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Breathe!
One thing which I learned from distance shooting is to breathe well. If you breathe shallowly or even worse, hold your breath the sensitivity of your vision, which is very dependant on the oxygen level in your blood disappears. Some people tend to hold breath/breathe shallow unconsciously when looking in any kind of eyepiece so as to reduce the movement of the head. I have seen a crystal clear black target dot fade to white between breaths when looking down an open rifle sight. A quick search turned up this hypoxia related information:
Visual sensitivity at night is decreased by 10 percent at 1,500 m (5,000 ft) and by 30 percent at 3,000 m (10,000 ft).
This relates to a sudden change in air pressure rather than someone who is acclimatised to living well above sea level- similar effect to holding ones breath I should think.
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