For most people, the left side is the best side to sleep on. It offers the widest range of benefits: improved digestion, reduced snoring, and better waste clearance from the brain. More than 60% of adults already sleep on their side, making it the most common position, and the research consistently favors lateral sleeping over back or stomach sleeping for several key health markers.
That said, “best” depends on your body. If you have shoulder pain on your left side, sleeping on your right is perfectly fine and still gives you most of the same advantages. Here’s what the evidence actually shows.
How Side Sleeping Helps Your Brain
Your brain has its own waste-removal system that works primarily while you sleep. This system flushes out metabolic byproducts, including the proteins linked to Alzheimer’s disease. A study published in The Journal of Neuroscience found that this cleaning process was most efficient when subjects were in the lateral (side) position compared to sleeping on the back or stomach.
Stomach sleeping performed the worst. When the head was in a more upright position, the brain retained waste longer and cleared it more slowly. Side and back sleeping both outperformed stomach sleeping, but side sleeping showed the most efficient overall transport. The researchers noted that the lateral position mimics the natural resting posture of most mammals, suggesting an evolutionary basis for why so many of us instinctively curl onto our sides at night.
Left vs. Right: Does It Matter?
For digestive comfort, the left side has a slight edge. Your stomach sits to the left of your abdomen, and lying on the left keeps the junction between your stomach and esophagus above the level of stomach acid. This makes left-side sleeping particularly helpful if you deal with acid reflux or heartburn at night. Gravity works in your favor, keeping acid where it belongs.
For brain waste clearance specifically, the research tested the right lateral position and found it highly efficient. So if reflux isn’t your concern, either side works well. The practical takeaway: choose the left if you have heartburn, choose either side if you don’t, and avoid the side that causes you pain if you have a shoulder or hip issue.
Side Sleeping and Snoring
Sleeping on your back lets gravity pull the tongue and soft tissues toward the back of the throat, which narrows the airway and increases snoring. For people with obstructive sleep apnea, this effect can be dramatic. Switching to a lateral position often cuts the number of breathing interruptions significantly.
One nuance worth knowing: not every lateral position is equal for everyone. Individual anatomy, nasal structure, and the severity of airway obstruction all play a role. Some people breathe better on one side than the other. If you’ve been told you snore heavily or you wake up feeling unrested despite getting enough hours, a sleep study can reveal whether positional therapy (simply staying off your back) would help.
Pregnancy and Sleep Position
Pregnant women are commonly told to sleep exclusively on their left side to avoid compressing a major blood vessel that returns blood to the heart. The concern is real in theory: the weight of the uterus in later pregnancy can press on this vessel when lying flat on the back. In practice, the risk is smaller than many people believe. Research published in the Journal of Obstetrics and Gynaecology Canada found that while some women develop lightheadedness when lying on their backs, only 2% to 4% of those symptomatic women experience significant compression, and even then, no evidence of harm to the fetus was found.
Side sleeping is still a good default during the second and third trimesters simply because it’s more comfortable as your belly grows. But if you wake up on your back at 3 a.m., there’s no need to panic. Your body will typically signal discomfort before any real problem develops, prompting you to shift naturally.
The Downsides of Side Sleeping
Wrinkles
Pressing your face into a pillow night after night does leave its mark. Research in the Aesthetic Surgery Journal confirmed that compression, tension, and shear forces applied to the face during side and stomach sleeping cause facial distortion over time, leading to sleep wrinkles. These wrinkles form from mechanical pressure, not muscle movement, which means treatments like Botox don’t help with them. If this concerns you, silk or satin pillowcases reduce friction, and sleeping on your back eliminates the issue entirely.
Shoulder and Hip Pressure
The most common complaint from side sleepers is waking up with a sore shoulder or numb arm. When your full body weight compresses one shoulder for hours, blood flow decreases and the joint can become irritated. If you already have shoulder pain, sleep on the opposite side and place a pillow in front of your chest to support the affected arm. This lifts it slightly and reduces pressure on the joint. A pillow between your knees serves the same purpose for your hips, keeping them stacked and preventing your top leg from pulling your spine out of alignment.
How to Set Up Your Bed for Side Sleeping
Pillow height matters more than most people realize. When you’re on your side, the gap between your head and the mattress is wider than when you’re on your back, so you need a thicker pillow to keep your neck straight. Research suggests a pillow height of about 4 inches provides the best spinal alignment, the most comfort, and the least muscle strain. Most side sleepers do well with a pillow in the 4 to 6 inch range, depending on shoulder width.
Down and feather pillows tend to compress too much under the weight of your head, leaving your neck tilted downward by morning. Firmer fills like memory foam, latex, or buckwheat hold their shape better throughout the night.
A few other adjustments that make a difference:
- Knee pillow: A pillow between your knees keeps your pelvis neutral and takes strain off your lower back.
- Mattress firmness: Side sleepers generally need a slightly softer surface than back sleepers. Your shoulder and hip need to sink in enough to keep your spine level rather than bowing upward.
- Arm placement: Avoid tucking your bottom arm under your pillow or head. Extend it slightly forward or hug a body pillow to keep circulation flowing.
When Back Sleeping Is the Better Choice
Side sleeping wins for most people, but back sleeping is better in specific situations. If you’re recovering from surgery on both shoulders, dealing with certain spinal conditions, or trying to minimize facial wrinkles, lying face-up distributes your weight most evenly and keeps pressure off your joints. Back sleeping also keeps your face off the pillow entirely, which can help with acne for people who are prone to breakouts along the cheeks and jawline.
The tradeoff is that back sleeping is the worst position for snoring and sleep apnea. It also requires a thinner pillow to keep your neck from being pushed forward. If you switch between positions throughout the night, which most people do, a medium-loft pillow in the 4-inch range is a reasonable compromise for both side and back sleeping.