ok i had to re-listen to make sure i was right but as with all songs i guess there are a few interpretations, but for what its worth heres mine.
He knows she's cheating, he wants to know the details but not right now.
with the light on he has reminders of her betrayal told in the following lines:
"i bet you still got a trace of his love in your eyes"
"i see the look in your eyes and i got the proof"
"there are more lies on your body"
these visuals constantly remind him so he wants to turn off the light and be "left in the dark again" so he can pretend he still doesn't know.
Not sure this makes sense but i know what i mean!
