For most people, the left side is the better side to sleep on. It reduces acid reflux, supports your brain’s waste-clearing system during sleep, and helps with snoring and sleep apnea. That said, the right side is preferable in certain situations, particularly for people with heart failure. More than 60% of adults already sleep on their side, so the real question for most people isn’t whether to side-sleep, but which side deserves priority.
Why the Left Side Wins for Digestion
The biggest advantage of left-side sleeping comes down to simple anatomy. Your stomach sits slightly to the left of your body’s midline, and the junction where your esophagus meets your stomach is positioned at the top right of your stomach. When you lie on your left side, gravity pulls stomach acid down and away from that junction, making it much harder for acid to travel back up into your esophagus.
Research from Amsterdam UMC confirmed that left-side sleeping not only reduces the frequency of acid reflux episodes but also helps stomach acid that does reach the esophagus drain back into the stomach more quickly. If you deal with heartburn or gastroesophageal reflux, especially at night, switching to your left side is one of the simplest changes you can make. Right-side sleeping, by contrast, positions your stomach above the esophageal junction, essentially giving acid an easier path upward.
Brain Waste Clearance During Sleep
Your brain has its own cleaning system that kicks into high gear while you sleep, flushing out metabolic waste products, including proteins linked to Alzheimer’s disease. A study published in the Journal of Neuroscience found that this system works most efficiently when you sleep on your side (lateral position) compared to sleeping on your back or stomach. Sleeping on your stomach was the worst position, characterized by slower clearance and greater retention of waste in the brain.
The study tested lateral sleeping in general rather than comparing left versus right specifically, so either side appears to offer this benefit over back or stomach sleeping. Given that most humans and many other mammals naturally gravitate toward side sleeping, this finding suggests the preference may have deep biological roots.
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 soft tissues toward the back of your throat, partially or fully blocking your airway. People with positional sleep apnea, where their breathing disturbances are at least twice as severe on their back compared to their side, can see meaningful improvement from simply staying off their back. One study found that using a chest-worn positioning device to encourage side sleeping cut the number of breathing interruptions per hour roughly in half.
There’s no strong evidence that one side is superior to the other for airway openness. The key is staying off your back.
When the Right Side Is Better
People with heart failure often find that sleeping on the left side makes their shortness of breath worse. When you lie on your left, your heart shifts slightly within the chest due to gravity, which can increase pressure and discomfort. Many heart failure patients naturally prefer the right side for this reason, and that instinct is worth following. The American Heart Association notes this is a common and well-recognized pattern.
If you have no heart condition, this isn’t a concern. A healthy heart handles positional changes without issue.
Pregnancy and Sleep Position
Pregnant women are frequently told to sleep exclusively on their left side to avoid compressing a major blood vessel called the inferior vena cava, which returns blood from the lower body to the heart. The concern is that lying on your back or right side could reduce blood flow to the baby. In practice, this advice is more cautious than the evidence supports.
Research published in the Journal of Obstetrics and Gynaecology Canada found that while some pregnant women do experience lightheadedness when lying flat on their backs, only 2% to 4% have significant blood vessel compression. Even among those women, no evidence of harm to the fetus was found. The left side remains a reasonable default during late pregnancy, but if you’re more comfortable on your right side, that’s fine too. The most important thing is avoiding prolonged back sleeping in the third trimester if it makes you feel dizzy or faint.
Protecting Your Shoulders and Hips
The main downside of side sleeping, regardless of which side, is pressure on the shoulder and hip you’re lying on. Over time, this can aggravate existing problems or create new ones. Shoulder impingement, where tendons in the rotator cuff get pinched beneath the shoulder blade, tends to flare up when you sleep on the affected side. If you already have shoulder pain, sleep on the opposite side.
A pillow between your knees makes a noticeable difference for hip and lower back comfort. It keeps your hips aligned and prevents your top leg from pulling your pelvis into a twisted position. This is especially helpful if you have lower back pain, hip bursitis, or knee discomfort. A firm, medium-thickness pillow works better than a thin one that compresses flat overnight.
Your pillow height matters too. Side sleepers need a thicker pillow than back sleepers to fill the gap between the mattress and their head, keeping the neck in a neutral line with the spine. If you wake up with neck stiffness, your pillow is likely too thin or too thick for the position you’re sleeping in.
A Practical Approach
If you have acid reflux, sleep on your left side. If you have heart failure, sleep on your right. If you snore, sleep on either side rather than your back. For everyone else, the left side offers the most overall benefits, but the reality is that most people shift positions multiple times during the night. Starting on your left side is a good default, but don’t lose sleep over it if you wake up in a different position.
If you want to train yourself to stay on one side, a body pillow placed behind your back can discourage you from rolling over. Some people also find that slightly elevating the head of the bed (using a wedge or bed risers) enhances the anti-reflux benefits of left-side sleeping, since you’re combining gravity’s effect on two axes.