18/6 means 18 hours on and 6 hours off, each day.

It's been established for many many many years now that cannabis is a C3 plant. It does not need a dark period.

C3 plants gather CO2 only during the light period when they are photosynthesizing. As long as the light is on, C3 plants gather and use CO2 for photosynthesis.

Some growers practice a version of anthropomorphism with their plants. They believe that since people need rest, plants do as well. Concerning cannabis, this is not true.

Every grower can make a personal choice about light cycle. They can save on their electric bill or prolong ballast/bulb life. 18/6 can be less of a "shock" when changing over to 12/12 for flowering than 24/0 or heat issues can be addressed by fewer hours of light, but basic botany has proven long ago that cannabis needs no dark period.

Ed Rosenthal, Mel Frank and Robert Clarke all have covered this extensively over their careers.

Marijuana Botany Chapter 1 - Sinsemilla Life Cycle of Cannabis
Need the dark
Fencewalker Reviewed by Fencewalker on . 24/0 vs 18/6 I am currently on a 24/0 light cycle since seeds with my plants. I'm two and a half weeks in. What I'm wondering is when, if ever, should I switch to an 18/6 light cycle before I go into the 12/12 flowering cycle at what I plan to be week 6 as long as the plants are large enough. Heres a pic of my plants as of earlier this evening: Rating: 5