Sleeping on your left side is the healthiest option for most people. It reduces acid reflux, keeps your airway open, and improves blood flow during pregnancy. That said, the “best” side depends on what your body needs. Some conditions actually favor the right side, and either side beats sleeping on your back for breathing and digestion.
Why Left Side Sleeping Wins for Most People
The left side gets the top recommendation because of simple anatomy. Your stomach naturally curves to the left, and when you lie on that side, your esophagus sits above the junction where it meets your stomach. Gravity keeps stomach acid where it belongs. Flip to the right side and that relationship reverses: your esophagus drops below the stomach opening, making it easier for acid to creep upward. A meta-analysis of clinical studies found that left-side sleeping significantly reduces the total time acid spends in your esophagus compared to both right-side and back sleeping.
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. It won’t replace other treatments, but it addresses the problem mechanically rather than chemically.
How Side Sleeping Helps With Snoring and Sleep Apnea
When you sleep on your back, gravity pulls your tongue and the soft tissues in your throat backward, partially blocking your airway. This is the main reason back sleeping worsens snoring and obstructive sleep apnea. Switching to either side keeps that tissue from collapsing inward.
The difference can be dramatic. In people whose sleep apnea is position-dependent (a large subgroup), breathing disruptions drop by half or more when they move off their back. One study found that people averaging 43 breathing interruptions per hour on their back dropped to about 8 per hour on their side. Other studies have shown even steeper reductions, with some patients going from dozens of events per hour in the supine position to nearly zero when sleeping laterally. For snoring and sleep apnea, left and right side perform similarly. The key is simply getting off your back.
Left Side Sleeping During Pregnancy
The American College of Obstetricians and Gynecologists advises against back sleeping in the second and third trimesters. The growing uterus presses directly on the inferior vena cava, the large vein that returns blood from the lower body to your heart. This compression can lower blood pressure and reduce blood flow to the fetus.
Left-side sleeping is the standard recommendation because it takes pressure off that vein most effectively, promoting optimal blood flow to the uterus and reducing swelling in the legs and ankles. Right-side sleeping is also acceptable and far better than back sleeping, but the left side offers the clearest path for circulation. If you wake up on your back, don’t panic. Just roll to your side and settle back in. The concern is about sustained back sleeping, not briefly passing through that position.
When Right Side Sleeping Is Better
The left side isn’t universally superior. One area where the right side may have an edge is brain waste clearance. During sleep, your brain flushes out metabolic waste through a system that relies on cerebrospinal fluid. Research on this process has found that clearance is most efficient in the right lateral position compared to sleeping on your back or stomach. This is still an evolving area of study, but it’s a notable point in the right side’s favor.
People with certain heart conditions sometimes find left-side sleeping uncomfortable because the heart shifts slightly closer to the chest wall in that position, creating a sensation of pressure or palpitations. If sleeping on your left side feels uncomfortable in your chest, the right side is a perfectly good alternative that still delivers the airway and general digestive benefits of lateral sleeping.
The Downsides of Side Sleeping
Side sleeping does put more pressure on your shoulder and hip than back sleeping does. If you spend the whole night on one side, you’re loading that shoulder joint for hours. People with shoulder bursitis or rotator cuff problems typically can’t tolerate sleeping on the affected side because it worsens pain. The same applies to hip bursitis: sustained pressure on an inflamed bursa makes it angrier.
The fix isn’t to abandon side sleeping. It’s to alternate sides when possible and use support strategically. A pillow between your knees takes torsional stress off your hips and lower back. If one shoulder is the problem, sleep on the opposite side. Some people also find that hugging a pillow in front of their chest prevents the top shoulder from rolling forward and pinching.
Getting Your Alignment Right
The biggest mistake side sleepers make is using a pillow that’s too flat or too thick. When you’re on your side, the gap between your head and the mattress is the full width of your shoulder. A pillow that’s too thin lets your head drop, bending your neck toward the bed. One that’s too high pushes your head upward. Either way, your cervical spine spends the night out of its neutral curve, which often shows up as neck stiffness or headaches in the morning.
Research on pillow height suggests that around 10 centimeters (roughly 4 inches) of loft tends to maintain the natural curve of the cervical spine for most people, though individual variation matters. Interestingly, studies have found that body measurements like shoulder width don’t reliably predict your ideal pillow height, so the best approach is to test what feels neutral. When lying on your side, your nose should be roughly aligned with the center of your chest, and your neck should feel straight rather than kinked in either direction.
Your mattress plays a role too. A firmer surface doesn’t let your shoulder sink in, which means you need a thicker pillow to bridge the gap. A softer mattress lets your shoulder compress into it, reducing the distance your pillow needs to cover. If you’ve recently changed your mattress and your neck feels off, your pillow may need to change with it.
Practical Tips for Staying on Your Side
Knowing the best position doesn’t help much if you can’t maintain it. Most people shift positions 10 to 30 times per night, and you can’t consciously control what happens after you fall asleep. A few approaches help. Placing a firm pillow or rolled towel behind your back makes it harder to roll onto your back without waking up. Body pillows that run from chest to knees give your limbs something to rest on, which makes side sleeping more comfortable and more likely to stick.
If you’re trying to switch from back sleeping, give it a few weeks. New sleep positions feel awkward at first, and it takes time for your body to adapt. Starting the night on your left side, even if you shift later, still gives you the benefits for the first and often deepest stretch of sleep.