Which Side Should You Sleep On? Left vs Right Explained
Most people who sleep on their side don't consciously choose a side. They settle into bed, find the position that feels right, and stay there. But the question of left versus right turns out to be more substantive than it sounds: the research on side preference has genuine clinical implications, particularly for digestion, circulation, and sleep apnoea, and the differences are large enough that the choice matters.
For most adults, one side is meaningfully better than the other.
What's the Short Answer?
For most people, sleeping on the left is generally better. The benefits are most pronounced for people with acid reflux, heartburn, or gastro-oesophageal reflux disease, but they extend to lymphatic drainage and, during pregnancy, fetal circulation. The right side has fewer documented advantages and, for some conditions, measurable disadvantages.
There are, however, some exceptions: People with certain cardiac conditions are sometimes advised to sleep on the right side, since lying on the left can produce uncomfortable awareness of the heartbeat. For most healthy adults without specific medical considerations, however, left side sleeping is the better default.
Why Does Left Side Sleeping Help With Acid Reflux?
The mechanism is anatomical. The oesophagus enters the stomach on the right side of the abdomen. When you lie on your left, the junction between oesophagus and stomach sits above the level of stomach contents, making it harder for acid to reflux upwards. When you lie on the right, the junction sits below the level of stomach contents, and gravity actively works to push acid up the oesophagus.
A 2022 study published in The American Journal of Gastroenterology monitored 57 patients with chronic heartburn during sleep, comparing the three main sleep positions. The researchers found that acid cleared from the oesophagus significantly faster when participants were on their left side compared with their right or back. The number of reflux episodes wasn't drastically different, but the duration of acid exposure was, and exposure duration is what produces oesophageal damage over time.
What Does the Research Say About Pregnancy?
Pregnant women in the second and third trimester are almost universally advised to sleep on the left side. The reason is the inferior vena cava, a large vein that returns blood from the lower body to the heart, which runs along the right side of the spine. Lying on the back or right side allows the weight of the growing uterus to compress this vein, reducing blood return to the heart and, by extension, blood flow to the placenta.
Left side sleeping keeps the uterus off the vena cava and supports better circulation to both mother and fetus. Most obstetric guidelines from around 20 weeks gestation onwards recommend left side as the default position.
What About the Brain's Waste Clearance?
This is a newer area of research. The glymphatic system, the brain's overnight waste-removal process, appears to work most efficiently when sleeping on the side rather than the back or stomach. Some animal studies suggest the left side may improve glymphatic flow, though the human evidence is still preliminary.
Are There Any Downsides to Left Side Sleeping?
For some people, yes. Sleeping on the left can produce a sensation of feeling the heartbeat too clearly, which keeps them awake. People with certain types of heart failure are sometimes advised to sleep on the right side for this reason, though the evidence for cardiac harm from left-sided sleeping in healthy adults is weak.
Shoulder pain on the dominant arm is another consideration. If your left shoulder is injured or sensitive, sleeping on it for hours each night isn't going to help recovery. In that case, right side sleeping with proper pillow support is reasonable.
What About Sleeping on the Right Side?
The right side has fewer documented advantages, but it's not a problematic position for healthy adults without reflux or pregnancy considerations. It tends to produce slightly more snoring than left side sleeping in some studies, and as noted, it's worse for acid reflux. But the daily impact for most people is modest.
People who naturally gravitate to the right side and don't have reflux or other contraindications don't need to forcibly switch. The benefit of left side sleeping for these people is real but small.
Your Pillow Matters More Than the Side
A common mistake is focusing on which side to sleep on while ignoring whether the pillow setup actually supports either side properly. A side sleeper without adequate pillow height kinks the cervical spine for hours regardless of which side they're on, while a side sleeper without a knee pillow rotates the hips forward and pulls the lower spine out of alignment.
Get the pillow loft right (medium to high, matched to your shoulder width), put a pillow between the knees, and the position you choose becomes a much smaller variable.
Bed Size Plays a Role Too
Side sleeping requires more horizontal space than back sleeping. Couples who both prefer side sleeping often find a standard double bed cramped, particularly if one or both partners use a body pillow or pillow between the knees. King size beds offer the additional room that supports proper side-sleeping posture without negotiation. We make a range of premium king size beds for couples at Simba, designed to accommodate two side sleepers without compromising on individual comfort.
The Bottom Line
If you don't have a strong preference, default to the left side; the evidence base is broader and the clinical benefits are more substantial, particularly for reflux. If you have a medical reason to favour the right side, that takes precedence. The pillow setup and bed size matter at least as much as which side you choose, and getting those right makes either position more sustainable.
FAQs
Prolonged compression can contribute to shoulder pain, particularly with insufficient pillow support. A proper loft and pillow firmness reduces the risk.
Compression of nerves and reduced circulation in the arm under your body. Try putting your lower arm forward rather than under your body.
Yes. From around 20 weeks, the left side is recommended for fetal circulation.
Side sleeping reduces snoring significantly compared with back sleeping. Left and right are roughly equivalent for most people.
Yes, using pillow barriers and consistent practice. It takes a few weeks for most people.
Disclaimer: This article is for general informational purposes only and is not intended as medical advice. Sleep position recommendations vary depending on individual health circumstances, including pregnancy, cardiac conditions, and acid reflux severity. Speak to your GP, midwife, or qualified healthcare professional for guidance specific to your situation.
