[ELI5] Why are our eyes sometimes blurry within the hour or so after we have woken up from a night’s sleep? Why is it sometimes one eye more than the other?
In: Biology
If you sleep with your head into the pillow or your arm over your eye, the pressure temporarily deforms your eye. Your eyes’ lenses can’t focus correctly onto the back of the eye when they are deformed. This means everything is blurry as if you needed glasses. Your eyes move back into their normal shape over time.
Glasses or contacts are needed permanently when eyes default shape isn’t correct to make up for the error.
Among other reasons, you might have junk on the surface of your eye, such as dust or loose eyelashes. There are [three sets of glands](https://www.merckmanuals.com/home/eye-disorders/biology-of-the-eyes/protective-features-of-the-eyes)—one for tears (water), one for mucus, and one for lipids. The three secretions mix together in a protective coating your eye.
You might wake up with an eyelash or other small bits stuck in there, surrounded by a small glob of mucus that keeps it from shredding your eye as you blink. You might also wake up with the coating having partially dried out, making things blurry for a moment.
Optometrist here:
The answer here can be for a variety of different reasons or any combination of reasons. Some of these include:
– eyes drying out at night. Many people sleep with their eyes partially open. This causes the surface of the eyes to dry out. A dry surface is blurry.
– Change in shape of the eye: if you typically sleep face down, your cornea is likely to become deformed as you sleep and it takes several minutes to a couple of hours for it to return to a normal shape
– Changes in blood sugar: for diabetics, vision can fluctuate with blood sugar level changes. If your blood sugar is drastically different in the morning when you wake up than it is most of the rest of the day, this is likely to cause vision changes.
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