At first I was thinking that maybe the change is triggered by direct moonlight, but untreated lycanthropy induces an irresistable urge to go into the moonlight, so people *can't* just stay indoors, but then I realized that that doesn't take into account that apparently you can lock a werewolf up before the moon rises in a basement and he'll still change. So that's right out.

Oh well. Fun while it lasted.