Because the condition causes one side of the face to droop, the eyelid may also be affected and may be difficult to close. If the eyelid droops enough to limit or block normal vision, your doctor may recommend surgery to tighten the levator muscle or attach the eyelid to other muscles that can help lift it. But you'd probably be surprised to learn that while 80 per cent of people SLEEP with both eyes closed, 20 per cent sleep with both eyes open (via Offen Eyes Associates). The evolutionary trick is called unihemispheric sleep, in which, essentially, half of an animal's brain sleeps while the other half stays awake, with the ability to keep one eye open for anything that looks like trouble.
Bottlenose dolphins know how to turn off half their brain and keep the other half ready while they sleep. Because one half of the brain is awake in unihemispheric sleep, the eye on the side of the body that controls the awake hemisphere of the brain can remain open during sleep. The study has revealed that humans also slip into unihemispheric sleep when they find themselves in new or unfamiliar situations. However, it seems unlikely that this is the only reason for this documented sleep state, as crocodiles have almost no known predators, apart from humans and perhaps larger crocodiles.
The second smallest of the manatees, the Amazonian variety, is another aquatic mammal that employs unihemispheric sleep. Although not as well known, thyroid problems can also cause them to sleep with their eyes open. Interestingly, the eye that was closed was kept hidden under their wing, presumably to preserve a semblance of darkness in their sleeping quarters. If you find yourself with restless sleep, dryness or irritation in your eyes, consult your doctor to find the right treatment options for you.
In fact, there are a variety of medical conditions that can make it impossible to close your eyes while sleeping. Only when there is something to consider in controlled circumstances is it observed to sleep with one eye open. Without sufficient lubrication, the eyes are more susceptible to infection and can become scratched and damaged.