Marijuana, hours of light and dark: the best cycles for indoor cultivation

How many hours of light and dark should marijuana plants take?

Modified on: 20/09/2022

Are you curious about the best light and dark cycles for indoor-grown cannabis plants? Here are the differences according to plant variety and growing season

Lighting schedules in indoor cultivation have a considerable influence on the successful growth of marijuana plants, from the moment the cannabis seeds are planted to the vegetative stage and then on to flowering.

It means that knowing and following your plants’ light and dark cycles (photoperiod) can make all the difference.

However, to get the best out of it, it should also be made clear that there are two types of plants: auto-flowering and photo-dependent.

Are you curious about the best lighting cycles for cannabis plants

In the following few paragraphs, we will talk about marijuana and the hours of light and darkness that this plant needs to grow best in indoor cultivation.

Indoor marijuana lighting

Marijuana and light hours: variety and growing period make a difference

Whereas when growing outdoors, the hours of light and darkness are determined by the sun’s rising and setting, in indoor cultivation, the growers decide how to light their cannabis plants.

If you read some articles on this subject here and there, you will probably have realised that some people recommend lighting the plants for 18, 16, 20 or even 24 hours. 

But what is the right recipe?

It all depends on the variety of plants you are dealing with (auto-flowering or photo-dependent) and their growth phase, mainly divided into vegetative and flowering stages.

If you are growing photo-dependent cannabis plants, you will need to reduce the number of hours of light at some point to induce the flowering phase; if you are growing plants from auto-flowering seeds, on the other hand, they will flower automatically after around 25 to 30 days.

So let’s find out in detail what the differences are.

Read also: Marijuana grow box kits: what are they?

Light hours for photo-dependent plants

Cannabis growers who plant marijuana seeds of a photoperiodic variety will need to recreate typical summer light conditions, i.e., about 18 hours of light and 6 hours of dark, from the beginning and throughout the growing period with lamps and various indoor growing systems.

Thanks to this cycle of light and dark, the marijuana plants will develop at their best, both in terms of their stems and leaves and their roots.

Once the cannabis plants have reached their ideal development, the hours of darkness can be increased.

When to switch from 18 to 12 hours of light?

Usually, you switch to a photoperiod of 12 hours of light and 12 hours of dark when the vegetative phase of a marijuana plant is finished (this can last a month or more depending on the genetics you are growing) to induce the plant to flower.

Most of the time, the flowers will appear about ten days after the time change and, during this time, the plant will continue to elongate well, but these are approximate times because, as already mentioned, each type of plant has its own rhythms.

At this point, you are probably wondering why you should choose to grow photo-dependent plants. Are there any advantages?

Although this category of cannabis plants requires more care and attention than dwarf or classic auto-flowering plants, the advantage is that it allows you to choose when you want to flower. 

The grower himself determines the maximum heights of the plants and can do this by simply switching between 18 and 12 hours of light.

Clock to mark the hours of daylight for marijuana

Light hours for auto-flowering plants

Indoor-grown auto-flowering cannabis plants do not need to change their lighting when switching from one growth phase to another, but they allow a photoperiod of 18 hours of light and 6 hours of dark from start to finish.

In the previous lines, we talked about 18 hours of light, although it must be said that some canapiculturists, to speed up the growth of the plant, advocate 20 hours of light.

Let’s say that there are no fixed rules for this, although the best cycles seem to be the more ‘natural’ ones.

Why is it important to keep the light hours constant over time?

When growing hemp plants indoors, it is essential to keep the light and dark cycles constant.

More precisely, it is crucial that the light hours in the indoor growing phase always remain 18 (or 20, in short, the number of hours you choose) and that they do not change from one day to the next. The same applies to the light hours in the indoor flowering phase.

Sudden changes in the photoperiod or irregularly switching on the lighting system could harm your marijuana plants in several ways:

  • they could result in hermaphrodite plants;
  • they could slow down their development;
  • or even cause disease and death.

To avoid these kinds of problems, experienced growers usually equip themselves with sophisticated and precise systems with timers to program the lighting system to turn on and off even when they are not in the area.

Read also: Can hemp inflorescences be used? Clarification


As we have seen, the lighting cycles for indoor-grown hemp plants vary depending on the time of year and the genetics of marijuana grown.

Auto-flowering plants are straightforward to manage. They grow successfully without varying the light and dark cycles according to their growth phase; photo-dependent plants, on the other hand, have to be cared for constantly to develop to their full potential.

We hope that you have enjoyed reading this short article and that it has answered your questions. We would like to take this opportunity to invite you to visit our SensorySeeds store, which specialises in selling cannabis seeds online.

Please browse through our many offerings and choose the ones you prefer to enrich your seed collection, but remember: cultivating cannabis is still prohibited in the UK.

Visit our today. We are waiting for you!