On occasion today, while walking swiftly around Gnomedex, I’ve noticed that I feel a little dizzy and my heart starts pounding. Am I falling love? Having a stroke? What’s up? Stupidly, I couldn’t figure it out, until it recently occurred to me that I’m 6225 feet higher than I usually am.
Then I remembered the first time I came to Lake Tahoe, when I was about 10. It was the one and only time I tried downhill skiing, and it didn’t last long. After taking a lesson in the morning, I skied down the bunny hill and promptly fainted at the bottom. My aunt, who was with me, had no idea what to do. She just lied, told me to put my head between my knees, and fed me M&Ms, assuring me that the sugar would rejuvenate me. So, it’s not that I’m now terribly out-of-shape–I just have a sea-level body.
DarrenBarefoot.com 
Altitude? Not a chance. You’re in your little heaven. Happens to all geeks who go to Gnomedex
This happened to me when I was in Mongolia. Sometimes I would just feel very tired, or dizzy. When I went hiking, I would become winded very easily. At first I thought I was just out of shape, but then I figured it out.
By contrast, when I used to come to Vancouver or Victoria to visit, the change in air pressure and level would make me very sleepy.
I get massive migraines if I go to the Upper Levels or Burnaby Mountain. I can’t deal with altitude at all…which is very sad when you live in BC.
Hey! Maybe that explains my low centre of gravity, too.