I dunno where to mention it so I'll mention it here - most of us suffered from altitude sickness to various degrees. My parents were actually incapacitated for a few days. It's not fun, and it's counterintuitive - when you have it, it's better to do some moderate physical activity instead of sleeping and resting, because the sickness is caused by not enough oxygen going through your blood. Sleeping does something like constricting it further, making it worse, whereas activity gets the blood pumping. But yeah, hard to think of execising when you have a pounding headache.