Meaning behind the TV?

Even though this movie makes sense without trying to interpret it, I somehow think, there's more to it.

It was quite noticable that there were two scenes which featured a shimmering TV: The nightmare scene and the scene where Zach is in Josh's room a few minutes before (spoiler warning!) he's trying to murder the two girls. I think there's gotta be a meaning behind this. Obviously it was intended by the director and why should Josh just leave his TV on when leaving the house anyway?! Does anybody know the meaning behind it?

I thought maybe the last act was just in Zach's imagination (he's paranoid and goes crazy) and the TV, which has already appeared earlier in a dream sequence should give the viewer the clue to figure this out. But maybe someone has a better interpretation of it? After all, the last scene makes it look like it wasn't just a fantasy (Allison sitting in the class room covering her scar) so probably I'm wrong. But I'm still curious about the meaning of the TV.

What do you think? :)