I drove from Las Vegas to a bit above Breckenridge in a single day and got mild altitude sickness. It doesn’t sound like much, but damn, I couldn’t hardly walk without feeling like I was about to keel over from the shortness of breath. Puked a lot too. Got dehydrated enough to pee brown. I remember being super out of it trying to make ramen on the stove and watching the water boil after just a minute, so I stuck my hand in out of disbelief — sure enough, it was just mildly warm. Weird stuff.