I just come across this thread that actually has some figures on the difference in root mass.

http://boards.cannabis.com/grow-faqs...ut-lights.html

here's a quote: " Light Cycles

The dark cycle is very important to plants. Respiration (this is when plants are making energy from sugar to oxygen) Continues in dark but at a much slower pace, almost independent of available oxygen because the leaves are not making oxygen in the dark.

In the dark there is a shift from leaf production to root production as the leaves transfer excess energy down to the branches and roots. Therefore, some dark time allows for better root structures. The "Experimental Root Growth 10 Days After Cutting" graph on page 33 shows that giving plants a 24-hour light cycle is not the way to supercharge growth"

This is from "How to supercharge your garden" With the graph on page 33 this follows...

"Light period for cuttings

Plants have a free running internal bio-rhythm of 21-27 hours. In this rhythm, they need dark time. Cuttings have a built in daily rhythm (age also) that they inherit from their parent. Cuttings will root better with a 6 hours to 8 hour dark period because this is the main time when leaves and the stems transfer energy down to the root zone for storage and growth."

The chart has hours of light per day on the bottom, and length of roots in millimeters.
-@ 4hours of light the roots after ten days were only 22mm long
-@ 8 hours of light after 10 days the roots were 25 mm long
-@12 hours of light we shoot up to 40mm long
-@16 hours it peaks at 50mm
-@20 hours it drops down to 37mm
-@ 24 hours it drops down to 29mm

There is also a shit load of info regarding root to yield ratio vs veg to yield ratio, and draws the conclusion that at the end of the experiments the root to yield ratio was higher....."


The only thing that's missing is the 18/6, but I think the evidence is there for the dark period. If it works that good on clones, I'm sure it's better for vegging plants. I went 24/0 this time and everything was fine, but my next grow (and especially for my clones), I'm gonna go 18/6.