Cycle the grow lights on and off appropriately. You cannot maximize plant growth by simply leaving the grow lights on 24 hours each day. In fact, this is a good way to stress and kill your houseplants. Instead, aim to leave grow lights on continuously for about 14 to 16 hours each day, which simulates spring sunlight. A good way to ensure that the grow lights are cycled on and off each day is to wire them to a timer. You can purchase a light timer at most home improvement stores.