For most people, sleeping on the left side offers more benefits, particularly for digestion and blood flow. But the best side depends on your specific health situation. People with heart failure, for example, often do better on their right side. Here’s what the evidence says for each major health concern.
Left Side Wins for Digestion
Your stomach sits slightly to the left of your midline, and its natural curve means gravity works in your favor when you sleep on your left side. In this position, your stomach sits below the opening to your esophagus, making it harder for acid to travel upward. 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 sleep on your right side, the opening between your stomach and esophagus sits lower than the pool of stomach acid, essentially making it easier for acid to slosh upward. This is why people with gastroesophageal reflux disease (GERD) or frequent heartburn often notice their symptoms flare up on the right side, especially after eating.
Pregnancy: Left Side Is Recommended
During the second and third trimesters, side sleeping is the standard recommendation. The American College of Obstetricians and Gynecologists advises sleeping on your side with one or both knees bent, using a pillow between your knees and another under your belly for support. The left side is particularly beneficial because it allows the most blood flow to the baby, since it keeps the weight of the uterus off the large vein (the inferior vena cava) that returns blood from your lower body to your heart.
That said, if you wake up on your right side, there’s no need to panic. The key recommendation is to avoid spending long stretches flat on your back, where the full weight of the uterus compresses that major blood vessel. Either side is far better than back sleeping in late pregnancy.
Heart Failure Favors the Right Side
This is the clearest exception to the “left side is better” rule. People with heart failure frequently experience worsening shortness of breath when they lie on their left side. The American Heart Association notes that this discomfort leads many heart failure patients to naturally prefer sleeping on their right.
The likely reason is mechanical. When you lie on your left, your heart shifts slightly due to gravity, and in someone whose heart is already enlarged or struggling to pump efficiently, that extra positional stress can make breathing feel more labored. If you have heart failure or another cardiac condition and notice one side feels noticeably worse, trust what your body is telling you.
Brain Waste Clearance During Sleep
Your brain has its own waste-removal system that ramps up during sleep, flushing out metabolic byproducts including proteins linked to Alzheimer’s disease. A study published in the Journal of Neuroscience found that this cleaning process works most efficiently when sleeping on the side (lateral position) compared to sleeping on the back or stomach. In the study, side sleeping outperformed both alternatives for clearing waste, while stomach sleeping showed the slowest clearance and the most fluid retention.
The research did not find a meaningful difference between left and right lateral positions for brain waste clearance. So for this particular benefit, either side works. The important thing is that you’re not sleeping face-down.
Sleep Apnea: Side Matters, but Varies
Side sleeping generally reduces the number of breathing interruptions in people with obstructive sleep apnea compared to back sleeping, because gravity pulls the tongue and soft tissues forward rather than letting them collapse into the airway. However, the difference between left and right isn’t consistent from person to person. In one case documented by the American Thoracic Society, a patient’s breathing interruptions dropped from 87 events per hour on the right side to just 17 on the left. That’s a dramatic difference, but it reflects individual anatomy rather than a universal rule.
If you have sleep apnea, a sleep study can reveal whether your events cluster on one side. For most people without a diagnosis, simply avoiding your back is the bigger win.
Shoulder Pain on Both Sides
The trade-off with any side sleeping is shoulder pressure. When you lie on your side, your body weight concentrates on a smaller contact area than when you’re on your back, and much of that pressure lands on your shoulder. Research measuring skin pressure has confirmed that the shoulder bears significantly more force in a side-lying position than in a back-sleeping position. Over time, this can contribute to rotator cuff irritation, especially if you sleep in a curled “fetal” position that narrows the space inside the shoulder joint.
This applies equally to left and right. If you already have shoulder pain on one side, sleep on the opposite side or use a supportive pillow arrangement: one firm pillow under your head to keep your neck aligned, and another between your knees to reduce lower-back strain. Hugging a pillow in front of your chest can also take some compression off the bottom shoulder by preventing your upper body from rolling forward.
Choosing Your Best Side
For a healthy adult with no specific conditions, the left side edges ahead overall. You get digestive benefits, reduced reflux risk, and good blood flow, all without any real downside beyond normal shoulder pressure. Switching sides during the night is completely fine and probably inevitable.
If you have heart failure, the right side is likely more comfortable and easier on your breathing. During pregnancy, the left side is preferred but the right is still a safe choice. And if your main concern is sleep apnea or brain health, either side beats sleeping on your back or stomach. The “best” position is ultimately the one that lets you fall asleep, stay asleep, and wake up without pain.