I think there's a difference between 'droopy' and 'turning down' or whatever you'd want to call that. Are the leaves stiff and turgid, or are they floppy? Do you have more than one light, or are you under a big bank of strong flouros? If the plants ARE stiff and perky, and they are getting light from a large area or multiple directions, and there aren't any spots, I would not worry.