Sleeping with your eyes open, or at least your eyelids only partially closed, is called nocturnal lagophthalmos. When your eyelids do not fully close, your eyes are exposed to the open atmosphere. Because they function to lubricate the surface of your eyes, your eyes begin to dry out significantly.
In very young patients, nocturnal lagophthalmos tends to be less severe than in older patients and is usually outgrown.
Lagophthalmos can also occur during the day. Patients with this condition do not have a full blink, so there is always a part of the eye that does not receive enough tear lubrication.