Neither the left nor right side is universally “best” for everyone. The better choice depends on what’s going on in your body. Left-side sleeping wins for acid reflux and pregnancy, while right-side sleeping may be preferable for heart failure and could edge out the left side for brain waste clearance. For most healthy adults without these specific concerns, either side works well, and both are significantly better than sleeping on your back or stomach.
Left Side Is Better for Acid Reflux
If you deal with heartburn or gastroesophageal reflux, sleeping on your left side is the clearest recommendation in sleep medicine. The reason is simple anatomy: when you lie on your left, your esophagus sits above your stomach. Gravity helps keep stomach acid where it belongs. Flip to your right, and the esophagus drops below the junction where it meets the stomach, making it much easier for acid to flow upward. Acid also takes longer to clear from the esophagus in the right-side position.
A systematic review and meta-analysis published in the National Library of Medicine confirmed that left-side sleeping is associated with improved reflux symptoms. If nighttime heartburn is disrupting your sleep, switching to your left side is one of the simplest and most effective changes you can make.
Left Side Is Recommended During Pregnancy
Pregnant women, especially in the second and third trimesters, are typically advised to sleep on their left side. The growing uterus can compress the inferior vena cava, the large vein that returns blood from your lower body to your heart. This compression is worst when lying flat on your back and can reduce blood flow to the fetus while causing a drop in the mother’s blood pressure. The left-side position takes pressure off this vein most effectively. In clinical practice, if a woman shows signs of fetal distress during labor, the standard move is to reposition her onto her left side.
Right Side May Be Better for Heart Failure
People with heart failure often find that lying on the left side makes breathing harder. The heart sits slightly left of center in the chest, and in the left-side position, it shifts further due to gravity, which can increase the sensation of pressure and shortness of breath. According to the American Heart Association, many people with heart failure naturally gravitate toward sleeping on their right side for this reason. If you have heart failure and notice more discomfort on your left, your body is giving you useful feedback.
Side Sleeping Helps Your Brain Clear Waste
Your brain has a waste-clearance system, sometimes called the glymphatic system, that flushes out metabolic byproducts during sleep. This includes proteins like amyloid-beta, which are linked to Alzheimer’s disease. Research 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.
Interestingly, the study found that the right lateral position showed slightly more efficient brain clearance than other positions in the rodent models tested. A separate review from the National Institutes of Health echoed this, noting that glymphatic transport was most efficient in the right lateral sleeping position, with more cerebrospinal fluid clearance occurring compared to back and stomach sleeping. These findings haven’t been fully confirmed in humans yet, but they suggest that side sleeping in general, and possibly the right side in particular, supports the brain’s nightly housekeeping.
Side Sleeping and Sleep Apnea
For people with obstructive sleep apnea, sleeping on either side is better than sleeping on your back. Back sleeping allows gravity to pull the tongue and soft tissues backward, narrowing the airway. Research shows that the number of breathing interruptions per hour drops when switching from back to side sleeping, particularly during the deeper stages of non-REM sleep.
There’s an important caveat, though. In people with severe sleep apnea, particularly those who are very overweight, the improvement from side sleeping alone may not be enough. The number of breathing interruptions can remain clinically high even in the side position. For milder cases and for people at a lower weight, positional changes tend to make a more meaningful difference.
How to Protect Your Spine and Joints
Side sleeping puts pressure on the shoulder and hip you’re lying on, which can lead to pain over time, especially if you favor one side exclusively. If you already have shoulder bursitis or irritation, sleeping on that shoulder will almost certainly make it worse.
Proper pillow placement makes a big difference in keeping your body aligned. Your head pillow should be thick enough to fill the gap between your ear and the mattress, keeping your neck straight rather than angled up or down. A pillow that’s too high forces your neck into a bent position that can cause stiffness and strain. Place a second pillow between your knees to prevent your top leg from pulling your pelvis out of alignment. You can bend your hips and knees slightly for comfort, but pulling them up too high will round your lower back.
If you’re waking up with shoulder or hip soreness, try alternating sides during the night or choosing a mattress with enough give to cushion those pressure points without sagging.
Which Side to Choose
Here’s a practical way to think about it:
- Sleep on your left if you have acid reflux, heartburn, or are pregnant.
- Sleep on your right if you have heart failure or experience breathlessness on your left side.
- Sleep on either side if none of those apply. Both are better than back or stomach sleeping for airway health and brain waste clearance.
Most people shift positions multiple times during the night without realizing it, and that’s perfectly normal. If you’re trying to train yourself to stay on one side, a body pillow placed behind your back can help prevent you from rolling over. For the majority of healthy sleepers, the best side is whichever one lets you fall asleep comfortably and stay asleep through the night.