system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come when it is extra bright outside, having one eye open makes seeing “doable” while having both open is uncomfortable?

Edit: My thought process is that using one eye would still cause enough uncomfortable sensations that closing / squinting both eyes is the only viable option but apparently not. One eye is completely normal and painless. 

This happened to me when I was driving the other day and I was worried I’d have to pull over on the highway, but when I closed one eye I was able to see with no pain sensation whatsoever with roughly the same amount of light radiation entering my 👁.

I know it’s technically less light for my brain to process, less intense on the nerve signals firing but I couldn’t intuitively get to the bottom of this because the common person might assume having one eye open could be worse?
gpt
You have a blink reflex when exposed to bright light. Ideally, you're exposed to a light that's too bright and you squint or close your eyes to prevent any damage. 

You might ask, "Surely only closing one eye shouldn't help, since the open eye could still be damaged, right?" And you'd be absolutely correct. This is a glitch of the human nervous system. 

Essentially, this glitch occurs because your brain registers bright light by adding together the amount of light received in both eyes. If one eye is closed, that eye is receiving the same amount of light, but the brain is only registering half of the original amount. 

Fun fact, some people (Ze Frank is a notable example) don't have this glitch. You can (but don't, it's not good for your eyes) test this by shining a light in one of your eyes, blocking the light from getting to the other eye, and watching to see if your non-lit pupil constricts. Normal people's pupils will both constrict if one eye is exposed to bright light. In the case of Ze Frank and others, only the pupil receiving the light will constrict.