There's been a lot of discussion of veg lighting schedules on many different forums. I've spent some time searching around to find some solid data. Almost every thread I can find speculates on two different ideas... either that plants need "sleep" so 18-6 is best/fine, or that more light equals more growth, so 24-0.
Well, I finally found a test done on another forum. I can't find the original post, only references to the study that was originally posted. So, I completely understand if no one wants to take it fully seriously. But the results of this study were that side-by-side comparisons with different lighting schedules (and light intensities) resulted in the conclusion that 20-4 was actually the most efficient lighting schedule for vegetative growth.
They compared 16-8, 18-6, 20-4, 22-2 and 24-0... as well as HID vs. T-5 fluoro. The results concluded that using T-5 vs. HID in the same time period, etc. found T-5's to produce only 49% of the yield that the plants veg'd under HID did. Not to say that T-5's aren't great (I have an 8-bulb, love it) but side-by-side in the same time, they get their ass kicked. Also, 20-4 came out on top, and was rated at the standard of 100%. In comparison it went something like, 18-6 being 2nd, 24-0 being 3rd, 22-2 being 4th and 16-8 coming in at 5th. I might have mixed up one of those, I'm just reciting from memory, but the T-5 to HID ratio is definitely on point, and 20-4 pulling a 100% vs. 89% ratio on 18-6 for final yield is also accurate.
So, I don't have a link. But take it for what it's worth. Something worth looking into... I've been running 20-4 since, only switch it up if it's an unusually hot day, etc.
