I'm not sure that asking for the science behind this on a metaphysically-based website may get you the answers you are looking for :) But I can suggest a metaphysical-style answer instead...

I think you'll find that when you are truly in a state of alignment with your broader self (a state that Abraham terms The Vortex) then it's not only eye strain that diminishes...your entire body operates from a heightened level.

The idea is a bit like that of Yoda in the Star Wars movies.

Throughout the movies, Yoda is depicted as an old, stooped-over creature hobbling along slowly with a walking stick. But once energized by The Force, he immediately has access to tremendous inner powers and his body is suddenly full of energy and vitality. He is then able to effortlessly fight the bad guys in the movies. As soon as the necessary fighting is over, The Force that was previously summoned would cease flowing and he reverts to hobbling with a walking stick again.

Yes, it's fiction but it is hinting at this idea of deliberate, temporary alignment with a state that can give us power beyond our normal abilities.

There are occasional stories in the media of people being able to perform superhuman feats, such as lifting cars off those who are trapped, through this temporary summoning of physical ability.

A similar real-life incident was once described by Abraham involving Esther Hicks. Her husband, Jerry, and her were going out jogging one time and, from what I recall of the story, Jerry was playfully mocking her jogging because he was fitter and faster. There was then some kind of mini-race between them and, to aid Esther a bit, Abraham "took over" Esther's physical body and during that time she was able to race at a phenomenal speed and beat Jerry.

The exact details of the story may be slightly inaccurate, because I heard it a long time ago and am recalling it only from memory, but the idea of Abraham (Esther's Vortex, you could say) energizing and revitalizing her body is accurate.

Now Abraham then explained that this state of heightened vibrational alignment where Esther could perform semi-miraculous feats (compared to her natural state) was not permanent.

This was because as the broader consciousness within her (Abraham) then withdrew, her body would then return to its normal vibrational level and she would then suffer aches and pains the next day from that exertion because of the vibrational mismatch.

So, coming back to your question, yes, in that state you could probably find that you do not need sunglasses and maybe you could even stare directly into the sun without problem (not recommended though) but then you would probably find that as soon as you were out of that state of alignment and reverted to your usual bodily vibrational place then that vibrational difference may have uncomfortable results for you afterwards.

With training, one could make those heightened vibrational states more habitual and I think this is what training in anything really is...deliberate acclimation to a particular, improved vibrational state that then results in physical manifestations matching that state on a more permanent basis.