plant photosynthesis has light-reactions and dark-reactions. but the dark-reactions dont actually happen in the dark, that is a misconception. they are light-independent, and can happen in either light or darkness. they, however, require the products of the light-reactions, so they usually happen during the day period as well. the exception is in succulents, which have their reactions occur in the dark to reduce photorespiration (the opposite of photosynthesis).
most plants will grow faster the longer the photoperiod they are given. there is, however, a diminishing return (algae is not subject to this diminishing return). after a certain point, its not worth the electricity it takes, unless its a very valuable plant. one valuable plant is canibis. weed farmers will leave their lights on 24/7 while the plant is in the vegetative growth phase, and only give it night cycles to induce flowering.