The eye condition glaucoma develops when a fluid called aqueous humor builds up in the eye, increasing pressure on the optic nerve and damaging it. Around 4.2 million people in the U.S. have the condition, although not all have been diagnosed.

And glaucoma is a leading cause of blindness in people ages 60 and older.

Glaucoma usually progresses slowly. You might not notice problems until you have blurry vision, small blind spots in your visual field, or eye pain.

Getting eye exams every one to three years can help you keep tabs on your eye pressure. If pressure is increasing, your ophthalmologist can prescribe eye drops that reduce fluid production or increase drainage, or recommend surgery to open up clogged drainage passages.

But according to a study in the British Journal of Ophthalmology, even the most conscientious patient can make his or her glaucoma worse by sleeping on stacked-up pillows. Two-thirds of study participants experienced moderately increased internal eye pressure when switching from a flat head position (no pillow) to using two pillows.

It seems that pillow-stacking constricts the jugular vein, reducing proper drainage of fluids in the eye even more.

While many doctors recommend glaucoma patients sleep with a stiff wedge pillow or raise the top of the bed, this is the first study to see what impact stacked pillows (which affect the body differently) may have.

Give flat-sleeping a try, avoid elevated triglycerides and blood pressure, and get plenty of DHA and EPA omega-3s to reduce your risk of glaucoma and help manage it if the condition develops.