Auto and Photoperiod Light Cycle Options

I read a lot about 24/0, 18/6, and 12/12.

I get that 24/0 and 18/6 is for vegging photoperiods, that 12/12 is for flowering photoperiods, and that 18/6 is an optimal cycle for autos, but I’m also reading arguments regarding maximizing vegatative growth at 18/6 vs. 24/0. It may be incorrect, but is intuitive to me that 24/0 might be optimal for vegging photoperiods.

There are some awfully smart people here. Can someone please point out a good source or summarize here the advantages and drawbacks to the various lighting cycle options?

I’d like to be a little smarter when I make my choices for both photoperiod and autos.)


Processes occur during night hours that don’t do as well if under constant light.


I agree with this. 24 hours of light might not be detrimental, but it doesn’t seem to be better than 18/6 or 20/4.

DJ Short swears by 11/13. Some people like 6/2x3.

11/13 is said to bring out phenotypes, especially in tropical Sativa.

6/2x3 can be convenient if your schedule is dictated by work/family rather than your hobby. It’s never more than 2 hours from the light period and your chance to tend to the plants.

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What matters is the Daily Light Index. As long as you’re hitting that, you’re golden. That determines how much light is needed over a certain period of time to make your plants happy. It takes into account PPFD, lights on hours, etc.

For instance, a plant receiving 24 hours of light needs less intense light than a plant only recieving 18 hours of light.

I know that clarified absolutely nothing. :grinning: There are all sorts of mathematics involved. Here’s a nice rundown:

Let’s see if @dbrn32 is around.


As a rule all living things need sleep. Without sleep…well youve heard of meth heads…that’s what your plant would become without sleep. It produces fast but it’s scrawny and dwindly…and it’s teeth fall out lol


If I was growing Autos I’d have a 18/6 schedule till it was time to flip to 12/12. @Myfriendis410 hit it perfectly.


I am believer of what @blackthumbbetty posted.

Scientifically, c3 plants can and will complete all neccessary processes under 24 hour light condition. If you don’t believe this, all you have to do try it. The only thing you will miss out on is internodal elongating from the plant sensing dark. Which puts me in the position of trying to maximize plant energy. If you can do that on 18 hour schedule, then it doesn’t make any sense to run lights longer. If you can’t, then you can run lights longer to make up for the difference.


Does this mean, for instance, that I can manage nodal spacing (at least in part) via the difference in an 18 vs. a 24 hour lighting cycle during veg?

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I don’t know if manage is term I would use. But you should see less stretch on a 24 hour light cycle. Biggest reason is that the same thing can happen from your light recipe.