I often find it physically painful to open my eyes past a squint after stepping out into the full summer sun. However, if I close one eye entirely, I can keep the other open without pain. This seems odd. Naively, one would assume the pain results from too much light entering the eye and doing some sort of damage to the interior, but if this were true, closing one eye would have no effect on the other. Does the fact that it does imply the discomfort is actually related to (e.g.) the sum-total of information reaching the optic chiasma?
A couple of factors play into this. First, you're halving the number of nociceptors that are signaling the brain. Second, there is a psychosomatic effect. Your brain continuously combines the visual data from both eyes to form one image. With one eye closed, your brain combines the brightness of both inputs to form an image that is roughly half as bright as reality. As a result, your brain not only ignores a portion of the nociceptor signals from the open eye, but it also doesn't assume the presence of pain like it normally would.
I've no publicly accessible sources to cite, as this is something I learned in my EMT class. Someone actually asked your same question in class
This happens to me as well. Complete guess (and probably somewhat of a poor answer) but I think it has something to do with focusing problems (similar to an extreme astigmatism) jerking the eye muscles around.
Closing 1 eye removes the need for a coordinated focus, eliminating the problem.