You may have heard the phrase “sleep with one eye open.” While it’s usually meant as a metaphor about protecting yourself, you may wonder if it’s actually possible to sleep with one eye open and one closed.

In fact, there are a variety of medical conditions that may make it impossible to close your eyes when you sleep. Some of these may lead to sleeping with one eye open and one eye closed.

There are four main reasons you might sleep with one eye open.

Unihemispheric sleep

Unihemispheric sleep is when one half of the brain sleeps while the other is awake. It mostly happens in risky situations, when some sort of protection is necessary.

Unihemispheric sleep is most common in certain aquatic mammals (so they can keep swimming while they sleep) and birds (so they can sleep on migratory flights).

There’s some evidence that humans have unihemispheric sleep in new situations. In sleep studies, data shows that one brain hemisphere is in less deep sleep than the other during the first night of the new situation.

Because one-half of the brain is awake in unihemispheric sleep, the eye on the side of the body that the awake hemisphere of the brain controls may stay open during sleep.

Side effect of ptosis surgery

Ptosis is when the upper eyelid droops over the eye. Some children are born with this condition. In adults, it results from the levator muscles, which hold up the eyelid, getting stretched or separating. This may be caused by:

  • aging
  • eye injuries
  • surgery
  • tumor

If your eyelid droops enough to limit or block your normal vision, your doctor may recommend surgery to either tighten the levator muscle or attach the eyelid to other muscles that can help lift the eyelid.

One potential complication of ptosis surgery is overcorrection. It may lead you not to be able to close the eyelid that was corrected. In this case, you may start to sleep with one eye open.

This side effect is most common with a type of ptosis surgery called frontalis sling fixation. It’s usually done when you have ptosis and poor muscle function.

This side effect is usually temporary and will resolve within 2 to 3 months.

Bell’s palsy

Bell’s palsy is a condition that causes sudden, temporary weakness in the facial muscles, usually just on one side. It usually has a rapid onset, progressing from the first symptoms to paralysis of some facial muscles within hours to days.

If you have Bell’s palsy, it will cause the affected half of your face to droop. It can also make it hard for you to close your eye on the affected side, which can lead to sleeping with one eye open.

The exact cause of Bell’s palsy is unknown, but it’s most likely related to swelling and inflammation in facial nerves. In some cases, a viral infection can cause it.

Symptoms of Bell’s palsy usually go away on their own within a few weeks to 6 months.

Medical emergency

If you have sudden drooping on one side of your face, call 911 or your local emergency services, or go to the nearest emergency room.

Damaged eyelid muscles

Some conditions can damage the muscles or nerves of one eyelid, which can lead to sleeping with one eye open. These include:

Sleeping with one eye open and sleeping with both eyes open can have similar causes. All potential causes of sleeping with one eye open listed above can also cause you to sleep with both eyes open.

Sleeping with both eyes open may also occur because of:

Sleeping with one eye open and sleeping with both eyes open lead to the same symptoms and complications, such as tiredness and dryness.

Sleeping with both eyes open isn’t necessarily more serious, but the complications it can cause happen in both eyes instead of one, which can be more serious.

For example, severe, long-term dryness can cause vision issues. Sleeping with both eyes open can therefore cause vision issues in both eyes instead of just one.

Many of the causes of sleeping with your eyes open are treatable. However, conditions that are more likely to lead to sleeping with one eye open, such as Bell’s palsy, are more likely to resolve on their own than many of the conditions that lead to sleeping with both eyes open.

Most people will feel the eye-related symptoms of sleeping with one eye open just in the eye that’s staying open. These symptoms include:

You’re also likely to not sleep well if you’re sleeping with one eye open.

Most of the complications of sleeping with one eye open come from dryness. When your eye doesn’t close at night, it can’t stay lubricated, leading to a chronically dry eye. This can then lead to:

Sleeping with one eye open can also cause you to be very tired during the day, since you won’t be sleeping as well.

Try using eye drops or ointments to help your eye stay lubricated. This will reduce most of the symptoms you may have. Ask your doctor for a prescription or a recommendation.

Treatment that will stop you from sleeping with one eye open depends on the cause. Corticosteroids may help with Bell’s palsy, but it usually resolves on its own within a few weeks to a few months. Ptosis surgery side effects and unihemispheric sleep also usually go away on their own.

While waiting for these conditions to resolve, you can try taping your eyelids down with medical tape. Ask your doctor to show you the safest way to do this.

You can also try adding a weight to your eyelid to help it close. Your doctor can prescribe an external weight that will stick on the outside of your eyelid.

In some cases, you may need surgery to resolve the issue. There are two types of surgery:

  • surgery on your levator muscle, which will help your eyelid move and close normally
  • implanting a weight in your eyelid, which helps your eyelid close fully

Sleeping with one eye open is rare, but it is possible. If you find yourself waking up with one very dry eye and don’t feel well rested, talk to your doctor. They may recommend a sleep study to see whether you’re sleeping with one eye open, and can help you get relief if that’s the case.