Cannabis Light Cycle Fundamentals: 24/0, 18/6 and 12/12

Cannabis Light Cycle

Which light cycle is best for vegetative growth, is an often-debated topic. Here we review the basics and explain why we prefer an 18/6 light cycle.

Cannabis Light Cycle Basics:

In the cannabis community, light cycles are expressed as a fraction with “Lights on” time over “Lights off” time.

  • 24/0: Lights are on 24 hours and off 0 hours
  • 18/6: Lights are on 18 hours and off 6 hours
  • 12/12: Lights are on 12 hours and off 12 hours. 12/12 timing is used during the flowering period with Photoperiod plants.

The light cycle is important because plants need light to photosynthesize. The more light we can give them, the better they are. With photoperiod plants, we are limited to only 12 hours of light during the flowering stage. However, there is confusion in the growing community about the duration of lighting that works best for vegetative growth for photoperiod and auto-flowering plants.

Understanding Basic Plant Processes

Plants use energy from light, to power their growth. Photosynthesis is really two basic processes, a light dependent cycle (the “photo” part of photosynthesis) and a light independent cycle, where carbohydrates are built (the “synthesis” part of photosynthesis). During the light dependent cycle, plants use light and water to create energy in the form of ATP and NADPH. This energy drives the synthesis of carbohydrates in the light independent cycle, which is often called the Calvin cycle. It is “light independent” because nothing within the Calvin cycle itself uses light energy directly. However, it relies on the energy created in the first half of the photosynthesis.

It is during this second stage that plants take carbon dioxide from the air and use it to create the ultimate product of photosynthesis, carbohydrates. Although they are called “light independent” the synthesis processes of photosynthesis only operate when they receive energy from the “light dependent” part of the cycle. This means that the synthesis of carbohydrates occurs only while the plant is receiving light.

Cannabis plants can be grown under continuous 24/0 lighting. They do not require a dark phase to perform their basic functions of growth during the vegetative stage. There are plants that only can absorb carbon dioxide at night and thus require a dark period to complete photosynthesis. However, Cannabis is a C3 plant, which means that it takes in carbon dioxide during the day and does not require a dark cycle to complete the photosynthesis cycle. That said, there is reason to believe that certain strains may benefit from a dark cycle. Furthermore, as we explain below, the benefits of 24/0 lighting are minor and may not be worth the additional costs for electricity.

18/6 For Vegetative Growth

Just thinking about photosynthesis and the Calvin cycle has led many growers to the conclusion that 24/0 lighting is best for vegetative growth. However, there are additional factors to consider. Like other living organisms, plants are governed by a circadian rhythm and a hormonal clock system that functions in sync with a light on, light off cycle. Research has shown that there are important connections between a plant's internal clock and genetic expressions, which can play an important role in the overall health and performance of a plant. A massive review on continuous lighting studies has shown that some plants perform negatively while others perform positively when grown under 24/0 lighting.

The potential benefits of 24/0 lighting are somewhat faster vegetative growth. However, it comes at the expense of additional electricity. Because of diminishing returns, the growth realized during the “additional” light time may be less efficient. You are using 25% more electricity, but you may not realize 25% faster growth. Because of potential negative responses and less efficient returns from electricity, we recommend vegetating all cannabis plants under 18/6 lighting.

12/12 For Flowering

While cannabis plants do not require any darkness during the vegetative stage, many growers know that most cannabis plants do require a uninterrupted dark period to produce flowers. The dark period for flowering has nothing to do with photosynthesis or the ability of the plant to use light energy and make carbohydrates. Rather, the dark period needs during the flowering period are a result of photoperiodism.

Photoperiodism is an evolutionary characteristic which plants developed to be able to determine when the seasons change. There are many plants that respond to day-length or “photoperiod” and the most common response is the induction of flowering. In this way, plants are able to produce flowers at the optimal time of year and before a winter freeze.

Most cannabis plants are photoperiod, which means that they will only produce flowers when a certain duration of dark is received. They actually require a period of uninterrupted darkness to trigger the photoperiod response. In most cannabis plants, the dark period must be 12 hours or half of the total 24-hour cycle. This limits the amount of energy that can be provided to plants during the flowering cycle and is one of the advantages of auto-flowering plants.

Light Cycles for Auto-Flowering Plants

Auto-flowering plants are day-length neutral, which means that they do not depend on light or dark intervals for the induction of flowering. They are bred by crossing cannabis with a day neutral plant that has a fast flowering cycle, ruderalis. This means that longer light cycles can be used during flowering and therefore you can provide more total energy to the plant during the flowering cycle. Rather than reducing the lights to 12/12, you can continue to provide 18/6 lighting to auto-flowering plants throughout their life-cycle. You can increase your yields, but of course you also increase your electricity use, so it will also increase the expense of the grow. As a result, it is not more efficient in terms of returns to electricity, but auto-flowering plants may allow you to harvest more within a certain time-frame, which may be more important to some growers.

Author: Dr Photon

"Being wrong is an opportunity for getting things right."

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The information on should not be considered as financial, legal, or medical advice.
You are responsible for knowing and following the local laws that pertain to cannabis cultivation, possession, and use. Decisions to grow cannabis should be made in consultation with a lawyer or qualified legal advisor. Decisions to use cannabis should be made in consultation with your doctor or medical professional.