Med thumb sleepingeyesopen

It’s a trait attributed to herd-watchers, Tommy gun-wielding gangsters and other such swaggering movie tough guys, men so attuned to watching their backs, they’ve learned to survey their surroundings while they rest. But is it really possible to sleep with your eyes open?

Before we delve into this, it is first necessary to learn about our sleeping peepers. We shut our eyes when we sleep for many of the same reasons as when we’re awake: To keep them lubricated, clean and safe from external irritants. Blinking creates a vacuum that draws fluid from our tear ducts and across the eye’s surface. Think of the whole process as a tiny irrigation system — our tears flush away dust and other particles that, left to accumulate, could cause severe damage. They also diffuse oxygen and other vital nutrients throughout the cornea, combat potentially-harmful bacteria and maintain a smooth optical surface that’s essential to good vision.

There is also evidence to suggest blinking serves as a sort of micro-(micro, micro) nap. A blink is what’s known as a semi-autonomic response, meaning it’ll happen if you want it to and it’ll happen if you don’t. Spontaneous blinking — the kind that happens without effort or external stimuli —occurs 15-20 times per minute. This number has befuddled scientists for some time, as most agree it’s more than enough to protect and lubricate.

In 2012 a group of researchers found a link to sleep that might explain the disparity: Writing in the Proceedings of the National Academy of Scientists, they argued that blinks activate a part of the brain that lets it rest, reset and refocus attention. Just as a night’s sleep readies us for a new day, blinking might ready the brain for, say, the next sentence in a book, a new scene in a video.

Granted, that’s just one study. But even casual blinkers will agree that for an eye to stay open, it must occasionally close. When it comes to sleep, the immediate advantage of shut-eyed shuteye is obvious: Our eyes can get pretty dry over the course of eight hours, and closure conserves energy that would be spent on blinking. At a neurological level, shutting your eyes blocks out light that would suppress production of melatonin. Scientists also believe that visual stimulation tends to cause the reticular activating system — the part of your brain that regulates sleep-wake transitions — to induce wakefulness. Less stimulus, more sleep.

Okay, there are some pretty compelling reasons why we sleep with our eyes closed. So what about the scores of people who don’t?

Nocturnal lagophthalmos is a condition that causes people to sleep with their eyes open, whether fully or partially. It is usually caused by damage to the eyelids or to the facial nerve, which governs eyelid closure. According to Dr. William Winter, a sleep scientist and medical director of the Martha Jefferson Hospital Sleep Medicine Center in Charlottesville, lagophthalmos is often a symptom of an underlying condition.

Given the choice, these people would probably choose to avoid ocular irritation. But it’s also proof that sleeping with your eyes open is at least physically possible

“Anything that paralyzes the face can impair the eyelids,” he said. “It’s associated with certain pathological things, like a Bell’s palsy, or a couple congenital disorders. It’s generally detrimental — people wake up with their eyes dried and red, and they usually end up taping them shut.”

Lagophthalmos may also be associated with keratopathy — calcium buildup on the cornea — or strokes, infections and other trauma. Dryness and irritation are the best-case side effects; on the other end of the spectrum are corneal tearing and bacterial infections. It’s hard to pinpoint exactly how many people suffer from the condition. Some sources say five percent, others say ten, but even these numbers are problematic, given that many of the afflicted may not even know they’re afflicted. “I had one guy tell me he sleeps with his eyes open, and he wasn’t aware of it until other people told him,” Dr. Winter recalled.

Whether patients know it or not, the key thing about nocturnal lagophthalmos is that it’s involuntary; given the choice, these people would probably choose to avoid ocular irritation. But it’s also proof that sleeping with your eyes open is at least physically possible. So can someone teach themselves to do it?

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According to Dr. Neil Kline, a sleep scientist with the American Sleep Association, the answer is a resounding no. “It is not possible to train yourself to keep your eyes open while asleep,” he says. “Although the sleeping brain is very active, it is not able to perform planned tasks during sleep. It's normal to waken several times during the night to perform tasks — like moving in bed to become comfortable and relieve pressure spots. We might not remember those activities in the morning, but they occur during brief periods of wake time.”

Dr. Winter suggests this news might not be so bad. “Even if you could somehow train your eye to stay open,” he said, “I’m not sure what the advantage would be. If someone’s breaking into your home — your eyes already open pretty quickly when you wake up. What, you’re going to react a fraction of a second faster?”

It’s a comforting point. Many animal species have evolved to sleep with their eyes open — or, more accurately, with half their brains awake — as a crucial means of avoiding predators. So what if we can’t sleep like a bird, with one eye always peeled? At least we don’t have to worry about becoming a bigger bird’s midnight snack.