Side sleeping, specifically on your left side, is the best position for most adults. It supports healthy blood flow, reduces acid reflux, and helps your brain clear waste more efficiently during the night. That said, the ideal position also depends on your body and any conditions you’re dealing with, so the right answer isn’t identical for everyone.
Why Side Sleeping Comes Out on Top
Side sleeping wins on multiple fronts. It keeps your airway open better than back sleeping, which makes it a natural fit for people who snore or have sleep apnea. It also keeps your spine in a relatively neutral position without forcing your neck into rotation the way stomach sleeping does.
The left side has a slight edge over the right for most people. Because of where your stomach sits anatomically, sleeping on the left uses gravity to keep stomach acid from creeping back up into your esophagus. The American Gastroenterological Association specifically recommends left-side sleeping for people with nighttime acid reflux, since this position makes it harder for acid to breach the sphincter between your stomach and esophagus. If you’ve ever noticed heartburn getting worse at night, switching to your left side can make a noticeable difference.
Left-side sleeping also promotes better blood flow overall. During pregnancy, this becomes especially important. In the third trimester, lying on your back allows the weight of the uterus to compress the inferior vena cava, a major vein that returns blood from your lower body to your heart. Sleeping on the left side avoids that compression, improves blood flow to the baby, and supports better kidney function. The American College of Obstetricians and Gynecologists recommends this position to reduce swelling in the legs and ankles as well.
How Sleep Position Affects Your Brain
Your brain has its own waste-clearance system that works primarily during sleep, flushing out metabolic byproducts, including proteins linked to neurodegenerative disease. A study published in the Journal of Neuroscience found that this system works most efficiently when sleeping on your side compared to sleeping on your back or stomach. In the study, the prone (stomach-down) position showed the slowest clearance and the most waste retention, while the lateral (side) position allowed waste to be removed most effectively.
This is one of the more compelling arguments for side sleeping beyond comfort. Your brain is doing critical maintenance work every night, and the position you sleep in appears to influence how well that process runs.
When Back Sleeping Makes Sense
Back sleeping has one clear advantage: it’s the only position that keeps mechanical pressure off your face. When you sleep on your side or stomach, your skin gets compressed, stretched, and subjected to shear forces for hours at a time. Over years, these forces contribute to sleep wrinkles that form at the points where skin buckles under pressure. Researchers have noted that these compression-related wrinkles are distinct from expression lines, though sleep patterns can reinforce them over time. If preventing facial aging is a priority, back sleeping is the only reliable way to minimize that mechanical damage.
Back sleeping also distributes your weight evenly across the widest surface of your body, which can feel good for people with certain types of back pain. Placing a pillow under your knees while on your back helps relax the lower back muscles and maintain the natural curve of your lumbar spine. A small rolled towel under your waist can add extra support if needed.
The tradeoff is significant, though. Back sleeping tends to worsen snoring and obstructive sleep apnea because gravity pulls the tongue and soft tissues toward the airway. It also aggravates acid reflux. And for anyone in late pregnancy, it’s actively discouraged due to the vena cava compression issue.
Why Stomach Sleeping Causes Problems
Stomach sleeping is the least recommended position. The core problem is your neck: because you can’t breathe through a pillow, you’re forced to turn your head to one side for hours at a time. Harvard Health Publishing notes that this position arches the back and rotates the neck, creating strain on the cervical spine that can lead to stiffness and pain over time.
Beyond the neck, stomach sleeping also puts your face under sustained compression, contributing to the same wrinkle-forming forces seen with side sleeping but often more intensely. And the brain waste-clearance research found that the prone position resulted in significantly more waste retention compared to side sleeping, suggesting your brain’s nightly cleanup is less effective face-down.
If you’re a committed stomach sleeper and can’t switch, using a very thin pillow (or no pillow) can reduce the angle of neck rotation somewhat. But the structural disadvantages of this position are hard to engineer away completely.
Pillow Placement for Each Position
The right pillow setup matters almost as much as the position itself. One small study found that a pillow height of about 4 inches offered the best spinal alignment, the most comfort, and the least muscle activity during sleep. The general recommendation is a pillow between 4 and 6 inches, adjusted for your body size.
For side sleepers, the pillow needs to fill the gap between your shoulder and your head to keep your cervical spine straight. A pillow between the knees is equally important: it aligns your spine, pelvis, and hips by preventing the top leg from pulling your body out of alignment. Drawing your legs up slightly toward your chest while side sleeping further helps maintain a neutral spine.
For back sleepers, a pillow under the knees takes pressure off the lower back by gently flexing the hips. The head pillow should be thick enough to support the natural curve of the neck without pushing the head forward.
For stomach sleepers, thinner is better. A thick pillow forces the neck into a sharper angle of rotation, so keeping the pillow as flat as possible (or placing one under the pelvis instead) can reduce some of the strain on the lower back.
Special Considerations for Heart Conditions
People with heart failure often find that the general “left side is best” advice doesn’t apply to them. Many experience shortness of breath that worsens when sleeping on the left side, likely because of the heart’s position and the additional pressure this creates. For these individuals, the right side is often more comfortable and functional.
Needing to sleep upright, whether propped on pillows or in a recliner, because lying flat causes breathlessness is a sign that something more serious may be going on with heart function. That pattern is worth bringing up with a cardiologist, as it can indicate fluid buildup that needs treatment.
Switching Positions Takes Time
If you’ve been a stomach sleeper for decades, you won’t become a comfortable side sleeper overnight. Your body has strong positional habits during sleep, and researchers have acknowledged that consciously changing sleep patterns is extremely difficult. A few strategies that help: using a body pillow to prevent rolling onto your stomach, placing a pillow behind your back as a physical barrier, and giving yourself a few weeks of adjustment before deciding whether the new position works. Most people shift positions multiple times during the night regardless, so the goal isn’t perfection. It’s spending more of your sleep time in a position that works with your body rather than against it.