The standard – 18 hours of light/6 hours of dark per 24 hour day for veg – and 12 hours light/12 hours dark for flowering – this generally is what is used indoors to emulate what the plants would need to occur outdoors to get the plants to produce flowers/buds.
Cannabis is what is commonly known as a short day plant. This means it flowers when the days get shorter, in the fall/winter.
The reason the term “short day” is used, is because botanists used to think the length of the day had the most important impact on when a plant would produce flowers. Later scientist discovered it is actually the length of the darkness that had the most significant impact on the hormones and such that regulate flowering.
In general, most strains need about at least a solid 10 hours of darkness to flower.
Some strains might flower with as little as 8 hours of darkness, but these would be rare indica or ruderalis dominant strains. And of course some ruderalis don’t need any darkness to flower, hence autoflowers
And pretty much all strains will flower with at least 12 hours darkness. This is why 12/12 is used for flowering indoors.
However there are a few equatorial sativa dominant strains out there that have been known to need a full 13 hours of darkness to really get flowering.
Most studies have shown that any less than 9 hours of pretty intense light, and the plant won’t be able to produce as much THC or “bud mass” and will result in a lower yield with less potent buds. And lower quantity and lower quality is not what most people are looking for…
Flowering in response to day vs night hours is called photoperiodism and it is the most significant influence on flowering.
Short day is actually a misnomer, and as I made clear above, it is the length of the night that has the effect on the plant’s biology. These plants should actually be called “long night” plants.
I somewhat inaccurately called them hormones, I used the term loosely more so as it is something a lot of people can understand on kind of how the biology works.
More accurately they are called phytochromes. Plants make 5 of them: PhyA, PhyB, and also C, D, and E.
The mechanism of photoperiodism in short-day plants is known as the ‘hourglass model’.
Phytochromes come in two varieties that are interconvertible. One absorbs red light (around 660 nm). The other absorbs far-red light (around 730 nm).
We can express these as Pr and Pfr, abbreviated for ‘phytochrome red’ and ‘phytochrome far-red’.
Absorption of red light by Pr converts it into Pfr. And vice/versa, absorption of far-red light by Pfr converts it into Pr.
In the dark, Pfr spontaneously and gradually converts back to Pr.
So the plant pretty much needs the full uninterrupted dark cycle (8-13 hours depending on strain) to convert all the Pfr back to Pr. and to carry out the supplementary reactions leading to the release of the flowering signal (“florigen”).
This is why “The Gas Lantern Routine” works and prevents the plant from going into flower with nearly 12 hours of darkness. Only a flash of intense 660-nm light and the Pr is immediately converted to Pfr, undoing all the night’s work.