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12hr / 8hr grow cycle

Since I can’t find any answers anywhere posted I decided to make my own topic. I have heard you can shorten the length of a day by 4 hrs, 12 hrs ON and 8 hrs OFF. Essentially this still gives the plants a 12 hr light cycle but the dark is shortened because plants don’t need 12 OFF, they can finish their dark period in 8 hrs and return to flowering after. This obviously is a hard thing to have on timers since almost all timers are based on a 24 hr day. I have in my arsenal of tools a device called ‘grobot’, this is a doser, ph, co2, and computer that runs rfid timers. Very curiously the software program has the options to run the outlets based on a 20hr day and can actually run 12/8 days, this means you can flower a 63 day flower in 51 days (23% faster). I haven’t tried this option out but am curious if anybody has heard of it or if they have actually done something like this before. All comments are welcome thanks


That sounds very interesting. I have never tried it but you have me intrigued.

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I don’t use my grobot anymore, but thinking of trying the 20hr day option. I can just use it as an advanced timer and not for everything else it does. I found it to be a subpar doser unit that didn’t always dose the correct amounts. The timer option for a 20hr day can only be achieved by software as it will make your day time lights come on 4 hrs sooner everyday!!! Plus it seems it would be hard to remember everyday when the lights are on!! :laughing: but it does give you 2 extra months a year in growing, so it could be worth it. Hopefully someone on here has done it before and can shed some light on the topic


I may be wrong but I’m pretty sure you have that backwards ,the most important thing is the 12 dark period (uninterrupted) to keep the plant in flower.i have seen this experiment ran ,it’s super fast but way small yeilds .the plant picks up momentum as it gos thru flower and to cut off those important last week’s of flower greatly reduces yeild.and potency a bit as well.great idea if u need smoke asap but I’d only do it with a plant or two imho. Hope this helps;)


I couldn’t find much information on 12/8. I seen articles on 12/1, apparently it is a way to vegetate them though. Involves 12 hours of initial light and an hour of lighting to break up the dark period. I think some people call it ‘gas lighting’ or something similar.

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i honestly thought it was more dark than light caused it to flower? i just kinda started looking into how much light per day and viceversa dark per day. like all through sept and oct sunrise is one later each day. i got the app lmao. only cause i want to do an LED grow. always just let nature take its course. but any how back to subject, so you would do 10 on and 12 off plus aint rest good lmao. i mean i feel great after a nap hahahha

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Ya it’s 12 dark and 8 light for that experiment ,I’m my experience;)

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I came across a form years ago where someone was flowering their plants in fewer than 12 hours of light. He was tracking his Daily Light Integral, basically the amount of PAR that hits the leaves over a given time. He would crank his light intensity up and illuminate the plants for less time.
Think of it like a 1000km road trip. You reach the destination (total DLI) in 12 hours if you drive normally, or in 6 hours if double the intensity.
I was just lurking in that forum and don’t have any Prabal personal experience, but his posts said he was having success over several runs of growing like that.

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Sounds like he still had significant dark periods, then had shorter, intense light periods?

There was some “Dr” who decreased light period farther, starting mid-bloom. Something like 12/12 to start bloom. Then decrease the light/increase the dark. 11.5/12.5, 11/13, 10.5/13.5, 10/14. He never went too low with the light on. His theory was this would save power, increase yield, and reduce harvest time. I saved that info on my old phone, but never did anything with it.

This might be it, though I don’t remember it being so polished. I think the doc has moved up in the weed world.

I, personally, am sticking to the usual.


Yes, longer dark periods with very short, intense light periods. Something like 6 hours of lights on and 18 hours lights off. Now that I’m thinking more about it I remember the grower having 3 or 4 flower rooms that ran on a flip/flop. With such short lights on time he was able to flower 30-50% more canopy with the same energy inputs.

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