Some are specialized to vertical or horizontal motion, others see only lines for edge sharpening, etc. You cannot do anything to improve its function, but you can do much to worsen eye sensitivity. Smokers suffer from poor night vision due to a poisoning of their bloodstream with carbon monoxide. High altitudes decrease oxygen pressure, and vision therefore suffers compared to sea level. Vision decreases if blood sugar levels are low, and an occasional snack while observing at night can help.
Alcohol diminishes both light sensitivity as well as brain function.
Vitamin A deficiency is simply not seen in this country and extra beyond basic needs does not help. If you plan an observing session after a day of skiing or beach, wear sunglasses. Finally master the technique of averted gaze and flicking your vision. Happy hunting for those faint and elusive NGC objects.