This is what sleeping on the left side does for our brain, stomach & glymphatic health

Tips:

Put a pillow under your knees to support the spine and lower back.
Spread the legs and arms out to reduce pressure on the joints.
Choose a pillow that offers good support for the neck, and avoid the ones that tilt your chin toward your chest.
Elevate the head with extra pillows or a wedge pillow to help reduce heartburn, headaches, and sinus buildups.

Sleeping on your stomach

Unfortunately for people who enjoy this position, experts warn against it. “If you are sleeping on your stomach and notice you are suffering back pain, there probably is a reason,” says Bill Fish, a certified sleep science coach. “Since the majority of the weight of the human body is around your center, that core pushes into the sleep surface further and basically puts strain on your spine in the wrong direction, causing back and neck pain.”

Still, here are some tips for those who can’t switch to their side:
Use a flat pillow or none at all.
Don’t tuck your arms under your pillow or head, since this could cause arm numbness or shoulder joint pain.
Alternate the sides of your head to avoid neck pain.
Don’t lift your leg to one side with a bent knee since this could worsen back issues.

Sources
“Side Sleeping: Which Side Is Best and How to Train Yourself to Do It.” CNET. Taylor Leamey. July 7, 2022
“Could body posture during sleep affect how your brain clears waste?” Science Daily. August 4, 2015
“How These 3 Sleep Positions Affect Your Gut Health.” Healthline. Jennifer Chesak. March 20, 2023