heart pounding against your chest. This is worth mentioning to your cardiologist if it’s a recurring issue.
Shoulder Pain: Avoid the Injured Side
This one is straightforward. If you have a rotator cuff injury, bursitis, or any kind of shoulder inflammation, sleeping on that side compresses already sensitive tissue against the mattress for hours. The sustained pressure can turn a dull ache into sharp pain that wakes you repeatedly.
Sleep on the opposite side instead, or on your back with a small pillow supporting the affected arm. If both shoulders hurt, back sleeping with proper support is your best option. A pillow between the knees can keep your spine aligned if you do sleep on the uninjured side.
Digestion: Left Side Helps Things Move
Your digestive tract has a specific layout that responds to gravity. The valve connecting your small intestine to your large intestine sits in your lower right abdomen. When you sleep on your left side, gravity helps waste travel upward through the ascending colon on your right, across the transverse colon, and down into the descending colon on your left. This natural path encourages a bowel movement in the morning.
Sleeping on your right side doesn’t cause digestive harm, but if you struggle with constipation or sluggish digestion, left-side sleeping may give your system a slight assist overnight.
Brain Waste Clearance: Side Sleeping Beats Other Positions
Your brain has its own waste-removal system that’s most active during sleep. A study published in The Journal of Neuroscience found that this cleanup process works most efficiently when sleeping on your side compared to sleeping on your back or stomach. In the study, sleeping face-down (prone) resulted in significantly more waste retention and slower clearance, particularly in areas like the hippocampus, which is critical for memory.
The research was conducted in rodents, so the exact translation to humans isn’t settled. But side sleeping in general, not one specific side, appears to give the brain the best conditions for flushing out metabolic waste, including proteins associated with neurodegenerative diseases.
Wrinkles: Both Sides Apply Pressure
Side sleeping of any kind presses your face into the pillow for hours, subjecting facial skin to compression, shearing, and stretching forces. Over years, this creates “sleep wrinkles,” lines that form where the skin buckles under pressure and that gradually become permanent. These are distinct from expression lines, though the mechanical forces from sleep can reinforce creases that already exist from facial movement.
If this concerns you, back sleeping eliminates facial compression entirely. A silk or satin pillowcase reduces friction but doesn’t eliminate the compressive forces that cause these lines.
How to Train Yourself to Sleep on a New Side
Most people shift positions dozens of times per night, so “choosing” a side really means choosing which side you fall asleep on and return to most often. A few strategies can help you stick with a new position:
- Pillow between the knees: This holds your hips in alignment, reduces lower back pressure, and makes side sleeping more comfortable overall. It’s the single most recommended adjustment for side sleepers.
- Body pillow along your back: Placing a firm pillow behind you creates a physical barrier that discourages rolling onto your back or opposite side during the night.
- Pillow under the belly (pregnancy): Supports the weight of a growing abdomen and reduces the pull on your lower back when side sleeping.
Expect an adjustment period. It can take several nights to a few weeks before a new position feels natural. Extra pillows help your body learn the new posture, and you can gradually remove them as the habit becomes automatic.