Most plants will not benefit from continuous light and may even perform more poorly. Tomatoes seem to not only do better with 8 hr darkness but prefer nights in the 60s F for temperature. In most cases you'l just be wasting electricity with the extra light.You'll benefit from doubling up on lights during the 14-16 hr that light are on to maximize intensity and if you have the room, spread your plants to allow light to reach all branches well. I've seen no benefit to fancy grow lights; a cheap shop light gets it done.

I cannot find any links to good pictures but Alaska vegetables can be quite huge, 90 pound cabbage, 10 pound carrots, that sort of thing. These large vegetables are often grown a little north of anchorage with 18-20 hours of good to fair sunlight.