Best Light Schedule for Autoflowers

What is a light cycle and which one is the best for your autoflowering plants.
18 March 2021
5 min read
Best Light Schedule for Autoflowers

  • 1. What is the best light cycle for autoflowers?
  • 2. Do autoflowers need darkness?
  • 3. 24/0 light cycle
  • 4. 20/4 light cycle
  • 5. 18/6 light cycle
  • 6. 12/12 light cycle
  • 7. In conclusion

Unlike photoperiods that need at least 12hrs of darkness to trigger flowering, autoflowers automatically enter the flowering cycle when they’re mature enough. They start producing buds based on age thus they don't depend on darkness to start the flowering cycle.

This means that the best light schedule for autoflowers is either 24/0, 20/4, or 18/6 so there are multiple options and the best one will depend on your growing environment and setup.

Tip: Light cycles are used to simulate seasons in nature, more light = summer, more darkness = winter.

1. What is the best light cycle for autoflowers?

Most growers agree that the optimum amount of light to give autoflowering strains is somewhere between 18-24 hours of light a day. As a grower you should adjust the light depending on the cultivar you’re growing.


Best Light Schedule for Autoflowers: close look at cannabis seedling

The best light cycle will depend on your growing conditions and growing setup.

There are basically four light cycles aka Light schedules: 24/0, 20/4, 18/6, and 12/12 (the first number is hours of light, the second is hours of dark). You can always adjust the cycle as long as they are receiving more than 12 hours of light a day (to achieve the best results). An example would be 19/5, 16/8, 22/2, etc.

Having that in mind, as mentioned above, there are growers experimenting with schedules like 16/8 but if you’re new to growing you should stick to the basic schedules mentioned above.

Light Spectrum

Now, despite cannabis plants relying on the amount of light they get, you should know that depending on the phase your plant is in, it will need a different spectrum and that can also affect your harvest.

As you may know, cannabis plants go through 2 stages, the vegetative and flowering stage, during these stages your autoflowers will obviously need light, but they will also need certain wavelengths of the spectrum.


Best Light Schedule for Autoflowers: best light spectrum for autoflowers

It's essential you provide the ideal light spectrum for each stage of plant growth and preferably, a full spectrum.

For example, during the vegetative stage, your plants will need a blue spectrum that ranges from around 400 to 550nm whereas during the flowering stage your plants will need a red spectrum, with wavelengths that range from 550-700nm.

This doesn’t mean that you won't be able to have a successful harvest if you don’t provide a full spectrum but you will definitely have better results by providing the whole spectrum from seed to harvest or at least the spectrum your plants need during each stage.

Have in mind that this applies to all Light fixtures, including light bulbs, usually good-quality LEDs will emit a full spectrum but you can get mixed spectrum bulbs that will get the job done.

2. Do autoflowers need darkness?   

There are growers who believe autoflowering plants need a dark period and won’t be as healthy if they get a 24/0 light schedule. There’s no real evidence of that but there may be exceptions. Also have in mind that a 24/0 will lower humidity, increase temperature, and increase the light bill.

Schedules like 18/6 will save you electricity and if the few hours of darkness indeed help the plant to grow better, it’s a win-win. We recommend starting with an 18/6 light cycle and if you see your autoflower has the potential to grow more, you can always try again in the next cycle.

3. 24/0 Light Cycle

Plants usually grow faster when they get more light. This schedule is a good choice if you live in a cold climate, keeping the lights turned on 24hrs a day will keep your autoflowers warm.


Best Light Schedule for Autoflowers: 24/0 Light Cycle for cannabis plants

24/0 Light Cycle for cannabis plants.

Obviously, this is the easiest autoflower light cycle as you don’t even need a timer, just turn on lights until harvest.


  • Doesn’t require a timer. 
  • May result in the best yields.


  • Uses more energy, can get expensive.

4. 20/4 Light Cycle

The 20/4 has become a popular autoflowering light schedule amongst growers because it’s in the middle between 18/6 and 24/0, providing a good growth without having to spend too much like when growing under 24/0.


Best Light Schedule for Autoflowers: 20/4 Light Cycle for cannabis plants

20/4 Light Cycle for autoflowers.

Despite obviously leaving the lights on for a bit longer than in 18/6, your plants still have some time to “rest”, allowing you to have slightly better results without having to spend too much like in a 24/0.


  • Allows autoflowers to rest. 
  • Better results than 18/6.


  • Uses more energy than 18/6, can get expensive.

5. 18/6 Light Cycle

This is the most common cycle for autoflowers. You're providing enough light for your plants to develop and a few hours of dark for them to rest, encouraging healthy growth. This can be good in hot climates.

You can leave your lights at night and have them turn off during the 6 hottest hours of the day. By using this schedule you’ll be using around 25% less electricity, which adds up to be a good economy at the end of every harvest.


Best Light Schedule for Autoflowers: 18/6 Light Cycle for cannabis plants

18/6 Light Cycle for autoflowers.

A lot of growers who have experimented with 24/0 and 18/6 light cycles say they didn’t notice any major difference in final yield. The only minor inconvenience with this schedule is having to buy and adjust a timer. They cost around 10 bucks so it shouldn’t be a problem at all.


  • Allows your plants to rest.
  • Uses less electricity, allowing us to save on electricity.
  • Can help lower temperatures and increasing humidity in hot climates.


  • Requires a timer. This shouldn’t be a problem at all, as this is the first thing you should buy when growing indoors.

5. 12/12  light cycle

The 12/12 schedule is normally used for photoperiods. It can also be used for autoflowers but it’s not that popular. Normally growers give autoflowers this light cycle when they have them growing in the same tent as photoperiods. By giving your autos 12/12 you’re not growing them to their full potential.


Best Light Schedule for Autoflowers: 12/12 Light Cycle for cannabis plants

12/12 Light Cycle for cannabis plants.

Because your plant is getting less light each day, it isn’t able to make as much energy to promote growth. It will underperform compared to the other light cycles, that being said, you can grow them just fine if you don’t mind reduced yields and an overall smaller plant. 


  • Uses less electricity, allowing us to save on electricity.
  • Allows you to grow autos and photos in the same grow tent.


  • It’s recommended to grow under 18/6 so 12/12 usually results in aery and underdeveloped buds.

Autoflower grow light schedule pros and cons

Light ScheduleProsCons
24/0Faster growth.May be too much for some strains.
20/4Good growth and allows plants to “rest”.Slightly less growth than 24/0.
18/6Less heat and cheaper.Slightly less growth than 20/4.
12/12 Cheapest. Underdeveloped buds.


6. In Conclusion

There isn’t really a proven best light cycle. Before going for any light cycle, think about electricity costs and the climate you’ll be growing in. If you are in doubt, start with 18/6. This cycle is the most used and should work for all autoflowers.

When growing outdoors you have to work with what you have as you can’t control the sun (obviously). Outdoor growers should be aware that with the change of seasons the amount of daylight may increase or decrease but autoflowers should grow fine in all seasons. 

Even though there are the most common light cycles, make sure you experiment with all of them and find what suits you better because every plant and grow setup is different so there's a chance that you have better results growing under an uncommon light cycle.


This post was most recently updated on March 18, 2021.

18 March 2021