What Light Cycle is Best for Cannabis Grow?

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Added 19 August 2020

Cannabis Plants Need Lots of Sunlight

What is a Light Cycle?

A light cycle is as it reads. It is the amount of time there is daylight and darkness over a 24 hour period. When we consider light cycles, there is a big difference between growing cannabis indoors or outdoors. To put it simply, most cannabis plants require a certain photoperiod in order them to recognise when they need to flower.

When a plant does not receive the correct amount of light during the various stages of it's life, it probably will not thrive. You can see the effects of this by checking out this grow that did not receive enough light.

Light Deprived Cannabis Plant

Lack of light and other nutrients can cause cannabis plants to start showing deficiencies.

Setting up your light cycle is a simple process that is quick to organise. There are a few basic rules about light cycles that need to be followed in order to get the best results. This article will give you a deeper understanding on the photoperiod a cannabis plant needs to produce a top quality harvest.

Outdoor Light Cycles for Growing

Germinate During the Early Spring

Germinating is best during early spring as the amount of light starts to increase outside.

To have a clear understanding of light cycles, we need to understand how light changes naturally outside. During the summer season, there is plenty of sunshine to grow cannabis outdoors.

Cannabis plants need generous amounts of light in order to survive. This is why the best time to start germinating seeds is early in the spring, as the daylight hours start to increase after the winter.

After the spring, the light hours gradually decrease during summer to around 12 hours per day. At this stage a plant should already be showing signs of flowering. Once you notice flowers developing, pay close attention to how the light is changing over the days. 

The number of weeks a cannabis plant will be in this cycle also depends on the strain and you should check light cycles in your region before germinating. If you plant too late into the season your plant may not have enough seasonal light for it to develop fully.

Plants will mature before winter arrives as the days get shorter and should be harvested before temperatures drop too much.

Germinate your seeds inside. Their 24/0 or 20/4 light cycle should begin once the seedling has broken the surface of it's growing medium. Plant them outdoors and they will follow the sunlight's natural course. Nature takes care of itself.

Seasonal Calendar for Growing Cannabis Outdoors

In the northern hemisphere, germinating generally starts in March/April. Plants are harvested after mid September. 

Indoor Light Cycles for Growing

Growing indoors means you are trying replicate the conditions of an outdoor environment. There are many factors to consider when setting up an indoor grow and the lighting is one of them.

Unlike the outdoors, you have full control over the photoperiod, meaning plants can grow all year round. Because they are light sensitive, most plants will not flower until the amount of daylight hours is reduced. In an indoor environment this would mean having to change the schedule from 18 hours of light to 12 hours of light.

Grow Room Light Cycles

Growers have full control over light cycles when growing indoors.

There are a number of different schedules growers use but we find the following schedule to be the best when it comes to growing indoors:

  • Sprouting - 24/0 = 24 hours of light and 0 hours of darkness. Prevents seedling stretching and helps to keep the conditions stable. 
  • Vegetation - 18/6 = 18 hours of light and 6 hours of darkness. This is the average light cycle most growers use to vegetate their plants.
  • Flowering - 12/12 = 12 hours of light and 12 hours of darkness. Simulates the outdoor summer/autumn hours so plants recognise when they need to flower.

 The following table lays this schedule out easily for you.

Stage Light Dark Weeks
Seedling 24 0 1-2
Vegetation 18 6 4-6
Flowering 12 12 8-14

Some growers also use other light cycles such as 20/4 for seedlings or 16/8 for vegging, and even 14/10 for flowering. We would advise against using these schedules and instead advise you to stick with what generally works best for most strains of cannabis. To see that is it possible under different light schedules, have a look at this grow of an outdoor autoflowering plant at 12 weeks on 14 hours of light. 

Once you get used to working with light cycles you can experiment, but this method will work for most strains when growing indoors.

Top tips for growing indoors:

  • Buy a timer for your lights. This means you won't have to manually turn your lights on and off. It also keeps the light cycle as accurate as possible. For safety reasons, use a timer that can handle the amount of power used by the lights.
  • Set your lights to be on mostly during the night to keep temperatures down. This is especially helpful during the summer when temperatures rise.
  • Make sure you know how many days your crop needs for the different light cycles. This way you know what date to switch the hours of light. Using a calendar makes this easy.
  • Keep the dark period completely dark. Even a small light leak can cause problems to your plant's cycle and it may not flower properly. Check this by closing yourself in your grow space with a light on outside. Patch up any light leaks or any equipment lights with tape.

Grow Room Lights Set on Automatic Timer

Grow room lights can be set to turn on and off automatically.

Light Cycles for Autoflowering Plants

Autoflowering plants are unique in that they do not require a dark period to flower. This means you can grow them under 24 hours of light. However, plants have a dark period naturally when they are outside so it might be best to give an autoflowering plant at least a few hours of darkness. Growing under 24 hours of light is not necessary (plus, it increases costs), so we recommend using a 20/4 or 18/6 schedule.

Because they do not need a dark period, you can easily have your autoflowering plants in the same room as your vegging plants. As autoflowering plants are usually smaller than photoperiod plants, they can be used to fill up areas of your grow room that would otherwise be empty.


It is important to study the nature of light and how it changes yearly. This is how indoor growers have managed to refine their indoor growing technique to be the most effective. Every grower wants to produce good quality bud and make their grow easier, so covering these basic tips will help you decide what works well for you. If you have any questions, feel free to leave us a comment below.

External Sources

Time and Date AS, Day and Night World Map

This article was updated August 2020



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When u grow in pots/buckets outdoor u can also have full control of the Light Cycle. Forced flowering is used frequently in the Northern Hemisphere to grow long flowering sativas, otherwise they would be destroyed by the cold temperatures and high humidity late September/early October.