There are different phases to gaining night vision. The pupil widens, hungry for light. Rhodopsin in the rods in your retina increases, slowly, changing the chemistry to make the seeing possible. You learn to look for edges instead of taking them for granted.

Playing kick the can in the dusk and dark, you learn this about night vision: it's all shades of grey. Cones are for color. Rods are for peripheries and darkness. The center of your vision, in the dark, is dimmer than the rest (there are fewer rods there). The looker can be made to believe her eyes are playing tricks on her, if she looks right at you. You can become a tree. You can become a rock. You can get closer and closer. Beware, though - motion makes things easier to see. So stand still.

Night vision is a gift. If you have it, you are invisible. You don't need a light. You can see the monsters that aren't there and tell them with authority to go away. You can walk quietly past sleepers and not trip over things left carelessly.

And so a flashlight, though well-intended, is a curse. Walking in the dark, you want to hide from people with flashlights, so they won't shine it in your face. Avert your eyes from headlights. It takes seconds to lose night vision and up to twenty minutes to regain it, all the while stumbling, fearful, and clumsy.

From Suzanne Vega's 1987 album, Solitude Standing.

By day give thanks
By night beware
Half the world in sweetness
The other in fear

When the darkness takes you
With her hand across your face
Don't give in too quickly
Find the thing she's erased

Find the line, find the shape
Through the grain
Find the outline, things will
Tell you their name

The table. the guitar
The empty glass

All will blend together when
Daylight has passed

Find the line, find the shape
Through the grain
Find the outline, things will
Tell you their name

Now I watch you falling into sleep
Watch your fist uncurl against the sheet
Watch your lips fall open and your eyes dim
In blind faith

I would shelter you
Keep you in l
But I can only teach you
Night vision
Night vision
Night vision

Night blindness, the inability to gain much (or any) night vision, has a number of potential causes, one of which is a Vitamin A deficiency. This is why mothers and others prescribe carrots for your eyes (not just a tale!). The deficiency might be dietary lack or something worse, like cirrhosis of the liver - at any rate, carotene may help. Some varieties of night blindness have been successfully experimentally treated with concentrated doses of Vitamin A, but it does not conclusively work in all cases.

It's not listed anywhere (scientists can't measure it) but an aspect of night vision is patience and the will to see. If you can't sit/stand quietly and wait, but have to turn on the lights, run, hide from the dark - this relation to the shapes around you won't come. You won't learn to look with your ears and your eyes, with your sight slightly averted so the central blind spot won't fool you. And you certainly won't win at kick-the-can.