During the day, two main metabolic processes occur in plants. Photosynthesis is the process that plants use to convert water and CO2 into sugar using light as an energy source. These sugars are then used to fuel metabolism. The plants use oxygen obtained from the air and burn sugars to power the cell. Respiration takes places continually, day and night.
Growth is the result of the plant using sugars to build cellulose and other tissue. Plants build tissue both during light and dark periods, but the growth is limited by the amount of sugar produced during photosynthesis.
C3 green plants such as cannabis only use CO2 when they are photosynthesizing, so they only need to be enriched during the lighted portion of the day and not during the dark cycle. During the dark period, cannabis, like all green plants, uses oxygen. Air circulation should continue to replenish the micro-environment at the leaf surface with fresh oxygen-rich air.
-Ed Rosenthal The Best of Ask Ed: Your Marijuana Questions Answered