What It Really Means When You Sleep With One Eye Open

"Sleep with one eye open" is a figure of speech usually used as a warning. However, sleeping with one eye open is more than just an advisory to take extra precaution — it can also be a result of a number of health-related conditions.

The purpose of closing our eyes when we sleep is to aid our bodies in blocking out light that would otherwise signal waking hours to the brain (via Psychology Today). But you would probably be surprised to know that while 80 percent of people sleep with both eyes closed, 20 percent of people sleep with open eyes (via Offen Eyes Associates). So what are some reasons this occurs?

In a study published in Current Biology, researchers found that several species of animals engage in what's called "unihemispheric sleep," a state in which one half of the brain is asleep while the other half remains alert and awake. This pattern of behavior is commonly seen in birds or aquatic mammals who, out of necessity, need to maintain attention when sleeping so they can swim or fly long distances. The study found that humans also engage in unihemispheric sleep when they find themselves in new or unfamiliar situations. As a result, the eye that remains open corresponds to the side of the brain that remains awake. During unihemispheric sleep, sleep is expected to be less restful than when in environments where we feel more at home.

According to Psychology Today, some people are unable to sleep with their eyes fully closed due to nerve or muscle problems in the face. These muscles can be affected by any number of things such as facial trauma, stroke, certain autoimmune disorders, or Bell's palsy. Healthline defines Bell's palsy as "a condition that causes a temporary weakness or paralysis of the muscles in the face. It can occur when the nerve that controls your facial muscles becomes inflamed, swollen, or compressed." Because this condition causes drooping on one side of the face, the eyelid can also be affected and can become difficult to close. Similarly, eyelid damage, whether from surgery or external injuries, can also pose difficulty in fully closing one's eye.

Although it isn't as commonly known, thyroid problems can also result in sleeping with your eyes open. Graves' disease, a specific form of hyperthyroidism, can produce redness, inflammation, or bulging eyes, obstructing our ability to fully close our eyes at night (via American Thyroid Association).

In some cases, sleeping with one eye open can lead to further vision complications. However, there are treatment options available. Some suggested examples include keeping your eye moisturized through the use of eye drops, or utilizing a medically prescribed weight or tape to help cover your eye at night (via Healthline). If you find yourself experiencing restless sleep, dryness, or irritated eyes, consult with your doctor to find the right treatment options for you.