I keep all of mine on a 12/12 schedule and yes, it needs to be dark. If you have the air conditioning on at night are you concerned about temperature or the light from the A/C? If you are concerned about temps, it will be fine so long as the enclosure is not placed next to the unit. If you are concerned about some lights coming off the A/C - that should be fine unless it's lighting up the entire room...which I doubt!