I've personally tried once and had one set of autos on 24 and another on 18/6. Now I don't know how accurate my results were, as there could have been many many factors that played out but I tried to grow them all the same using the same nutes and everything. My plants on 24hr were, i don't know how to put it... just not as healthy, lush and green. They seemed to have a bit more problems but like i said this could be attributed to any other number of factors even differences in pheno types of a strain. But like dude said above plants do things at night they don't in the day or atleast I would believe so.
I kinda look at like would you be stronger and better if you didn't sleep and worked 24hrs or would you feel better with a few hours of sleep?
when growing inside I try and grow to mimic mother nature since that is were the plant originaly came from. but other than that one experiment I don't have any other facts but from my experience I would give them some dark.