There was a rather lengthy discussion about light cycles a couple of months back. One point that a number of people seemed to agree on is that going from 24/7 to 18/6, the plant thinks the days are getting shorter so it wants to grow faster.
Ok, i'm not sure if i have over looked this on this thread or not but there was a very lengthy discussion on roots and the what goes on in a 24 hour period using different light schedules. To boil this doing to keep it short, here is what happened during that test (this is a hydro test if i remember correctly)

24hour period:
24/0 - 29cm root growth
20/4 - 33cm root growth
18/6 - 38cm root growth

the only thing i dont know is what nutes were used during that 24hour period...would be nice to get an experiment to find out during which stages of growth the plant uses which nutes more specificly....for example during the first week after pop'n the bean which nutes does it use the most and in what quantities..... If anyone has that info i would love to know it...

Hope that info helps, and if its a repeat, sorry...