Dear Brandon L,
I see you wrote a second question and will address this one with the next.
Dr. Feldman
Sandy T. Feldman, M.D., M.S.
ClearView Eye and Laser Medical Center
San Diego, CA
Changes to night vision (or day vision) at altitude have to do with the partial pressure of oxygen at altitude. As you go up in altitude you go down in pressure and at some point (quite low for smokers) vision becomes affected. Insufficient oxygen in the blood is insufficient oxygen in the brain and eyes. Eyesight is just about the first thing to be affected. Not uncommon for pilots flying at night, even in a pressurized airplane, to take a few whiffs of 100% O2 before starting descent for landing
USAF pilots/aircrew get refresher physiological training every three years including exposure to high altitude--25,000 feet if I remember right--with oxygen off, and start to do simple tasks that require them to think. It gets darker and darker and harder and harder to think until you have to give up, or the decision is made for you, and put on your mask and 100% oxygen. The scene brightens up immediately after a breath or two.
Search on "aerospace physiology" should give you some good sources of info.
My $0.02