it should be dark during the incubation period, because you don't want the mycelium to start making pins before the cake is covered AND if you have your jars right-side-up then the mushrooms will grow the wrong way. That is, if they are a phototropic variety.Basically, it doesn't need light and light will just confuse the fungus more than it needs to be, leaving more chance for failure.
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"He who makes a beast of himself gets rid of the pain of being a man."
-Dr. Johnson
"I should play God and just shoot you myself."
-Tool
happy shrooming!