Cannabis Light Cycle Explained - Indoor & Outdoor

Cannabis Light Cycle Explained - Indoor & Outdoor

Like all plants, cannabis needs light in order to grow. Light gives plants energy, which it uses to create the carbohydrates needed for growth, and a light cycle will determine how much light it will receive per day. That said, your cannabis plant will need different light cycles during its lifetime in order to produce the best yields. So, let's have a look at what the marijuana light cycle is and how it works.

Marijuana Light Cycle and the plant stage

Before we talk about the different plant stages and the light cycles for each, we need to discuss how lighting is understood in the grower community. Light cycles are often expressed in the following format: Light Hours/Dark Hours. So, when we talk about a light cycle of 18/6, we’re talking about 18 hours of light followed by six hours of darkness. Likewise, 24/0 would be a light cycle involving 24 hours of lighting with no dark periods.

There are two main stages in the plant’s life that require light cycles: the vegetative stage and the flowering stage. The vegetative stage is where the plant is still growing and is yet to produce any buds. Many growers use an 18/6 light cycle during the vegetative stage, although some prefer 24/0 when growing indoors. In order to begin the flowering process, the plant will need at least 12 hours of darkness. At this stage, you will need to switch to a 12/12 light cycle.

Therefore, the two stages we need to focus on are:

  • Vegetative Stage This is where the light cycle is used to ensure the plant growth is as large and robust as possible.
  • Flowering Stage This is where the light cycle is used to ensure the plant produces the best yield possible.

We will discuss these stages and their light cycles in depth below.

Vegetative stage - Marijuana Light Cycle 18/6

The vegetative stage of your cannabis plant’s life cycle is where it does all its growing. As such, this is the most important stage when it comes to producing a successful yield. You will notice during this stage that the stems and leaves of your plant are growing larger and taller, however the plant will not produce any buds at this stage. As your plant continues to grow, you can train its shape and size to your liking. The best way to improve your plant’s growth is by applying the correct light cycle.


When it comes to indoor light cycles, there is some debate whether 18/6 or 24/0 is best. Unlike other plants, cannabis does not need any dark periods to complete photosynthesis during its vegetative stage. Therefore, some growers will argue that a 24/0 light cycle will lead to better growth.

Although 24/0 can lead to faster growth, cannabis plants still run according to an internal clock and are generally healthier with some downtime. Plus, a 24/0 light cycle will use more electricity, which will be wasted if the plant is not performing at its best. For this reason, we recommend sticking with an 18/6 light cycle for an indoor crop.

In addition, you will need to consider the size of your plant. Some strains of cannabis such as Jack Herer and Northern Lights can grow especially tall even without training. As such, make sure your grow room has a high ceiling and is well maintained.

Keeping an 18/6 light cycle will ensure your plant remains in its vegetative state for as long as you want. Cannabis needs at least 12 hours of darkness before it can start flowering. Therefore, keep the light cycle at 18/6 until you are satisfied with its growth, after which you can switch to 12/12 to begin the next stage. The length of the vegetative period should last between 4-8 weeks.


Many growers prefer to begin the vegetative process indoors with controlled lighting before moving the plants outside. Usually, they begin growing during early to mid-Spring with an 18/6 or 24/0 light cycle, then move the plants outside in late Spring or early Summer.

The reason for this is because keeping your plants inside at this stage helps protect them from any sudden cold snaps that could kill them. Once the weather is warmer, you can safely move them outside, at which point they will usually remain vegetative up until the end of Summer.

Flowering stage - Marijuana Light Cycle 12/12

The next stage of your cannabis plant’s life cycle is the flowering stage. This is where all the training you carried out in the vegetative stage should pay off with a large yield. The flowering stage is triggered when the plant experiences at least 12 hours of darkness. This is due to cannabis plants being photoperiod, allowing them to determine the best time to start flowering based on the length of the day. In other words, once the days start getting shorter, the plant knows that this is the right time to produce flowers before the winter comes.


Once your plant has achieved its ideal shape and height, you should switch to a 12/12 light cycle to begin the flowering process. This helps mimic its natural growth pattern in the wild. Once the flowering process starts, you should notice buds begin to form. You may even see notice the plant increase its overall size twofold, so ensure you have enough space in your grow room. If you have trained your plant well and applied the correct light cycles, you should enjoy an excellent yield.


If your plant is outdoors, then the flowering stage will begin naturally. Your plant will start producing flowers sometime after the summer solstice, when the days will begin getting shorter. You may notice the plant will continue to grow during this time and may even double. If outdoors, this should present little problem. Keep in mind that any lighting during the scheduled dark period can adversely affect the flowering period. This can include streetlights and car lights, so ensure the plant is kept away from such light pollution. It can take between three and six months of growing before you reach this stage, but if done well you should be rewarded an excellent yield.

Grow Weed Without Adjusting the Cannabis Light Cycle

For beginners, taking the marijuana light cycle into account can be quite an adventure and, especially when growing indoors, can present challenges. However, it can be a lot simpler. Buy autoflowering seeds!

Autoflowers, do not depend on a light schedule and flower based on age. This often happens after a vegetative phase of just 3 to 4 weeks. As a grower, you don't have to adjust a lighting schedule for this. Moreover, autoflowers are mold resistant, don't require much attention and the completion of the plant takes a lot less time. 

Autoflowers that we can recommend to beginners are the AK-47 autoflowering seeds, but also the Bruce Banner auto, Gelato Auto and Critical Purple auto are very suitable strains for beginners.