system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes nosebleeds?

I had a recent nosebleed, and I'm curious. Please stick with the sudden-rush kinds of nosebleeds. Thanks.

1. What causes nosebleeds?
2. What is the evolutionary purpose of nosebleeds?
3. What would happen if you let a nosebleed run without stopping it?
4. Why do I most often get a nosebleed while I'm sleeping? And why do I suddenly jolt awake when I get one?
gpt
1. Nosebleeds are caused by small blood vessels close to the surface of the skin in your nose breaking and bleeding. The lining of your nose is covered with mucus; this is something everyone already knows. This mucus, combined with nose hairs, serves to protect your body from inhaling things that could make you sick. When, for whatever reason, the mucus layer dries up, the blood vessels are more prone to breaking without the slippery mucus for protection.

2. There isn't an evolutionary purpose to the bleeds themselves, they're a side effect of having the blood vessels so close to the lining of the skin. The evolutionary purpose behind THAT is to help warm the air as you breath it in so the cold air doesn't shock your system so much. Have you noticed that when it's cold outside, breathing air through your mouth for a long time is uncomfortable in your chest, but breathing through your nose only makes your nose cold? That's because that mucus and those blood vessels help warm the air for your body.

3. Without stopping any bleed you would eventually get light-headed and pass out. That's true of any bleed. BUT, nosebleeds are not usually serious enough bleeds for that to happen, let alone to cause permanent damage, even if left open for a long time. They tend to clot on their own as long as you don't keep disturbing them. 

 Interesting aside: A lot of people are told to tilt their head back and pinch their nose when it starts bleeding. This is wrong, and potentially dangerous, because not only are you stopping the blood from clotting by shunting it away into your throat, but if you aren't paying attention it could go down the "wrong pipe" into your airway (rare). The proper way to stop a nosebleed is to pinch your nostrils shut tight while tilting your head forward. The blood will pool downward, and with no place to go the clotting factors in your blood will clump together and it will clot faster.

4. While you're sleeping you breath through your nose (usually). This can dry out the mucus layer and make you more prone to nosebleeds. A dry mucus layer and the friction of the air constantly passing over the skin can cause a bleed if the blood vessel is thin enough and close enough to the surface. As to why you jolt awake, probably because your body can sense that something is wrong. The truth is, the science behind how/why we wake up is actually pretty unknown.