Different smell when lights off?

hey so i was just wondering if you guys have noticed a different smell, almost immediately, like 5 minutes after the lights go off? i notice one smell when the lights are on, the classic skunky smell that i love :D but then when the lights go off its a more planty smelling odor.
i was just wondering if there is actually a reason for this, do the plants actually give off a different smell at night? is there a biological purpose for this? etc? just curious :jointsmile: