For most people, the left side is the best side to sleep on. It reduces acid reflux, supports digestion, and improves circulation. More than 60% of adults already sleep on their side, making it the most common position, and small adjustments to which side you choose can make a real difference for specific health conditions.
That said, the “right” side depends on your body. People with certain heart conditions do better on the right, and some situations call for sleeping on your back instead. Here’s how to sort it out.
Why the Left Side Wins for Most Sleepers
The left side gets the most universal recommendations, and it comes down to anatomy. Your stomach curves to the left, so when you lie on that side, gravity keeps stomach acid pooled away from the opening to your esophagus. Research from Amsterdam UMC confirmed that left-side sleeping not only reduces acid reflux episodes but also allows any acid that does reach the esophagus to drain back into the stomach more quickly. If you deal with heartburn or GERD, this one change at bedtime can noticeably reduce nighttime symptoms.
Left-side sleeping also supports your body’s waste-clearing systems. Your brain has a network that flushes out metabolic waste during sleep, and lateral (side) positions appear to be more efficient at this than sleeping on your back or stomach. The lymphatic system, which drains fluid from tissues throughout the body, also benefits from left-side positioning because the thoracic duct, the largest lymphatic vessel, runs along the left side of the spine.
Left Side During Pregnancy
Pregnant people are consistently advised to sleep on their side, especially the left. The reason is a large vein called the inferior vena cava, which returns blood from the lower body to the heart. It runs along the right side of the spine. Lying on your back compresses this vein, reducing blood flow. Lying on the left side takes pressure off it entirely, allowing the most blood flow to reach the placenta and baby.
This becomes most important in the second and third trimesters, when the uterus is heavy enough to create real pressure. If you wake up on your back or right side, there’s no need to panic. Simply rolling back to your left is enough. Many people find a body pillow between the knees helps them stay in position through the night.
When the Right Side Is Better
People with heart failure often instinctively avoid sleeping on their left side. There’s a good reason for this: the left lateral position shifts the heart closer to the chest wall, which can make an enlarged heartbeat feel uncomfortable and increase shortness of breath. Research published in the Journal of the American College of Cardiology found that heart failure patients spontaneously avoid the left side during sleep, likely as a protective strategy against discomfort and further strain on the cardiovascular system. Left-side sleeping in these patients has also been associated with increased stress-response nervous system activity.
If you have heart failure or another structural heart condition, the right side typically feels more comfortable and places less mechanical stress on the heart. This is worth discussing with your cardiologist, but your body’s instinct to avoid the left side is well-founded.
Side Sleeping and Snoring
If you snore or have obstructive sleep apnea, sleeping on either side is significantly better than sleeping on your back. When you lie face-up, gravity pulls the tongue and soft tissues of the throat backward, narrowing or blocking the airway. Rolling onto your side opens that space back up. A meta-analysis of positional therapy studies found that shifting from back sleeping to side sleeping reduced the number of breathing interruptions per hour by about 54%.
For snoring and apnea specifically, left versus right doesn’t matter much. The key is simply getting off your back. Tennis balls sewn into the back of a sleep shirt, wedge-shaped body pillows, or wearable positional devices can all help train you to stay on your side if you tend to roll over.
The Trade-Off: Wrinkles and Eye Pressure
Side sleeping does have a couple of cosmetic and health downsides worth knowing about. The compression, tension, and shear forces of pressing your face into a pillow night after night contribute to sleep wrinkles. These are distinct from expression lines caused by smiling or squinting. Sleep wrinkles form from repeated mechanical compression and get worse over time as skin thins and loses elasticity. They can’t be treated with Botox because no muscle contraction is involved. If this concerns you, silk or satin pillowcases reduce friction, and specialty pillows with cutouts minimize facial contact.
There’s also a small effect on eye pressure. Lying on one side can raise the pressure in the eye on the downward side. For most people this is insignificant, but if you have glaucoma, it’s worth paying attention to which eye has more damage and avoiding that side. A 2010 study found that people with glaucoma who slept on a 30-degree wedge pillow had significantly lower eye pressure overnight compared to lying flat. Elevating the head slightly can help regardless of which side you choose.
How to Set Up Your Pillow and Mattress
The most common mistake side sleepers make is using a pillow that’s too flat. When you’re on your side, your shoulder creates a gap between your head and the mattress that needs to be filled to keep your spine straight. Your ears should line up with your shoulders, and your chin should line up with your sternum. If your pillow is too thin, your head tilts down and strains your neck. Too thick, and it pushes your head up.
Side sleepers generally need a firm, high-loft pillow in the range of 4 to 6 inches tall. People with broader shoulders need the higher end of that range; smaller-framed people can go lower. A pillow between the knees also helps by keeping the hips aligned and reducing lower back strain. If you’re a back sleeper, a medium-loft pillow of 3 to 5 inches works better, and stomach sleepers need the lowest profile possible, around 2 inches or less, to prevent the neck from craning backward.
Your mattress matters too. Side sleepers put concentrated pressure on the shoulder and hip, so a medium to medium-soft mattress that lets those points sink in slightly will keep the spine straighter than a very firm surface. If you wake with shoulder pain or hip numbness, your mattress is likely too firm for side sleeping.
Quick Guide by Condition
- Acid reflux or GERD: Left side. Keeps stomach acid below the esophageal opening.
- Pregnancy: Left side. Takes pressure off the vein returning blood to the heart.
- Heart failure: Right side. Reduces displacement of the heart and discomfort.
- Snoring or sleep apnea: Either side. Reduces airway collapse by roughly 54% compared to back sleeping.
- Glaucoma: Avoid the side of the more affected eye, and consider elevating the head with a wedge pillow.
- Wrinkle prevention: Back sleeping is ideal, but a contoured pillow that reduces facial contact helps side sleepers.
- No specific condition: Left side is the safest general default for digestion, circulation, and brain waste clearance.