Answer: Although most experts recommend that you keep the lights on about 16 hours per day, I keep mine on for 24 hours per day when starting seedlings. So far, I haven't had any problems. I don't have a timer, and I have a feeling that if I turn them off at night, I won't remember to turn them on in the morning before I leave for work. If you were growing plants permanently under the lights--as opposed to seedlings that will get transplanted outdoors in a month or so--it might make more difference.
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