Why don’t the whites of your eyes tan?

Without googling, because they don’t contain melatonin?

Because I always rub suncream in them. Stings a bit, but smells all coconutty.

Without googling, because they don’t contain melatonin?

There must be an evolutionary reason why the rest of our bodies have melatonin – why not eyes? By design they are fairly exposed to the rays of the sun.

also, why don’t you get sunburn on your eyeballs?

You can/do. It’s called photokeratitis.

Welders would recognise eyeball sunburn as Arc-eye. I think our protruding foreheads are quite effective at shielding our eyes from direct sunlight, and your eyelids are the final line of defence.

Had arc eye, was an unpleasant evening to say the least, took 2 days before i could actually see properly.
I was more careful after that I can tell you.

There must be an evolutionary reason why the rest of our bodies have melatonin – why not eyes? By design they are fairly exposed to the rays of the sun.

I’m sure I’ve read somewhere that the retina actually regulates the amount of melatonin (and therefore melanin, which gives us our tan) produced based on the UV rays it detects. Melanin diffuses UV rays and thus a surface tan protects deeper cells against the sun’s harmful rays. If the eyes tanned it would potentially reduce the amount of UV rays that reach the retina and therefore mean less melatonin/melanin produced, resulting in more damage due to UV rays. Over however many million years the eye has been evolving, the creatures with tanned eyes died of cancer and so didn’t spread their eye-tanning genes.

Disclaimer – I’ve not studied any form of genetics/evolution/physiology for about 13 years, so the above may be utter guff.

What I can’t figure is how harmful UV rays from the sun can travel through millions of miles of space, through miles of our atmosphere and, at the moment they strike our vulnerable little bodies, be defeated by a micron thick smear of ambra solaire.

That smear of lotion is nowhere near as effective at blocking out the uv rays as a pair of biking gloves. Who doesn’t love brown arms with white hands?

The UV rays don’t encounter much through the millions of miles of travel through the vacuum of space. A lot of the strength and intensity of the UV radiation is taken out by our atmosphere (ozone layer). It’s worth noting that you can’t get a sunburn behind glass, as glass absorbs the UV radiation, or underwater as water has the same effect (why fish never get sunburn). I wonder if the thin layer of moisture coating your eyeball also acts as a barrier.

Richard Hawley has a suggestion