Sleeping on your left side does offer a few specific health advantages, particularly for digestion and during pregnancy. But it’s not universally “better” for everyone, and for some people, it can actually be less comfortable or even counterproductive. The real answer depends on what your body needs.
The Digestive Advantage Is Real
The strongest case for left-side sleeping comes down to anatomy. Your stomach naturally sits on the left side of your body, and when you lie on that same side, gravity keeps stomach acid pooled away from the opening to your esophagus. In a study of 57 people with chronic heartburn monitored during sleep, lying on the left side didn’t reduce how often acid backed up into the esophagus, but acid cleared significantly faster compared to lying on the back or right side. If you deal with nighttime heartburn or gastroesophageal reflux, this one positional change can make a noticeable difference in how quickly that burning sensation fades.
Beyond acid reflux, the left-side position may help with general digestion. Because of how your digestive tract is arranged, gravity assists waste movement from the small intestine into the large intestine when you’re on your left side. This is also why doctors sometimes recommend lying on your left side to relieve gas or bloating.
Side Sleeping and Brain Waste Clearance
Your brain has its own waste-removal system that becomes most active 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 was most efficient in the lateral (side-lying) position compared to sleeping on the back or stomach. The researchers proposed that side sleeping may have evolved specifically to optimize this waste removal during rest.
It’s worth noting that this research was conducted in rodents, and the findings applied to side sleeping in general, not specifically the left side. Still, it’s a compelling reason to favor a side position over sleeping flat on your back or facedown.
What Changes During Pregnancy
Pregnant women have long been told to sleep on their left side, but recent research has refined this advice. A large meta-analysis published in The Lancet found that going to sleep on the left or right side appears equally safe. What matters is avoiding the supine (flat on your back) position after 28 weeks of pregnancy. Women who fell asleep on their backs had 2.6 times the odds of late stillbirth compared to those who fell asleep on their side. The researchers estimated that if every pregnant woman past 28 weeks settled to sleep on either side, late stillbirth rates could drop by roughly 5.8%.
So if you’re pregnant, left-side sleeping is fine, but right-side sleeping is just as safe. The key is simply not falling asleep on your back in the third trimester.
Snoring and Sleep Apnea
Side sleeping in general helps keep your airway open, which is why it’s often recommended for people who snore or have obstructive sleep apnea. When you sleep on your back, gravity pulls the tongue and soft tissues toward the back of the throat, partially blocking airflow. Rolling to either side reduces this effect.
Whether the left side specifically is better than the right for sleep apnea isn’t well established. One case study from the American Thoracic Society showed a dramatic difference between left and right positions for a single patient, but the researchers themselves noted the data was limited. For most people with positional sleep apnea, the benefit comes from getting off your back, regardless of which side you choose.
When Left-Side Sleeping Can Backfire
If you have heart failure or certain heart conditions, left-side sleeping may not be ideal. Research has found that sleeping on the left side causes the heart to shift and rotate within the chest, which shows up as changes on heart monitoring. People with dilated cardiomyopathy (an enlarged, weakened heart) tend to naturally prefer sleeping on their right side, likely because the left-side position creates a sensation of pressure or discomfort as the heart presses closer to the chest wall.
This doesn’t mean left-side sleeping is dangerous for people with healthy hearts. The positional shift is minor and your body adapts. But if you have a known cardiac condition and find left-side sleeping uncomfortable, there’s a good physiological reason for that, and sleeping on your right side is a reasonable alternative.
Protecting Your Shoulders and Spine
One downside of any side sleeping position is the pressure it places on the shoulder underneath you. The shoulder can collapse into the mattress or scrunch up toward your neck, creating misalignment that leads to morning stiffness or pain over time. This applies equally to left and right side sleeping.
To minimize this, use a firm pillow that keeps your ears in line with your shoulders. Your chin should stay neutral, not tucked into your chest. A medium-firm mattress helps prevent your shoulder from sinking too deeply. If you alternate sides throughout the night, you’ll distribute the pressure more evenly rather than loading one shoulder every single night. Placing a pillow between your knees also helps keep your hips and lower spine aligned.
Which Side Should You Actually Sleep On
For most healthy adults, left-side sleeping offers a slight edge for digestion and acid reflux. If heartburn is something you regularly deal with at night, it’s one of the simplest changes you can make. During pregnancy, either side works, just avoid your back after 28 weeks. If you have heart failure, the right side is likely more comfortable and places less strain on your heart.
The honest reality is that you move throughout the night regardless of what position you start in. Most people shift between positions dozens of times during sleep. Rather than obsessing over one “perfect” position, the more practical approach is to set yourself up in the position that addresses your biggest concern, whether that’s reflux, snoring, or shoulder pain, and let your body do the rest.