Cannabis is
photoperiodic.
That means that it is the period or duration of lighting phase by which it "knows" whether to grow leaves (vegetative phase) or flowers (regenerative or sexual phase). These growth characteristics are controlled by hormones that are switched on or off by light, and these hormones are what tell the plant whether it should be growing leaves or flowers. It is not the daylight period that induces changes, but the dark period that controls this.
If you have a plant that has gone into flower and the dark phase is interrupted by sufficient light (moonlight is not enough to induce), by as little as a couple of minutes, it *will* cause the plant to go back into vegetative phase.
This growth has a distinct appearance, beginning with the plant growing triple-lobed leaves, often that don't have the normal serrations, which then become singles, then goes back to triples, and finally normally lobed leaves. Little to no bud growth will occur during this period.
Each phase change takes about two weeks for the plant to work through. So if you've got a plant in flower and you see it's started to throw triple-lobed leaves instead of the normal 5, 7, 9 or 11, you know that something has caused it to go back into a vegetative phase, aka reveg. If your plan is to get it back into flowering, know that the whole process will take approximately 6 weeks to get through before you see normal growth again.
I have found that reducing an 18hr lighting photoperiod by as little as an hour can induce flowering, but interrupting it for as little as 15 minutes 1x/night is enough to stop this. This is *not* the same thing as when a rootbound plant begins throwing pistils.
I have not played around a whole lot with photoperiods outside using a reduced daylight (reduced by 1/2hr each day) during flowering and stopped when I hit an 8hr day, so if there are others who've played a bit with photoperiods and what their effects are with other observations, I think this is the place to put them.
