when growing from seed it is absolutely necessary for dark period....from seedling the amount of dark time it receives helps determine the sex of cannabis.....check the link in my sig

as for clones that have been sexed 24 hour light or 18/6 and anything in between is the choice of grower for vegging....i have a strain that if it gets any darkness it will preflower so i just run 24/7.....but it is very easy w/LED's in veg...when i ran 600w MH i ran 18/6 so the room could cool down.

it is the light source that determines the amount of root growth.....when i grew under MH i could not keep my clones in peat pots for too long without the plant getting leggy and stretchy and not much root action shooting out of the peat pot having to transplant into larger pots.....now using all LED in the grow cycle i can keep clones in peat pots in a 1020 tray for as long as needed and the root mass is crazy compared to MH...nodes stack tighter also:smokin:

thats my .02 from my experience and observation of using different light sources and schedules in the grow phase :hippy:
stra8outtaWeed Reviewed by stra8outtaWeed on . 24 hour lights Folks, I see lots of peoples using 24 hour veg times. All in good in some situations for a short period of time, but the plant needs some total darkness every day to work on root development. You will slow root proliferation by using 24 hour lighting, increase your electrical costs, increase the 'wear and tear' on the lighting system and needs rest daily too...... I think you will find that most of the experienced growers have learned there is no additional benefit from 24 hour Rating: 5