Autoflower Light Schedule: 18/6 vs 20/4 vs 24/0
Share
Autoflower Light Schedule: 18/6 vs 20/4 vs 24/0
One of the best things about growing autoflowers is that lighting becomes refreshingly simple: you pick a schedule, and you never touch it again. Unlike photoperiod plants, which you must flip from long days to a 12-hours-on, 12-hours-off cycle to force them into bloom, autoflowers flower automatically as they age, no matter what the lights are doing, so there is no flip to time and no dark-period discipline to maintain. That leaves just one real question, and it is the one every autoflower grower asks: which schedule should you run, 18/6, 20/4, or 24/0? And a close second, if the light does not trigger flowering, does the schedule even matter? We sell the lights, tents, and gear an autoflower grow needs, and notably we do not sell seeds, so we have no stake in talking you into anything and will give you the straight version. This guide compares the three main autoflower light schedules, explains why you never flip to 12/12, and covers how light hours, intensity, and distance actually affect your grow. For the full seed-to-harvest picture, see our complete autoflower grow guide. We don't upsell.
The 30-Second Answer
- Autoflowers flower on age, not light, usually around three to four weeks from seed, so you never flip to 12/12 and you run one schedule the whole grow.
- 18/6 is the balanced default: strong growth, a dark rest period, and lower heat and power than running the lights longer.
- 20/4 gives a little more growth for more heat and power, while 24/0 gives maximum light hours but no rest period and the highest heat and power, with no flowering benefit.
- Light still matters for growth and yield, just not for triggering flowering, so more hours mean more energy, but pick one schedule and keep it from seed to harvest.
Set your schedule once, keep it steady, and let the plant's own clock handle flowering. We stock the lights and gear. We don't upsell.
Why autoflowers don't need a light-cycle change
To choose a schedule confidently, it helps to understand the one genetic quirk that defines autoflowers. Most cannabis is a short-day plant, and peer-reviewed research confirms that indoor growers keep it vegetative under long days of at least 16 hours of light and then invoke a 12-hour photoperiod to transition it into flowering. That is the entire basis of a normal photoperiod grow: you control when the plant blooms by controlling the light. Autoflowers throw that system out. Thanks to Cannabis ruderalis genetics, inherited from a hardy subspecies native to the short, harsh summers of high northern latitudes where the days never reliably shorten in time to cue flowering before frost, autoflowers are day-neutral, switching from vegetative growth to flowering based on their age rather than on the light cycle, typically around three to four weeks from germination. The practical consequences are liberating: you never flip the lights to 12/12, there is no benefit to doing so, and you simply run one generous schedule the whole way through. It also means light leaks, which are the number one cause of photoperiod plants turning hermaphroditic, are essentially a non-issue for autoflowers, a genuine perk. The comparison below lays out your three schedule choices.
Does the light schedule still matter?
If the light does not control when an autoflower flowers, it is fair to ask whether the schedule matters at all, and the answer is yes, just for a different reason than with photoperiod plants. The number of light hours still feeds the plant's growth, even though it does not trigger flowering: more hours of light give the plant more total energy to build foliage and, later, buds, while the dark hours let it rest and move nutrients around. This is grounded in how cannabis responds to light overall, since research shows that cannabis yield rises with the light intensity reaching the canopy, and total daily light, a product of both intensity and hours, is what drives growth. So with autoflowers, the schedule you choose mostly affects growth speed, heat output, and your electricity bill, rather than whether or when the plant flowers. That reframes the whole decision: you are not choosing a schedule to control the plant's life cycle, which runs on its own internal clock, but to balance how much growth you want against how much heat and power you are willing to accept. Understanding this makes comparing 18/6, 20/4, and 24/0 straightforward, since it comes down to that tradeoff rather than to any effect on flowering.
18/6: the balanced default
An 18-hours-on, 6-hours-off schedule is the most widely recommended choice for autoflowers, and for good reason: it strikes the best balance between the competing factors. Eighteen hours of light provides plenty of energy for vigorous growth throughout the plant's life, more than enough to support strong vegetative development and healthy bud production, while the six-hour dark period gives the plant a genuine rest, a window in which it can carry out processes that happen in darkness and move resources around. At the same time, running the lights 18 hours rather than 24 meaningfully reduces both the heat your lights dump into the grow space, which is often a real challenge in a small tent, and your electricity costs, which add up over a full grow. This combination of robust growth, a beneficial rest period, and lower heat and power is why 18/6 is considered the sensible default and the one most growers should start with. If you are unsure which schedule to run, or you simply want the most reliable, forgiving option, choose 18/6, run it from the day your seed sprouts to the day you harvest, and you will not go wrong. It is the schedule we would point most autoflower growers toward.
20/4: a little more light
A 20-hours-on, 4-hours-off schedule is a popular middle ground for growers who want to push growth a bit harder without going all the way to constant light. The reasoning is simple: two extra hours of light per day compared with 18/6 give the plant a little more total energy, which can translate into slightly faster growth, while a short four-hour dark period still preserves at least some of the rest window that a fully lit schedule eliminates. The tradeoffs are proportional to the benefit, since those extra light hours also mean somewhat more heat in your grow space and a somewhat higher electricity bill than 18/6. For growers with good temperature control and a desire to nudge their plants along a touch faster, 20/4 is a perfectly reasonable choice, and many experienced autoflower growers run it successfully. It sits logically between the balanced 18/6 default and the maximum-light 24/0 option, offering a modest growth gain while retaining a small dark rest. As with every autoflower schedule, the key is to commit to it for the entire grow rather than switching, and to keep an eye on your temperatures, since the additional lit hours add heat that a tight space may struggle to shed.
24/0: maximum light, no rest
Running the lights 24 hours a day with no dark period at all is the most aggressive autoflower schedule, and it is also the most debated. Its appeal is straightforward: the maximum possible light hours mean the maximum total daily light, which can drive fast growth, and because autoflowers do not need any darkness to flower, they will bloom on schedule under constant light just fine. However, the drawbacks are real and are why many growers avoid it. First, there is no rest period at all, and plants generally benefit from a dark window in which to rest and carry out nighttime processes, so eliminating it entirely is a genuine tradeoff rather than a pure gain. Second, running lights around the clock produces the most heat, which can be difficult to manage in a small tent and can stress plants, and it costs the most in electricity of any schedule. Third, and importantly, the extra hours over 18/6 or 20/4 offer no benefit to flowering, only to growth, so you are paying maximum heat and power for a marginal and debatable growth gain. Some growers do run 24/0 and report good results, but for most people the lack of a rest period, the heat, and the cost make it a poorer choice than 18/6 or 20/4. If you do run it, watch your temperatures closely.
The one rule: set it and keep it
Whichever schedule you choose, the single most important rule of autoflower lighting is to set it once and keep it unchanged for the entire grow, from the seedling stage through vegetative growth and all the way through flowering to harvest. This is a point worth emphasizing, because it differs sharply from photoperiod growing, where changing the schedule is the whole mechanism of control. With autoflowers, the schedule never needs to change, and constantly switching between 18/6, 20/4, and other variations only stresses the plant for no benefit, since none of those changes affects flowering. What does change across the plant's life is not the schedule but the light intensity and the plant's size: a tiny seedling needs gentle, low-intensity light, while a large flowering plant can handle and wants much stronger light, so you adjust the intensity and the distance of your lights, not the hours. Pick your schedule based on the growth-versus-heat tradeoff we have covered, commit to it, and then leave it alone, focusing your attention instead on giving the plant the right light intensity for its stage. If you want to experiment with a different schedule, do it on your next grow, not partway through the current one.
Light schedule and intensity by stage
Although the schedule itself stays constant, how you deliver that light shifts as the plant grows, so it helps to think through each stage. During the seedling stage, keep the same light hours as the rest of the grow, but use low intensity, because a young seedling has a small root system and few leaves and is easily stressed or stretched by light that is too strong or too close; softer light, kept a bit further away, is ideal, as our seedling lighting guide covers. As the plant enters its short vegetative phase, it grows quickly and can handle progressively more intensity while you hold the same hours, stacking the nodes that will later carry buds. Through flowering, again on the same schedule, the plant wants strong, steady light to fuel bud development, so this is when full intensity pays off, and understanding the relationship between intensity and total daily light, through metrics like PPFD and DLI, helps you dial it in, as our PPFD and DLI guide explains. The theme across all three stages is the same: the schedule does not move, but the intensity climbs from gentle at the seedling stage to strong in flower, matched to what the plant can use. Getting that intensity progression right matters more than any tweak to the hours.
Light distance for autoflowers
Closely tied to intensity is the distance between your light and your plants, which is a common source of trouble and easy to get wrong in either direction. If a light is too close, it can burn the plants or bleach the tops and stress them, while if it is too far away, the plants receive too little of its energy and grow slowly and stretchy as they reach for it, so there is a sweet spot to find. The right distance depends heavily on the type of light: as a rough starting point, LED grow lights can often sit around 30 centimeters, roughly 12 inches, from the top of the canopy, while hotter HID lights should generally be kept at least 60 centimeters, about 24 inches, away, though you should always follow your specific light's recommendations, since wattage and design vary widely. Crucially, this distance is not set once but adjusted throughout the grow, because as your plants grow taller and stronger they both approach the light and become able to handle more intensity, so you raise the light or the plants as needed to maintain the right gap. Our guide to the best LED grow lights helps you match a light to your space. Watch your plants for signs of too much light, like bleaching, or too little, like stretching, and adjust accordingly.
Do you ever flip autoflowers to 12/12?
A frequent question from growers used to photoperiod plants is whether they should, or can, flip their autoflowers to a 12/12 schedule at some point, and the clear answer is no, you should not, because there is simply no benefit to it. Autoflowers will flower on their own regardless of the light schedule, so flipping to 12/12 does not make them flower any better or any sooner; the flowering is already handled by the plant's age. What flipping to 12/12 would do is reduce the total light hours from 18 or more down to 12, cutting the plant's daily energy and therefore reducing its growth and ultimately its yield, all for no gain. In other words, running 12/12 on an autoflower gives you a smaller harvest with no compensating benefit. The entire appeal of autoflowers is precisely that you never have to flip the lights, which removes one of the most common timing mistakes photoperiod growers make, so embracing that simplicity is the point. Keep your generous schedule, whether 18/6, 20/4, or 24/0, running unchanged the whole grow, and let the plant handle flowering itself. The only situation in which 12/12 enters the picture is if you are also growing photoperiod plants, which we turn to next.
Can autoflowers and photoperiods share a tent?
Because autoflowers and photoperiod plants have different lighting needs, growers often wonder whether the two can be grown together in one space, and the answer is a qualified yes, as long as the schedule favors the autoflowers. Since autoflowers thrive on a long schedule like 18/6 or 20/4 and photoperiod plants stay in vegetative growth as long as they receive at least around 18 hours of light, running an 18/6 schedule works for both: the autoflowers grow and flower normally on their internal clock, while the photoperiod plants simply remain in veg, held there by the long days. This lets you raise both types in the same tent for a while. The catch is timing at the end: the photoperiod plants will not flower until you eventually flip the whole tent to 12/12, which you can only do once the autoflowers have finished and been harvested, since dropping to 12/12 earlier would not benefit the autos and would cut their light. So the workable approach is to run 18/6, let the autoflowers complete their fast cycle and harvest them, and then flip the remaining photoperiod plants to 12/12 to bloom. The tradeoff is that the photoperiod plants spend the autoflower cycle vegging rather than flowering, but for many growers combining the two this way is a convenient use of space.
Common autoflower light mistakes
A handful of lighting mistakes trip up autoflower growers, most of them stemming from habits carried over from photoperiod growing. The most common is flipping to 12/12 unnecessarily, either out of habit or in the mistaken belief it will improve flowering, when in fact it only cuts light and reduces yield. Another is switching schedules partway through the grow, chasing an optimal cycle, which merely stresses the plant since none of the changes affects flowering; the fix is to pick one schedule and keep it. Over-lighting seedlings is a frequent error too, since young autoflowers are delicate and hard light too close will stress and stretch them, so gentle, low-intensity light early is essential. Getting the light distance wrong in either direction, too close and burning the tops or too far and causing stretchy, slow growth, is common and calls for watching the plants and adjusting. Some growers expect the schedule to speed up or delay flowering and are frustrated when it cannot, forgetting the plant runs on its own clock. And running 24/0 in a space without adequate cooling invites heat stress. Avoid these by choosing one generous schedule, keeping it constant, using gentle light on seedlings, dialing in the distance, and managing heat. We don't upsell, and simplicity really is the autoflower advantage.
What We'd Tell You at the Counter
If you came in wondering which light schedule to run on your autoflowers, we would tell you to keep it simple and run 18/6, because it gives you strong growth, a real rest period for the plant, and less heat and power draw than running the lights longer. We would explain that the whole reason autoflowers are easy is that they flower on their own timeline, no matter the lights, so you set your schedule once and never flip to 12/12 the way you would with a photoperiod plant. If you have great cooling and want to push a little, 20/4 is fine, and some folks run 24/0, but you are paying more heat and power for a marginal growth bump and giving up the rest period, so we would not push you there. We would remind you that what actually changes as the plant grows is the light intensity and how far the light sits, not the hours, so start gentle on seedlings and bring the light in as they get stronger. And we would point you to our full autoflower grow guide for feeding, training, and the rest. We sell the lights and the gear, and we do not sell seeds, so we will just give you the straight version. We don't upsell.
Frequently asked questions
What is the best light schedule for autoflowers?
For most growers, 18/6, meaning 18 hours of light and 6 hours of darkness, is the best and most recommended autoflower light schedule. It provides plenty of light for strong growth throughout the plant's life while giving a genuine dark rest period, and it produces less heat and uses less electricity than running the lights longer. Some growers run 20/4 for slightly faster growth at the cost of more heat and power, and a few run 24/0 for maximum light hours, though that eliminates the rest period and produces the most heat and cost with no benefit to flowering. Because autoflowers flower based on age rather than light, none of these schedules changes when the plant blooms, so the choice comes down to balancing growth against heat and power. Whichever you pick, run it unchanged from seed to harvest. If in doubt, choose 18/6.
Do autoflowers need 12/12 to flower?
No. This is the key difference between autoflowers and photoperiod plants. Photoperiod cannabis is a short-day plant that only begins flowering when its daily darkness lengthens, which is why indoor growers flip those plants to a 12-hours-on, 12-hours-off schedule to trigger bloom. Autoflowers, thanks to their Cannabis ruderalis genetics, are day-neutral and flower automatically based on their age, usually around three to four weeks from seed, regardless of the light schedule. So you never flip an autoflower to 12/12; in fact, doing so would only reduce its total light hours, cutting its growth and yield, with no benefit since the plant is already going to flower on its own. Instead, you run a generous schedule like 18/6 for the entire grow. Not needing to flip the lights is precisely what makes autoflowers simpler to grow than photoperiod plants.
Is 18/6 or 24/0 better for autoflowers?
For most growers, 18/6 is the better choice. Both schedules will grow and flower autoflowers successfully, since autoflowers do not need any darkness to bloom, but they differ in their tradeoffs. A 24/0 schedule provides the maximum light hours and thus maximum total daily light, which can push growth, but it eliminates the dark rest period that plants generally benefit from, produces the most heat, which can be hard to manage in a small tent, and costs the most in electricity. An 18/6 schedule still provides ample light for strong growth while preserving a genuine rest period and running cooler and cheaper. Since the extra hours of 24/0 offer no flowering benefit and only a marginal, debated growth gain, most growers get the best overall results from 18/6. Choose 24/0 only if you have excellent cooling and specifically want maximum light hours, and watch your temperatures.
Does the light schedule change autoflower timing?
No, the light schedule does not control when an autoflower flowers or how long it takes, because that is set by the plant's genetics and age rather than by light. An autoflower will begin flowering on its own around three to four weeks from germination and finish its whole cycle in roughly ten to twelve weeks, regardless of whether you run 18/6, 20/4, or 24/0, and you cannot speed this up or slow it down much by changing the lights. What the schedule does affect is growth: more light hours provide more energy for the plant to build foliage and buds, influencing size and potentially yield, along with how much heat your lights produce and how much power they use. So think of the schedule as a growth and efficiency choice, not a timing control. If you want to influence an autoflower's final size, focus on strong genetics, adequate light intensity, good nutrition, and healthy roots rather than on the light schedule.
Can I change the light schedule mid-grow?
You can, but you generally should not, because it offers no benefit and only risks stressing the plant. Since an autoflower's flowering is driven by age rather than light, switching from, say, 18/6 to 20/4 or 24/0 partway through will not make it flower differently or better, and constantly changing schedules can stress the plant at a time when it has little ability to recover. The recommended approach is to choose one schedule before you start and run it unchanged from the seedling stage all the way through to harvest. If you want to try a different schedule to compare results, do it on your next grow rather than mid-cycle. The only adjustments you should be making during the grow are to light intensity and the distance of the light from the plants as they grow, not to the number of light hours, which should stay constant throughout.
How far should the grow light be from autoflowers?
The right distance depends on your light type and its strength, and it should be adjusted as the plants grow. As a rough starting guideline, LED grow lights can often sit around 30 centimeters, about 12 inches, from the top of the canopy, while hotter HID lights should generally be kept at least 60 centimeters, roughly 24 inches, away, but you should always follow the specific recommendations for your light, since wattage and design vary. The goal is to find the sweet spot where the plants receive strong light without being burned: too close and you risk burning or bleaching the tops, too far and the plants stretch and grow slowly reaching for light. This distance is not fixed, since as plants grow taller and stronger they get closer to the light and can handle more intensity, so raise the light or lower the plants to maintain the gap. Watch for bleaching, a sign of too much, or stretching, a sign of too little, and adjust.
Can autoflowers and photoperiods grow together?
Yes, autoflowers and photoperiod plants can share a tent, as long as you run a light schedule that suits the autoflowers, and understand the timing tradeoff. Running an 18/6 or 20/4 schedule works for both types at once: the autoflowers grow and flower normally on their age-based clock, while the photoperiod plants stay in vegetative growth because they are receiving the long days they need to hold off flowering. The complication comes at the end, since the photoperiod plants will not flower until you flip the whole tent to 12/12, and you can only do that after the autoflowers have finished and been harvested, because dropping to 12/12 sooner would cut the autoflowers' light for no benefit. So the practical method is to run 18/6, harvest the autoflowers when they finish their fast cycle, and then flip the remaining photoperiod plants to 12/12 to bloom. The downside is that the photoperiod plants spend the autoflower cycle vegging rather than flowering.
Whether you are setting up your first autoflower tent or dialing in an efficient light routine, the LED lights, timers, and gear that make autoflower lighting simple live in our catalog, and our team is glad to help you match a light to your space. Autoflower lighting really is refreshingly simple, so pick 18/6, keep it steady, and pair this with our complete autoflower grow guide and our guides to choosing a grow light, light intensity, gentle low-stress training to make the most of your light, feeding lightly to avoid nutrient burn, and the full week-by-week grow guide. Because, as always, we don't upsell.
For informational and educational purposes only. This article is general horticultural guidance and is not legal advice. Cannabis cultivation laws vary by country, state and locality, and growing cannabis may be illegal where you live. Always understand and comply with the laws and regulations that apply to you before growing any cannabis plant.