Your body isn’t symmetrical, so there really is a difference between the left and right sides.
I’m one of those people who can’t get comfortable at night until I’ve rotated through every possible sleeping position, ideally twice. But when I was pregnant, I kept hearing the message that we’re all supposed to sleep on our left side, specifically. On the other hand, there’s research suggesting the right side may be better for people with certain health conditions. Let’s consider all the factors and see whether there’s an optimal side for you.
If you have a sleeping position that is comfortable, and you don’t have any medical issues suggesting that you should change it, you’re probably fine to sleep any way you want.
That said, side-sleeping is often recommended as a good thing. If you’re pregnant, sleeping on your stomach stops being an option. Sleeping on your back while pregnant may impede blood flow, since some of our major blood vessels, the aorta and the vena cava, run along the back of our abdominal cavity. Once the uterus gets big enough, it can put pressure on these blood vessels.
If you have obstructive sleep apnea, you’ll probably be advised not to sleep on your back, since people tend to have more apneic episodes on their back. Sleeping on your back is also generally understood to make snoring worse. (PSA: If you haven’t been tested for sleep apnea, but find that you sleep better when you aren’t on your back, you might want to look into getting tested.)
And if you struggle with back pain, side-sleeping tends to be more comfortable than sleeping on your back or your stomach—although this varies from person to person.
The main reason to sleep on the left side has to do with the shape of the stomach. If you’re envisioning your stomach as a sort of round ball, get that image out of your head right away. Your stomach is not only smaller than you’re probably thinking, it’s also asymmetrical. The esophagus (food pipe) enters it on the right-hand side. (Here’s a diagram from the University of Rochester Medical Center showing exactly where it is and how it’s shaped.)
Because of this shape, lying on your left side means that food and digestive juices have an easier time staying inside the stomach where they’re supposed to be. If you have GERD or you get heartburn easily, sleeping on your right side can make it more likely for stomach acid to end up in the esophagus, making your heartburn worse.
(By the way, most of us have our bodies organized this way, but every now and then somebody’s organs will be in a different position than what is in textbooks. A review in the International Journal of Internal Medicine notes that a mirror-style reversal of organs is so rare that “[e]ven a busy surgeon may expect to encounter this anomaly only once or twice in a lifetime.”)
When you sleep on your left side, your stomach might be happier, but your heart changes position. The heart is located slightly to the left of center in your chest, and there is a structure of connective tissue called the mediastinum that is in the very center of your chest.
When you lie on your right side, the mediastinum keeps the heart in place. But when you lie on your left side, your heart sags and rotates slightly. That’s not a problem for most of us, most of the time, but the movement is significant enough that it can be detected from electrocardiogram readings.
For this reason, it’s thought that the heart beats more efficiently when you’re lying on your right side than on your left. This in turn may help you breathe more easily if you have sleep apnea. And this may be why research in animals has shown that side sleeping may improve the clearance of “glymphatic” waste products from the brain—one of the important body-maintenance tasks that happens while we sleep.
First, if your doctor has advised you to sleep in a certain position, follow their advice. That said, if they want you sleeping in a position that you find uncomfortable or impractical, ask them why they recommend that position, and also how much it matters. It may be that the recommendation is just a suggestion for something that they hope will make you more comfortable, not a medical necessity. The only way to know the difference is to ask.
For the rest of us, it’s generally fine to decide on your sleeping position through experimentation. While the shape of our hearts and stomachs may suggest that one position is better than others, we have complex bodies made of many different parts. If you have an old shoulder injury that gets aggravated by sleeping on that side, you may be better off avoiding that position.
Ultimately, outside of certain positions for certain medical conditions, most of us can choose the sleeping position that is most comfortable—and for most of us, it really won’t make much of a difference. The Sleep Foundation advises that the best side for you “depends on your personal preference as well as the support of your mattress and pillow.” For troubleshooting, they suggest the simple rule: “If you experience discomfort or pain, try a different sleeping position.”
