I would tend to disagree with phytokind, i'm doing a grow in my loft and had the same concern and thought i'd turn my exhaust off at lights out, when the plants were small all seemed ok but as they grew when i'd check them in a morning the leaves were quite limp so i decided to put the exhaust on for 15 mins every 2 hours and the plants picked up in a day or two. The best advice is to try it and see! You might have better circulation than me and not need the exhaust on. Also you may need to get a heater for when the lights are off, your hps will keep them warm when they are on but you don't want the temp getting too low when they're off. Again you really need to see how your plant responds, some strains don't mind cold temps as much as others, don't let mine get below 60f tho.

Peace :rasta: