Lying on your left side keeps your stomach positioned below your esophagus, helps waste move through your colon with gravity, and improves blood flow during pregnancy. It’s one of the simplest things you can do to improve digestion, reduce nighttime acid reflux, and potentially help your brain clear waste more efficiently while you sleep.
It Reduces Acid Reflux
The most well-supported reason to sleep on your left side is acid reflux. Your stomach sits slightly to the left of your midline, and when you lie on your left side, gravity pulls stomach acid away from the opening where your esophagus connects. On your right side, that acid pools closer to the junction and is more likely to creep upward.
A study of 57 patients with gastroesophageal reflux measured how long acid stayed in contact with the esophagus in different sleeping positions. Left-side sleepers had a median acid exposure time of 0.0%, compared to 1.2% on the right side and 0.6% on the back. Perhaps more telling, the time it took for the esophagus to clear acid after an episode was about 35 seconds on the left side versus 90 seconds on the right. That means even when acid does reach the esophagus, your body deals with it roughly two and a half times faster when you’re on your left.
If you regularly wake up with heartburn or a sour taste in your mouth, starting the night on your left side is one of the easiest changes you can make.
It Helps Waste Move Through Your Colon
Your large intestine follows a specific path through your abdomen. Waste travels up the right side (the ascending colon), across the top (the transverse colon), and down the left side (the descending colon) before reaching the rectum. When you lie on your left side, gravity assists this natural flow, pulling waste downward through the descending colon toward the exit.
This is why some people find that sleeping on their left side encourages a bowel movement in the morning. It’s not a dramatic effect, but if you deal with sluggish digestion or constipation, the position works with your anatomy rather than against it.
It Improves Blood Flow During Pregnancy
During pregnancy, left-side sleeping becomes increasingly important as the uterus grows. The inferior vena cava, the large vein that returns blood from your lower body to your heart, runs along the right side of your spine. As the uterus expands, lying on your back or right side can compress this vein, reducing blood flow back to the heart and, in turn, to the placenta.
Sleeping on your left side shifts the uterus off the vena cava, allowing blood to flow freely. This delivers more oxygen and nutrients to the baby. Healthcare providers worldwide recommend it as the ideal sleeping position during pregnancy.
That said, an NIH-funded study found that sleeping on your back or side through the 30th week of pregnancy did not appear to increase the risk of stillbirth, reduced birth size, or blood pressure complications. The researchers noted their data only covered up to 30 weeks, so the guidance to favor left-side sleeping becomes more important in the final trimester, when the uterus is heaviest and most likely to compress blood vessels.
It May Help Your Brain Clear Waste
Your brain has its own waste-removal system that becomes most active during sleep. This system flushes out metabolic byproducts, including the proteins associated with Alzheimer’s disease, by circulating cerebrospinal fluid through brain tissue.
A study published in The Journal of Neuroscience found that this waste clearance system worked most efficiently in the lateral (side-lying) position compared to lying face-up or face-down. In the prone position, where the head was most upright, waste clearance was slowest, with more fluid retained rather than flushed out. The lateral position closely mimics the natural sleeping posture of the animals studied, which may explain why most mammals instinctively curl onto their sides to sleep. This research was conducted in rodents, so it hasn’t been directly confirmed in humans, but the anatomy of the system is similar.
It Can Reduce Snoring and Sleep Apnea
When you sleep on your back, gravity pulls the tongue and soft tissues of the throat backward, narrowing your airway. This is why snoring tends to be loudest in the supine position. For people with obstructive sleep apnea, switching to side sleeping can make a meaningful difference.
A Cochrane review found that positional therapy (keeping someone off their back) reduced apnea events by an average of about 7 per hour. That’s not as effective as a CPAP machine, which achieved roughly 6 additional events per hour of reduction beyond positional therapy alone. But for people with mild to moderate sleep apnea, or those who can’t tolerate a CPAP, side sleeping is a practical first step. Either side works for this benefit since the mechanism is about keeping the airway open, not stomach positioning.
When Left-Side Sleeping Isn’t Ideal
Left-side sleeping isn’t right for everyone. People with heart failure often experience shortness of breath that gets worse when they lie on their left side. This happens because the position shifts the heart slightly, increasing the sensation of pressure in the chest. Many heart failure patients find the right side more comfortable and easier to breathe on.
People with left shoulder injuries or chronic pain may also find left-side sleeping aggravates their symptoms. The weight of your body pressing down on an already inflamed shoulder joint for hours can slow recovery and disrupt sleep quality, which defeats the purpose.
How to Stay Comfortable on Your Left Side
The main challenge with side sleeping is keeping your spine, neck, and hips aligned. Your pillow needs to be thick enough to fill the gap between your shoulder and head. For most people, this means a full-size pillow or even two, since a flat pillow lets your neck tilt downward and creates strain.
Placing a pillow between your knees is equally important. Without one, your top leg drops across your body, pulling your hips out of alignment and putting direct pressure on the inner knee. A knee pillow keeps your hips stacked and reduces strain on your lower back. This is especially useful during pregnancy, after hip surgery, or if you have any lower back issues.
If you tend to roll onto your back during the night, placing a body pillow behind you can act as a gentle barrier. Some people also tuck a tennis ball into a pocket sewn onto the back of their sleep shirt, which creates enough discomfort to prompt a return to the side without fully waking up.