The light cycle can change autoflower plant growth, the end yield, bud density, branch formation, and even the plant’s height so growers should know why plants in certain light cycles do best and in what stages of plants growth what cycles would be best!

And that’s what I’m aiming to answer with this post!

But firstly, you need to know that autoflowers will flower under any light cycle. Which means that you can grow them under a 24/0 light cycle or even on a 12/12 light schedule. However, there are other aspects to consider than just if the plants will grow!

What is the best light cycle for autoflowers?

Cannabis is a plant that requires no dark period which is why it is categorized as a C3 plant. Being a C3 plant means that it gathers and uses CO2 during the light period but it doesn’t gather CO2 during the dark period like C4 plants do.

From this, we conclude that cannabis plants thrive if they get continuous light through their growth stages so a non-stop 24/0 light cycle should be better for autos than say a 16/8 or 18/6 light schedule.

In the vegetative stage of the plant’s growth (that happens in the first few weeks of autoflowering plants growth) you can increase the growth by up to 30% if you leave the lights on all the time. But in later growth stages, this additional growth isn’t so evident therefore despite the fact that increased growth will be visible you won’t get a 30% higher yield.

Although this additional light is beneficial for autoflower plants, it will also cost more.

Let’s assume that a grower has a 400W HPS lamp and the electricity price is 10 cents for KWh. In this scenario, if the grower would run his HPS 80 days nonstop then they would be burning 768 kilowatts ((24 h * 80 days * 400 W)/1000) which would cost him $76.80. However, if they would use an 18/6 light schedule they would be burning just 576 kilowatts that cost $57.8 and they would save $19 just with this one HPS lamp.

On top of that, the non-stop light would also decrease the lamp’s life span that could also be a potential saving.

After seeing these numbers every grower can do the math and see how much more money they would burn for his setup.

However, keep in mind that, depending on the autoflower strain, the dark period can decrease the plants’ growth by about 20%. This means that you can calculate how much expected yield would you lose if the autoflower plant would get 20% less energy.

After doing some math, we can see that a full 24/0 light schedule is far more beneficial for autoflowers because they will grow bigger, and faster and you will ultimately get more yield. And the electricity costs aren’t that big compared to that increase in yield.

So I would suggest you run your lights non-stop since this way you will get more for your investment in energy as probably the plant is much much more valuable than those few dollars that you spend on electricity.

Using different autoflower light cycles for cooling your grow room

Light cycle changes can also decrease the overall heat signature of your grow room. So, if you have any heat problems you can slightly reduce them by using a light schedule with a dark period since  the grow room will have time to cool off a bit during the dark period.

However, there are two potential pitfalls associated with not using a 24-hour light schedule for autoflowers to cool down your grow room.

The first is that your grow area should be well isolated and continuous light should not be a problem. Therefore, if a continuous light schedule can overheat your grow room you should definitely think about the grow room design and tweak it so the heat is not a problem anymore!

The second thing that’s wrong with the option of turning lights off for cooling the grow room is that autoflowers don’t like a temperature difference that’s more than 10 degrees, So, if your daytime temperatures are reaching 30°C but at night your temperature drops below 20°C you can severely stress your little plant and stun it.

So you can use light cycles for grow room cooling if you have no other choices but for most cases, I would not recommend it.

Using different light cycles to get bigger plants

In nature, autoflowering cannabis plants get different light exposure cycles and the most stretching usually happens in the night time. At night cannabis plant tries to find more light and it stretches toward the sky to do that.

This doesn’t mean that in the light cannabis growth stops, this just implies that at night cannabis stretches more.

Even if autos stretch more at night, the overall growth is much faster in daylight as cannabis produce energy only when it gets light shining on those green leaves.

So growers could use light cycle changes to get bigger plants by letting them stretch out in the dark period ultimately getting bigger plants. But a consistent dark period will not necessarily yield the most amount of bud.

I would say that if you really want to stretch your plant then a dark period should be mixed with continuous light periods so that the autoflower plant could stretch at the night and get a long light schedule for more vigorous growth to branch out, make more leaves, and produce denser buds.

But if cannabis produces energy only in the light then logically one might deduct that the best conditions for it are continuous light throughout its life to produce the most amount of energy to grow leaves, stems, and ultimately big buds.

Contrary to popular belief you should set the light schedule for more light if you want smaller plants. And keep in mind that smaller doesn’t mean that they will produce less – they would actually have denser buds with closer branches, and therefore have more total mass and they won’t be stretched.

Using differed light schedules for a bigger yield

Autoflowering cannabis plants will produce denser buds if the light period is continuous since the plant produce all its energy from photosynthesis and therefore more energy means more mass and denser buds.

The dark period between the light cycles can increase stretching that will lead to fluffier buds. However, this increased stretching can only be beneficial if you have a very humid environment where dense buds could easily sustain mold or fungus. And that can be easily treated by decreasing the humidity levels in your grow space.

A truly stable autoflowering seeds that are 100% autoflowers will definitely yield more if it is under a continuous light cycle.

But not all seeds are perfect and many first, second and some more generation unstable crossbred seeds will not completely flower under 24 hours of light and will produce small yields. In this scenario, the dark period between light is actually a good idea as you’re not so perfect autoflower will get some dark period and will produce better buds.

Should you deprive your autoflowers of light before harvest?

Many growers say that a dark period increases the resin gland production on cannabis plants and that plants which are deprived of light for the last couple of days will give you stickier and whiter buds.

Yet, this theory has not been proven scientifically. And as much as I can tell it really doesn’t matter if you give your plans a light shock in form of a two-day dark period or not. They will give you the same amount of buds.

If we think about this myth some more then we can boil it down to the truth – cannabis plants produce energy while they are exposed to light. Which means that you will get bigger and denser buds if you supply more light.

So for autoflowers if you give them 48 hours of light as opposed to the 48 hours of darkness at the end of their life cycle they should produce more from that extra energy!

The only real-world solution for the dark period at the end of the plant’s life cycle is in the situation when you need to harvest prematurely and you can’t wait for the real harvest.

In this situation giving your autoflower plants a complete light deprivation stress (also called light depping) may trigger them to form denser buds as the plants think their life is coming to an end so they produce more resin to protect their seeds!

What will an interrupted light cycle during veg do to my plants?

One more thing to remember here is to give your plants a stable light schedule and don’t change it during its growth so your autoflowers don’t get confused and can grow to their full potential.

It Is undoubtedly true that automatic plants will flower under any light schedule but cycle changes can reduce the yield and induce stress in those plants.

However, if you have frequent light leaks or unplanned electricity outages then autoflowers are undoubtedly more tolerant to those occurrences than regular photo-sensitive cannabis and they won’t convert back to vegetative growth as it can happen with regular weed plants.

Autoflower light cycle outdoors

For outside growers, light cycles aren’t a big deal since you can’t change the period when the sun shines and when it doesn’t. So the only thing you can do is calculate the best possible time to plant your weed.

Say your autoflower ganja plant will automatically flower after 25 days and it will be ready to harvest in 70 days. You can divide this time and plant your cannabis plant outdoors 35 days before the summer solstice (June 21st) since the nights are very short as midsummer approaches and it is the time when your plants will get the most amount of light.

If you need more help with growing your plants you can check out my complete autoflower grow guide where I give a thorough explanation of how to grow your autos!




  1. I’m approaching the final three weeks of a AutoMazar/Autoextreme grow with a 20/4 light cycle. Is it too late to go 24hrs. light to maximize yield?

    • Happy to hear that! I think that there will be no visible improvements if you go to 24/0 at this point , I would suggest to ending this grow with the same light cycle and in the next one use the 24/0!

  2. iv’e got 11 day old auto cheese that ive got on 18/6 and there doing really well should I go to 24/0 at this point?

    • Hi!
      If I were in your shoes I would go for 24/0 as the electricity costs would not increase that much but your plant could receive more light.
      Maybe go for 20/4 to not stress that plant as much ,as it has grown for those 11 days in an environment where there is a dark period.
      Is that Dinafen Cheese auto? If so then I can’t find what Dinafem says the perfect light cycle is so you can go 24/0 or 20/4 but 16/8 seems a bit too little light.

      • doctorspliff on

        midowo hello, I do autoflo dinafem, cycle lumier Is Always 20/4 for their seeds at the moment I do autoamnesia critical and 2.0 super perfume

          • doctorspliff on

            they have a captivating fragrance, especially auto amnesia, critical 2.0, I’m in heaven, harvest in 20 days 😀

  3. the seeds were NL auto cheese and I went to 24/0 on the 3rd and the growth has been great since then. in fact its quite incredible the difference its made. ive always been an outdoor grower so this is first indoor and af grow so I went with a 24-8-18 veg nute at 1/4 power right now wit a couple drops of superthrive inbetween feedings and at 16 days all four are right around 6 to 7 inches with stems as thick as a sharpie and super bushy. I do have another question im growing under cfl lights each plant has 3 26 watt 4600k 1450 lumen bulbs and one 55 watt 2700k 3860 lumens there in a small closet heat and humidity have been excellent so my question is can I nute more than once a week? if so what do u recommend? there in pro mix and I added couple tbs of bone meal and 1/2 cup perlite there in 2 1/2 gallon pots. thanks for any help….

    • great to hear your progress with autos.
      are u still using that 1/4 strength? I can suggest to increase that to 1/2 but for the feeding frequency if they are healthy and growing maybe don’t change that schedule.
      If I were in your shoes I would stick to what is working as nutrient overdose can be a real problem for autos.

  4. yes my last feeding was at 1/4which was a tsp per gallon was gonna go to tsp and 1/2 this week at feeding and bump the superthrive another drop. yeah ive read that autos LD 50 is funny when it comes to nutes and they can be picky. I have some experience with nutes and nute burn that’s y I added more perlite for drainage just in case I must flush. also ive ordered some auto widow cheese gonna use my mh and hps but ill be honest the cfls penetrate just as well although I have 16 bulbs. when do you suggest nute change to flower? ive read different theroies on that subject and haven’t came to a conclusion yet would like your input. also ive never used my hps just my mh to ready clones for the outdoor garden when growing an indoor auto should I use mh ,hps or both? again thnkk you for sharing your knowledge

    • I have also used regular CFL bulbs and they worked OK, but now I am using T5 fixture and it is much much better! About the MH or HPS, I think only HPS would be better, and you can read more about this on our Facebook page where Dave was speculating on one of his comments that autos don’t really need that much blue light.
      As for the flowering nutes, I usually change at the moment when I see first pre-flowers, but sometimes I give 1/2 flower and 1/2 vegetative for one or two feeding and then change. I believe we can’t be completely sure how any particular strain will react on that micro element change so its all trail and error.

    • That depends on the strain, but if you have a true auto-flowering plant then giving it more light will result in bigger and denser buds.

  5. if i have an auto that states on its site that 20/4 is optimal, will it hurt if i do 24? it’s wos nl & big bud

    • If it says 20/4 then you probably should maintain that light schedule but if that strain is stable then 24/0 should give even better results.

  6. Sorry for the drop in but does indica strain after week 1 drop firs leaves to gain light to new set?

    • Hi, I haven’t heard that Indica plants do that, the leaf dropping could be due to some stress.

  7. I bought clones in November. Took clones off them in December that went to 18/6. Well it January and those clones are teens now and are showing female already. I guess they were autoflowering. Anyway to delay them from flowering? Can I still clone them?

    • They may be some Super-Autoflower strains as regular autoflowers are usually ready to harvest in 70 to 80 days. If they are auto-flowering then taking clones will not improve the situation and you have to let them flower and get some new regular photo-sensitive strains.

  8. Stupid question…can or should you top an auto at all, or is it better to just use the LST method. Also, can the LST method be used on non autos.

    • You can grow autos on a 12/12 light schedule but that will diminish the yield. Autflowers are not photo-sensitive so that light cycle won’t affect their growth time.

  9. I have a autoflower tutankarman from pyramid seed company I’ve got her under a 180w ufo led and was wondering if the light cycle was still the same as a hps in flower or would i get away with 20\4 or 24 if enyome could help me would be must appreciated thank you in advance

    • I think 20/4 would be the best but you can definitely give your plant 24/0 if it is a true auto flower!

    • It depends on the growing conditions and the strain you are using. Fastest auto flower plants will take from 60 to 70 days from seed to harvest but super autoflowers can take up to 120 days to fully mature.

  10. Ted crane on

    My auto from Dutch passion auto flower in dwc. They have all died but 2. When should I nute and what should I use

    • Hi Ted, Sry to hear that.

      I am primarily a soil grower so I won’t be able to tell you the best nutrient schedule for your DWC grow.

  11. Robert Delgado on

    I am starting three THC BOMBS in a dwc and have them on a 600 watt HPS.

    Should I change bulbs to MH since they are still seedlings and then switch back to HPS when they enter the flowering stage?

    • Hi Robert,

      I think that you can let the plants grow under that HPS light, but if you have spare MH bulbs of course it would be best to start your plants under them.

  12. Im just starting out with amnesia haze autoflo fem shes only about 9 days old and is very tall like almost 8 inches but still at the first “true” leaf stage is this a problem??

    • Hi, what is your light source and what is your growing medium. The most likely cause is that you have too little light and the seedling stretches for that light, but it can also be that the growing medium has too much nutrients and that stresses out your little plant.

      • I was using 4 small cfl bulbs and black earth soil i shifted to an led grow light and noticed a few small burn marks in the upper leaves she stopped climbing though i have the light about 15″ away from the top its been 2 days since the new light … im also starting a new one using coco coir and perlite mix hoping it may work out better … for nutrients im using REMO micro REMO bloom REMO grow and REMO MagNifiCal

  13. Michael Stonebridge on

    Hey Buddy,

    Great site – thanks and well done 😉

    I am a complete rookie with Autos, never done one before ever, but I have decided to “chuck” in a RQS Stress Killer CBD into my Veg Tent for a little fun and extra/different something to smoke.

    As such matey my question(s) to you is this:
    *Would she grow as you mention above under 24/0 cycle under a 250w?
    *Would I be better moving her between my Veg Tent & Flower Tent – ie “lives” in the Veg but goes in the Flower for 12 a day when its “sunny”…?

    Sorry if all that makes no sense (lol stoner-gram) or it’s too much to reply to but keep up the good work on here anyways!
    Many Thanks

  14. I have 600w l.e.d growing blueberry x jack herra using foxfarm soil. should I start seedlings under full spectrum light. first time grower veteran burner thank you

  15. Im thinking of getting the lumatex ZEUS 600w led to help boost yield , using a 150cmx150cm tent , ill be growing 4 to 5 autos ?, and not sure of what pot size to go for also do you think the light would be worth the investment .????? im using easy grow tabs for feeding with guano tea once a week but open to any suggestions on pot size , how many in the room is to many and what you think of the easy grow tabs ..

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