ever since I was a child, I've always suffered from nose-bleeds after being exposed to sunlight for long periods of time (1 hour+).
Neither the temperature or humidity level of the day plays a role in this, as my nose only bleeds when I've been exposed to bright sunlight.
I also sneeze allot in bright sunlight.

This has been happening as far back as I can remember, I even had nasal spray prescribed to me because the physicians thought it was caused by allergens. Only, the nosebleeds happened at all times of the year - before, during, and after allergy season.

My question, then, is as follows:

Why (or how) does sunlight cause nosebleeds (regardless of temperature or humidity levels)?

P.S: The last nosebleed happened this morning; I spent several hours in the sun yesterdday - the temperature was in the mid-to-upper 70's F.
and it's been relatively humid here in the Appalachian Mountains.

I hope you can provide an answer to my query, as I've been wondering what's wrong with me since I was 10 years of age. Still wonder now.
My husband and I joke that I have an allergy to sunlight.

interesting and speaking as a clinician and a scientist I can't think of an obvious mechanism whereby exposure of your nostrils to sunlight could trigger nose bleeds which will be due to small vessels on the nasal mucosa. Clearly a fair number of doctors over the eyes have considered this each time you have mentioned it and presumably no-one has come up with any bright ideas.

The allergy idea occurred to me and I too might have tried a steroid nasal spray just to see if it had an effect but clearly your doctors have thought of that already!

Assuming this is limited to the nose (ie you don't bleed elsewhere) and you have no other associated symptoms, I suspect this will be with you for the rest of your life and will not be adequately explained.

This may or may not be relevant, but my younger sister had chronic nose bleeds. There was no clear cause, but there did seem to be a link between aspirin use (a blood thinner) and the bleeds. The suggestion was that the mucosal blood vessels were in some manner thin-walled or perhaps too close to the surface, and the combination of blood thinners with extra pressure (like energetic sneezing/swelling) compromised them and caused the bleeding.

thanks and yes asprin or non-steroidals increase the risk of bleeding in the body. Its the relationship to sunlight that has me stumped!