Well.. when fish sleep or are at rest, they tend to lay on the gravel and turn very pale. Thats when the lights are off. If the lights were off and it was dark in the room, and all the sudden you barge in a turn on the lights, thats your explanation, no need to worry.
But if they dont come around after about 15 to 20 minutes, something else is at work. What are your water parems? What size tank? Its inhabitants? The only thing I can think of right now is
maybe Neon Tetra disease, but that only fades the colors on certain parts of the fish, not the whole fish.
So post back with the information and we will be more likely to help.
Dakota