While the phenomenon isn’t entirely understood, the most common cause is a sleep disorder called nocturnal lagophthalmos, which basically means an inability to fully close your eyelids at night. Sometimes this is triggered by skin disorders or even botched cosmetic surgery, but usually it’s an issue with your facial nerve. Luckily, most people who sleep with their eyes open don’t suffer any serious side effects. As a result, treatment can be as simple as using fake tears or eye ointment before bed and after waking to ensure the eye stays lubricated. Sleeping with a mask over the eyes can help as well.