Adjust your lighting hours down to what they'll receive outside.
why would you ever do that??? they will grow much better under 24 hours light and shouldn't have any problems adjusting to less hours, if anything gradually shortening their day period would confuse them into thinking fall might be arriving, and would probably yield a higher amount of hermies. as long as you take the time to harden them off to the sun. to do that you need to give them sunlight till they start to look a tiny bit droopy and bring them inside, then go for longer the next day ect...till they can handle a full day of strong sun, the time it takes to do this will vary depending on how much light they are getting idoors, if you are vegging them with florescent, expect at least a week to safely harden them off, my first batch this year stayed out too long and the sun bleached half the leaves pure white, luckily i was able to save them and they are only a tad bit behind.
if you can't get away with putting a batch of sprouts on your balcony or back garden each day you could bring them out permanently and stash them under a thick pine tree, and each day move them out a little bit farther into the light, so they receive a little bit more filtered sun each day, also if you care to devote the effort a burlap tent will allow some light in and can be pulled aside a few hours longer each day. one other thing i have found was to plant each seedling, then cut the top and bottoms off of 2 liter sprite bottles (any green one will work)and tape it to a stake, then put around the individual sprout. i initially tried this to ward of woodchucks and rabbits,(it works!) but came back after a few days to find the ones with clear 2 liter bottles were all bleached white and fried from the sun, the ones with green bottles had no damage at all, just like with beer, the green and brown bottles filter the sun and protect the plant, those plants were not hardened off at all but were vegged under much higher wattage than yours so i don't know if i would depend on it completely.