Its from the book Marijuana Horticulture: The Indoor/Outdoor Medical Grow Bible on page 175 in the "Light, Lamps, and Electricity" section. It says:

"Some growers leave the HID on 24 hours a day. Marijuana can efficiently process 16-18 hours of light per day, after which it reaches a point of diminishing returns, and the electricty is wasted. (See Chapter 16 Breeding)
However, I didn't find anything further in Chapter 16, so who knows? :wtf: It does go on to say:

I talked with Canadian and Dutch growers who claim their plants flower under a 6 hour dark and 12 hour light photoperiod. This expedited, 18 hour photoperiod regimen is supposed to work, but I'm not sold on it. Growers say that their harvest is undiminished, and that they are getting 25 percent more marijuana in the same time. I have not visited their grow rooms to verify these claims. No electricity is saved by adoption this regiment.
So yeah, IDK. :jointsmile: