The UV light polymerizes collagen in presence of vitamin B. They did experiments by repairing cornea that way:
https://pmc.ncbi.nlm.nih.gov/articles/PMC3018104/
"The aim of this study is to evaluate the safety and effectiveness of riboflavin-ultraviolet type A (UV-A) light rays induced cross-linking of corneal collagen in improving visual acuity and in stabilizing the progression of keratoconic eyes.
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The eyes were saturated with riboflavin solution and were subjected for 30 min under UV-A light
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Cross-linking was safe and an effective therapeutical option for progressive keratoconus."
I think that this is probably one of the reasons why suntanned skin usually looks like it is in better condition mechanically-wise.
Another my favorite Sun exposure related correlation - vitamin D deficiency and autism, as couple studies on Somali immigrant population in Minnesota and Sweden - where such dark skinned population naturally gets very low on vitamin D - showed such correlation as autism rates in that population is higher than back there in Somali (and that would explain the correlation of low sunlight expo.
And my favorite pet theory is that Neanderthals with their large eyes adapted to the Northern latitudes were significantly impaired by spike of UV radiation - getting highly increased rate of early cataract and other eyesight damage - during that thousand years of magnetic field polar swap 40K years ago, and that caused them to lose to the Cro-Magnon who was coming out of Africa with more dark and smaller eyes more adapted to higher UV levels which are natural to Africa.
As a person w/ keratoconus I have read a fair bit about this treatment, corneal collagen crosslinking with riboflavin (C3R).
It does not repair the damage caused by keratoconus. It stabilizes the cornea and slows or halts progression. The collagen in the cornea naturally crosslinks (likely due to UV exposure) over our lives. My our late 30s our corneas are stable. For someone with keratoconus, where the cornea becomes progressively misshapen, stabilizing the tissue with UV (enhanced by riboflavin) slows or prevents further damage.
I was just a few years too early in my diagnosis to benefit from C3R. My eyes have been stable for the last 10 - 12 years. I wish I could have had C3R when my condition was first diagnosed back in my 20s.