Lying on your left side keeps your stomach below your esophagus, reduces pressure on major blood vessels, and can ease breathing during sleep. It’s most strongly recommended for reducing acid reflux and during late pregnancy, though side sleeping in general carries benefits for snoring and sleep apnea. Here’s what each of those benefits actually looks like in your body.
It Keeps Stomach Acid Where It Belongs
The most well-supported reason to sleep on your left side involves the anatomy of your stomach and esophagus. Your stomach sits slightly to the left of your midline, and the tube connecting it to your mouth (the esophagus) enters from above on the right side. When you lie on your left, gravity pulls the pool of stomach acid away from that opening. Your esophagus sits above your stomach in this position, so acid has to travel uphill to reach it.
Flip to your right side and the geometry reverses. Your stomach ends up positioned above the esophagus, and acid flows more easily toward the opening. Studies have consistently shown that right-side sleeping triggers more heartburn episodes, longer reflux events, and greater acid exposure in the esophagus. If the muscular valve at the bottom of your esophagus is weakened or relaxes too easily (the underlying issue in GERD), right-side sleeping makes the problem significantly worse. A systematic review and meta-analysis confirmed that left-side sleeping is associated with improved reflux symptoms overall.
This is one of the simplest, most effective lifestyle changes for nighttime heartburn. If you regularly wake up with a burning throat or sour taste, switching to your left side costs nothing and can make a noticeable difference within a few nights.
Why It Matters During Pregnancy
For over 60 years, healthcare providers have advised pregnant women to lie on their left side, and the reasoning comes down to one large blood vessel: the inferior vena cava. This vein runs along the right side of your spine and carries blood from your lower body back to your heart. As the uterus grows, it gets heavy enough to compress this vessel when you lie flat on your back. That compression reduces cardiac output and can lower blood flow through the placenta to the fetus.
The consequences of sustained compression range from mild to serious. On the mild end, you might feel lightheaded, nauseated, or short of breath (sometimes called supine hypotensive syndrome). On the more concerning end, reduced placental blood flow can affect oxygen and nutrient exchange with the baby, potentially contributing to fetal heart rate changes and growth restriction. Studies in awake pregnant women have measured reduced heart output in the supine position compared to the left lateral position.
That said, the guidance has become more nuanced than the blanket “always sleep on your left side” advice many people have heard. Current evidence indicates that before 28 weeks of gestation, sleeping position doesn’t appear to affect pregnancy outcomes. Starting at 28 weeks, the key recommendation is to avoid falling asleep on your back. Going to sleep on the right side appears to be equally as safe as the left. So if you’re pregnant and can’t get comfortable on your left, the right side is a perfectly good alternative. The real risk is prolonged back sleeping in the third trimester.
Side Sleeping Reduces Snoring and Apnea
If you snore or have obstructive sleep apnea, sleeping on either side is substantially better than sleeping on your back. When you lie face-up, gravity pulls your tongue and the soft tissues in your throat backward, narrowing the airway. This increases the frequency and severity of breathing pauses during sleep.
The numbers are striking. In one analysis, the average number of apnea and hypopnea events per hour (a standard measure of sleep apnea severity) was 34.2 in the supine position compared to 15.1 when sleeping on the side. Among people whose apnea is strongly position-dependent, the difference was even more dramatic: 43 events per hour on the back versus just 8.2 on the side. Apnea episodes that occurred while back-sleeping were also more severe by every measure, including longer breathing pauses, lower blood oxygen levels, louder snoring, and more disruptive arousals.
This benefit isn’t specific to the left side. Either side works for keeping the airway open. But if you also deal with reflux, the left side gives you both benefits at once.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that becomes most active during sleep. This system, called the glymphatic system, uses cerebrospinal fluid to flush out metabolic byproducts, including proteins linked to neurodegenerative diseases. Gravity affects how blood and cerebrospinal fluid move through the brain, so your sleep position plays a role in how efficiently this cleanup happens.
Interestingly, the research here doesn’t favor the left side specifically. Animal studies have found that glymphatic transport is most efficient in the right lateral sleeping position, with greater cerebrospinal fluid clearance compared to sleeping on the back or stomach. This is worth noting because many popular health articles claim left-side sleeping is best for brain detoxification, but the available evidence actually points to the right side, or to side sleeping in general, as the more effective position for waste clearance.
When the Right Side Is Actually Better
Left-side sleeping isn’t universally ideal. For people with congestive heart failure, the right side may be preferable. Research published in the Journal of the American College of Cardiology found that heart failure patients naturally spent twice as much time on their right side compared to their left or back. This appears to be a self-protective instinct.
When heart failure patients lie on their right side, the overactive stress signaling to the heart quiets down and the calming branch of the nervous system becomes more active, essentially normalizing a balance that’s disrupted by heart failure. There are also practical mechanical reasons: the right-side position frees the left lung (where fluid tends to accumulate in heart failure) from compression, reducing the workload on breathing. It may also help the stomach empty more efficiently, since the stomach’s natural exit curves to the right.
People with certain heart conditions, particularly an enlarged heart, sometimes feel palpitations or discomfort when lying on the left side because the heart shifts closer to the chest wall. If you notice your heartbeat feels uncomfortably strong or irregular on your left side, the right side is a reasonable default.
How to Make Left-Side Sleeping Comfortable
If you’re not a natural left-side sleeper, the switch can feel awkward at first. A few adjustments help. Place a pillow between your knees to keep your hips aligned and reduce lower back strain. Your head pillow should be thick enough to keep your neck in a neutral line with your spine, not angled up or drooping down. If you tend to roll onto your back during the night, placing a firm pillow or rolled towel behind you can act as a gentle barrier.
For pregnant women, a full-body pillow that supports the belly and tucks between the knees can make side sleeping far more sustainable through the third trimester. Some people also find that slightly elevating the head of the bed (by a few inches, using blocks under the bed frame) amplifies the anti-reflux benefits of left-side sleeping by adding gravity’s help.
Shoulder pain is the most common complaint from side sleepers. If your shoulder aches after sleeping on one side, a mattress with enough give to let your shoulder sink slightly can help. Alternating between left and right throughout the night is also fine for most people, unless you have a specific reason (like active reflux symptoms) to stay on the left.