The Eye and Dark Adaptation

TO MAXIMISE YOUR SENSITIVITY to dim light you should allow sufficient time for your eyes to get used to the dark. You must also allow for continued dark adaptation during the observing period. During the first minute, the eye's sensitivity increases ten fold. In 20 minutes it increases 6000 fold and forty minutes of dark adaptation increases sensitivity 25,000 times!
This means that a faint object which was overlooked in an early part of your observing run may be readily seen later, so don't expect to see faint objects at their best until at least a half hour into an observing session.

Note that light adaptation is much faster that dark adaptation, so even a brief exposure to light will destroy your night vision. It is also best to consistently avoid all light for as long as possible before observing. Prolonged exposure to bright sunlight reduces your ability to dark adapt for a couple of days; wear dark glasses at the beach. In the long run, ultraviolet light ages both the eye lens and retina, reducing sensitivity. So if you wear eyeglasses outdoors, ask your optometrist for UV-filtering lenses. Night vision is also impaired by alcohol, low oxygen and low blood sugar. So don't observe while drinking, smoking or fasting. In fact, nicotine retards dark adaptation to such an extent that even a single cigarette smoked half an hour before your eyes start dark adapting will slow down this process.