For most people, the left side is the best side to sleep on. It benefits digestion, keeps airways more open, and supports your brain’s natural waste-clearing process during sleep. That said, certain conditions like heart failure can make the right side a better choice, so the “best” side depends partly on your health.
Why the Left Side Wins for Most Sleepers
Sleeping on your left side positions your stomach below your esophagus, letting gravity work in your favor. The muscular ring connecting your esophagus to your stomach sits higher than your stomach contents in this position, which means acid drains away from your throat rather than pooling near it. If you deal with heartburn or acid reflux, this alone can make a noticeable difference in how you feel at night and in the morning.
The left side also has an edge when it comes to brain health. A 2015 study published in the Journal of Neuroscience found that the brain’s waste-clearance system works most efficiently in the lateral (side-lying) position compared to sleeping on your back or stomach. This system flushes out metabolic byproducts, including proteins linked to neurodegeneration. The researchers proposed that side sleeping may have evolved specifically to optimize this overnight cleaning process.
Digestion and Acid Reflux
Your stomach naturally curves to the left. When you lie on your right side, the junction between your esophagus and stomach sits lower than the pool of stomach acid, making it easier for acid to creep upward. Flip to the left, and that junction rises above the acid level. For people with gastroesophageal reflux disease (GERD), switching to left-side sleeping often reduces nighttime symptoms without any medication changes.
Even if you don’t have diagnosed reflux, sleeping on the left side after a large or late meal can help prevent that uncomfortable burning sensation. It’s one of the simplest adjustments you can make to improve nighttime comfort.
The Best Position During Pregnancy
Left-side sleeping becomes increasingly important as pregnancy progresses past the first trimester. Your body’s largest vein, which 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 can compress this vein against your spine, reducing blood flow to both you and the baby.
Sleeping on the left side prevents the growing uterus from pressing on this vein, allowing maximum blood flow to the uterus and placenta. That translates directly into more oxygen and nutrients reaching your baby. Measurable differences in placental blood flow have been observed based on sleep position alone. While sleeping on your right side is still far better than sleeping on your back during pregnancy, the left side provides the greatest circulatory benefit.
When the Right Side Is Better
People with heart failure often find that sleeping on the left side worsens shortness of breath. The American Heart Association has noted that this leads many heart failure patients to prefer sleeping on their right side. The heart sits slightly left of center in the chest, and lying on that side can increase the sensation of pressure or breathlessness for people whose hearts are already struggling to pump efficiently.
If you have a painful left shoulder, sleeping on your right side is also the practical choice. Lying directly on an injured or inflamed shoulder compresses the joint and can worsen impingement. When sleeping on the non-affected side, stack two pillows in front of your chest to support the injured arm, keeping it elevated and reducing strain on the shoulder joint.
Side Sleeping vs. Back and Stomach
Both left and right side sleeping outperform back sleeping in one key area: airway openness. When you lie on your back, gravity pulls your tongue and soft tissues toward the back of your throat, which can partially block your airway. This is the primary reason back sleeping worsens snoring and obstructive sleep apnea. Side sleeping keeps the airway more open by shifting those tissues to one side.
Stomach sleeping is generally the least recommended position. It forces you to turn your head to one side to breathe, holding your neck in a twisted position for hours. This sustained rotation strains the muscles along the back of the neck, shoulders, and upper back. Stomach sleeping also pushes your spine into an exaggerated arch, puts downward pressure on your knee joints, and holds your feet in an unnatural position. If you’re a committed stomach sleeper and wake up with neck or back pain, your sleep position is a likely contributor.
How to Make Side Sleeping More Comfortable
The most common complaint from side sleepers is shoulder or hip discomfort, but the right pillow setup solves most of these issues. Your pillow should be roughly 4 inches tall to keep your head, neck, and spine in a straight line. One small study found that this height offered the best spinal alignment and greatest comfort while requiring the least muscle activity. If the pillow is too high, it bends your neck sideways. Too low, and your head drops, pulling on the muscles of your upper neck and shoulder.
Placing a pillow between your knees takes pressure off your hips and keeps your pelvis aligned with your spine. This is especially helpful if you have lower back pain. The pillow doesn’t need to be thick; even a thin one prevents your top knee from pulling your spine out of alignment.
Avoid down or feather pillows if neck support is a priority. They compress easily and offer little structural support compared to memory foam or latex options. A pillow that holds its shape throughout the night keeps your spine neutral even as you shift positions.
If you tend to roll onto your back during the night, placing a firm pillow or rolled towel behind you can help train your body to stay on its side. Some people also find that hugging a body pillow stabilizes their position and makes side sleeping feel more natural.