As many as 20% of people sleep with their eyes open. The condition is officially known as nocturnal lagophthalmos. It gets its name from λαγωός, the Greek word for “hare.” This stems from the belief that hares sleep with their eyes open.

In 2010, Gregory Machon was kicked off a US Airways flight because of nocturnal lagophthalmos. Machon said he pressed the flight attendant call button to request a change of seats. Before the flight attendant could get to him, however, he fell asleep. The flight attendant tried to speak to Machon, thinking he was awake because his eyes were open. When he did not respond, she concluded there was something wrong with him. The flight attendant consulted with two medical professionals who happened to be on board — a veterinarian and a pediatric nurse. Neither of them were familiar with any natural condition that would cause someone to sleep with open eyes.

Machon said, “They brought up numerous possibilities for what could be ‘wrong’ with me, and discussed my medical status very openly in the aisle, with all the other passengers listening intently. Even after I told them that it happens to me relatively often, the very insistent flight attendant joined by the the veterinarian and pediatric nurse (with their combined expertise) concluded that I should be checked out by EMTs.”

Medical personnel evaluated Machon and declared him fit to fly. The flight attendant said she considered Machon to be a liability and did not want him seated in her section. He was kicked off of the flight and had to reschedule for the next day.

While noctural lagophthalmos affects one out of five humans, all dolphins have a related sleep condition. They sleep with one eye open and half of the brain asleep at a time. This allows half of the consciousness and one of the eyes to always be on the lookout for predators.

Read more fun facts about sleep.

Read more fun facts about aviation.