There are a few reasons why someone's vision could look blurry at night. A phenomenon called night myopia — a temporary type of nearsightedness — is the most common cause of blurry vision that only seems to happen at night.
What is night myopia?
Night myopia is nearsightedness that is only noticeable in low-light settings. It's a common problem — one study of professional drivers showed that one in four (about 25%) had some level of night myopia. The average age of those drivers was 21 years.
The same study suggested that night myopia can also affect safety: Drivers were more likely to get into an accident at night if they experienced some level of blurry vision in low light.
Researchers aren't exactly sure why night myopia happens, but there are two main theories:
Possible cause: Uncorrected myopia
Some people think it occurs when people already have "regular" myopia that hasn’t been corrected yet.
Since the eyes' pupils open wider at night to let more light in, blur increases and makes the effects of nearsightedness much more pronounced. But when the pupils get smaller again in the light of day, the blurring issue clears up and no longer seems problematic.
Possible cause: Lack of focusing ability in the dark
The other theory is that the eyes are less likely to focus correctly in the dark, or in response to certain images in the dark. This causes the eyes to set their focus to a shorter distance with a more nearsighted range of clarity.
If you think you have night myopia, schedule an eye exam. You might have myopia , which is very common and usually easy to correct. By correcting your nearsightedness, you might just improve (or remove) the blurry vision you experience at night.
Can anything else cause blurry or reduced vision at night?
Several things can cause your night vision to get worse overall. While night myopia might make it seem like only your distance vision is blurry at night, some things make everything harder to see in low-light conditions.
Causes of general night blindness (nyctalopia) include:
Glaucoma – As glaucoma reduces a person’s peripheral vision, their night vision is also affected. Many people with advanced glaucoma choose to stop driving at night because of their difficulty seeing in the dark. This worsens as the central vision also begins to deteriorate.
Cataracts – Behind each pupil, there's an oval-shaped structure called a lens that helps you focus on objects. A clouding of this lens, called a cataract, is relatively common as people get older. Reduced night vision is one of the symptoms of cataracts.
Aging – Eyes go through a few changes as people age. The pupils shrink and let in less light, and the eyes become more dry, leading to reduced visual clarity. Both of these can make it harder to see in the dark.
Diabetes – This widespread condition can lead to diabetic retinopathy when it isn't controlled well enough. This causes changes in the blood vessels along the retina and may lead to poor night vision and eventually vision loss.
Retinitis pigmentosa – Worsening night vision is one of the first signs of this genetic disease. The condition is rare, affecting between 1 in 3,500 and 1 in 4,000 people.
Vitamin A deficiency – Night blindness is one of the first signs of Vitamin A deficiency (VAD), one of the leading causes of blindness in the world. Nighttime falls, clumsiness and difficulty driving may also indicate low levels of vitamin A.
Bright light exposure prior to being in the dark – If you are exposed to bright light just before stepping into a dark space, you may experience a delay in “ dark adaptation.” One example of this is going from a bright sunny parking lot into a dimly lit movie theater. It can take your eyes 20 to 30 minutes to adjust, depending how bright it was outside compared to the dark theater. Another example is using a camera flash to take a picture in a dark room.
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