A marijuana plant needs energy, just like humans and animals. The plant takes the energy through the leaves (chlorophyll), from the light. This is known as photosynthesis. Along with carbon dioxide (CO2) from the air and water (H2O) from the soil, the light is converted into energy.
But not all light is suitable. The light needs to have a specific wavelength (color) and intensity (strength) in order to make the photosynthesis occur properly. It even requires more precision. Depending upon the processes occurring in the marijuana plant, the plant needs a certain light. Blue light, in particular, is needed for the growth process, while the bloom process requires orange and red light. This is the reason why not every lamp is sufficient and you need to use a special grow and bloom lamp.
Exposing the marijuana plant to light sets the following chemical reaction in motion;
Light energy + carbon dioxide + water <---> sugars + oxygen
The arrow in the center indicates that the reaction can go two ways. The arrow to the right means that during the daylight period the CO2 and water are converted into sugars. The marijuana plant gets the energy needed for this, via the chlorophyll from the leaves, from the light. The oxygen that remains is a waste product for the plant and is excreted through the leaves. This is called the carbon dioxide assimilation process.
The longer the energy is available, the longer the factory can run and the marijuana plant can grow or flower. The stock of nutrients in the water is limitless, and you are in complete control of the length of the daylight period. It seems obvious to keep the night as short as possible, preferably to skip it altogether, but it's not that simple.
At night other necessary processes take place, such as the production of proteins. The energy needed for this involves the same chemical reaction, but then from right to left. The sugar stored energy and oxygen is converted into water vapor and CO2. This is known as dissimilation, or the breathing of the plant. The plant uses this energy to make other proteins which are not formed with the assimilation during the day. For this reason a marijuana plant requires a day and a night period.
Research shows that a daylight period of 18 hours is the maximum and best during the growth period. In fact, it's an imitation of the longest day in summer, when the marijuana plant experiences its most important growth period.
During flowering, the daylight period may not be longer than 12 hours. In this case the fall season is imitated, when the plant goes through its most important flowering period. Giving it light for longer than 12 hours may result in the plant going back into growth. So you have to make sure that your marijuana plant has enough available light, and that it's of the right color and at the right moment.