When I was a kid, I was afraid of the dark. Anything could be happening in darkness. A mindless zombie could be stumbling toward me. A demon could be clawing up from the reaches of Heck and discover the passageway between the netherworld and my bedroom closet.
This technique is in keeping with my own geeky "lets learn something" (or "teach a child to fish rather than the location of the local fish market") parenting philosophy. Certainly, there are complex psychological issues at play here but a simple fear of darkness may be aided by this technique. Most important is the fact that you are acknowledging your child's fear and helping him or her understand and use his own body processes to help him adapt to the situation. It's impossible to promise you will always light up the dark but it is possible to help a child understand how his eyes adjust to it.
Step 1: You Need
One eye patch - This can be made a variety of ways like this. It can also be purchased in a toy aisle, costume shop, or pharmacy.
Step 2: Background
Explain to the child the purpose behind pirate eye patches. They were not simply a fashion accessory or a means to cover the an empty eye socket. Eye patches enabled people to keep one eye adapted to darkness at all times.
Once a person has been in the dark for 20-30 minutes, their eyes adjust to the darkness. For pirates, being on the deck of a ship with eyes adapted to the sunlight meant having very poor vision when going below deck where light was scarce. Because they couldn't simply flip a switch, they kept one eye ready for the dark by keeping it in the dark--behind an eye patch.
Experiment with the eye patch. Allow the child to wear it over one eye for at least 30 minutes. Explain that the uncovered eye is the "light" eye (adjusted for light) and the covered eye is the "dark" eye (adjusted for darkness). Keep the child in an area with bright light to ensure that the contrast between the light-adapted and darkness-adapted eye is as great as possible. Then take him or her into a room that can be darkened. Assure him that you are right there and hold hands or sit next to the child to make him comfortable.
Then turn off the lights and have the child take off the patch. Have him open one eye at a time, back and forth, and discuss his impressions. Take it very slowly. There may be little or no improvement in his fear. Ask how much he can see with both eyes. The "light" eye will make the room appear almost entirely dark while the "dark" eye will enable him to see shapes and discern the major features of the room. This is what the darkness looks like after adjusting. Not so bad, is it?
Step 3: Lights Out!
When the child goes to bed, have him or her take off the eye patch just as the lights are extinguished. Explain how he should close his "light" eye and use the "dark" eye to see better in the dark until the "light" eye has adapted. Then remind him to compare the vision of each eye while the other is closed and see the contrast. This might help alleviate some fears of the dark.
The eye patch should come off before bed, but only just before. You don't want a strap on a child's head at night. The elastic can slip down around a child's neck while in bed so it should be completely removed. Besides, once the lights are out, there is no point to wearing it anymore.
Good luck! At the very least, your child will understand a little more about human biology. For further information that older children may be interested in, and an explanation how consumption of vitamin A is important in eye adaption to light conditions, you may also want to refer to this article: "How Eyes Adjust to Darkness."