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Why Do We Close Our Eyes When We Sleep?

The science of sleep has fascinated many. Some animals, for instance, can sleep with one or both eyes open but still get sufficient sleep.

This phenomenon has been observed in some birds, rabbits, and dolphins which, for them, is basically to look out for predators.

In humans, however, sleeping with eyes open is an eye and sleep disorder called nocturnal lagophthalmos.

But the question is, why do humans close their eyes during sleep?

Are we genetically predisposed to doing so?

Like other vertebrates, the human body is a biological entity that functions optimally with 100% homeostatic potential, and sleep and wakefulness are crucial to ensuring a feel-good factor.

At sunset, the parasympathetic nervous system kicks in to slow down involuntary body function. Progressively, the body’s muscle activity, blood flow, and heart rate slow down, and you realize you have to close your eyes to take the much-needed rest.

Have you ever wondered why you get this compelling instinct to close your eyes when you feel sleepy? This behavior is not isolated to humans. Animals do too, and there is a reason.

So why do we have to close our eyes when we sleep? And does this phenomenon determine the quality of sleep we get?

 Read on to explore expert insight into why that is so.

Why Do People Close Their Eyelids During Sleep?

Humans close their eyes during sleep for several reasons.

First, it is a physiological response by the central nervous system that lubricates and protects the eyes from light.

The retina of the eye contains a light-sensitive protein that not only processes light but also signals the brain to regulate the cardian rhythm (the body’s sleep-wake cycle).

Secondly, the eyelids instinctively close during sleep to protect the eyeball from dust, debris, or any other foreign particles that could harm or irritate your eyeballs.

 Additionally, the closing function of the eyelid limits the excessive evaporation of tears, which keeps the eyes moist.

During sleep, the body’s autonomic nervous system takes over. Once your retina senses darkness, the brain signals to the rest of the body that it is time to sleep.

As such, the eyelid muscles relax and close during sleep. All this unwinding is a physiological reaction that promotes sleep and relaxation throughout the body.

Contrary to popular opinion, closing the eye is neither a necessity nor a precursor to sleep.

 Some vertebrates such as birds and dolphins have a specialized brain structure called the unihemispheric sleep system that helps them sleep with one eye open at all times. Similarly, some people do sleep with their eyes open.

Can You Sleep With Your Eyes Open?

Like stretching or yawning, closing the eyelids is an involuntary body activity that happens instinctively.

Closing the eye is critical to ensuring eye health and getting quality sleep.

But for whatever reason, some people sleep with their eyes open or partially open – a condition called nocturnal lagophthalmos.

During the day, your eye retina processes light and uses neurotransmitters to relay this information to the hypothalamus, which stimulates wakefulness.

When your body perceives darkness, the brain stimulates the production of melatonin (sleep hormone), signaling that it is time to sleep.

However, not all people can close their eyes during sleep.

Experts are unsure how many people suffer from nocturnal lagophthalmos, but the National Sleep Foundation estimates that 1 in 5 people suffer from this eye disorder.

As a result, patients are more susceptible to sleeplessness, eye dryness, or damage to the cornea.

Nocturnal lagophthalmos is generally classified as an autoimmune disorder, but can also point to an underlying medical condition, including thyroid disease.

 Most patients don’t usually suspect its symptoms early on simply because it is nearly impossible to tell whether your eyes are closed or open while sleeping.

However, they are typically uncomfortable in the morning and can lead to ocular complications in severe cases.

Common symptoms of nocturnal lagophthalmos include:

  • Watery eyes.
  • Blurry vision.
  • Eye dryness.
  • Redness.
  • Sore, painful eyes.
  • Scratchy and irritated eyes.

However, it is worth noting that not being able to close your eyes during sleep would mean you have an underlying medical condition.

Some people are known to sleepwalk with their eyes open. If you suspect to have the aforementioned symptoms, be sure to speak with an ophthalmologist or optometrist.

Causes Of Nocturnal Lagophthalmos

Nocturnal lagophthalmos is an autoimmune disorder that decreases the eyelids’ ability to protect the cornea.

This can dehydrate the eye, and limit its ability to discharge dust or block light during sleep.

The disease has been linked to underlying medical conditions but is also caused by:

  • Obstructive Sleep Apnea (Osa): Obstructive sleep apnea is a common cause of nocturnal lagophthalmos. It is typically associated with floppy eyelid syndrome – a condition that interferes with eye closure. OSA is common among glaucoma and optic neuropathy patients.
  • Infections: Nocturnal lagophthalmos can result from common infections, including polio, diphtheria, leprosy, Lyme disease, stroke, tumor, mumps, or chickenpox.
  • Eye Scarring: Your eyelids’ ability to close during sleep can also be limited by chemical burns or physical trauma that causes scars.
  • Invasive Cosmetic Procedures: Some common cosmetic procedures designed to remove fat, tighten the eyelid, or the skin tone surrounding the eyelid can interfere with eyelid muscle tone, limiting their ability to close during sleep.
  • Eye Proptosis: Proptosis, also called exophthalmos, is an eye condition that causes bulgy eyeballs. This condition renders the eyelids smaller and may not cover the eyeballs fully.
  • Facial Nerve Disorder: Common nerve problems such as Bell’s palsy can reduce eyelid muscle tone, thereby limiting their ability to function or cover the eyeballs.


Closing your eyes when sleeping is the body’s way to protect the eyes and usher in restorative sleep in its calibrated 24-hour clock cycle.

When we sleep, our eyes close naturally as a physiological response, and this is vital in getting a good night’s sleep.

 That said, not all people get to enjoy the luxury of lubricated, debris-free eyes, or quality sleep.

These people have eyelids that cannot close to cover their eyeballs during sleep, and suffer from a condition called nocturnal lagophthalmos.

Although it can be diagnosed and treated, this condition limits the eyelids’ ability to block light, lubricate, or protect the cornea.


Kenyalogue Contributor


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