Autoflower Light Schedules: 18/6 vs 20/4 vs 24/0
Compare the three main autoflower light schedules: what works, what costs, and what is folklore.
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Schedule matters far less than genetics, light intensity, and VPD. All three work fine on autoflowers because they don't depend on photoperiod to flower. The real difference is electricity cost and—possibly—heat management. Most breeders recommend 18/6 as a safe default, but there is no peer-reviewed study proving one schedule outyields the others. Pick one and optimize everything else.
What It Is
An autoflower light schedule is the daily on/off cycle you run lights for autoflowering cannabis from germination to harvest. Unlike photoperiod cannabis, which transitions to flowering when night length exceeds a threshold (typically 12 hours of darkness), autoflowers flower based on age and genetics—a trait inherited from Cannabis ruderalis populations that evolved in short northern summers [1][2] Strong evidence. Because flowering is not triggered by day length, you can run the same light schedule from seed to harvest without ever changing the timer.
The three standard schedules growers debate are:
- 18/6 — 18 hours light, 6 hours dark. The most widely recommended.
- 20/4 — 20 hours light, 4 hours dark. A middle ground.
- 24/0 — Lights on continuously. No dark period.
Why Growers Use Each One
18/6 is the default for several practical reasons. It mirrors natural day length in high-latitude summers where ruderalis evolved, gives plants a respiration and recovery window during darkness, and reduces electricity use by 25% compared to 24/0. Most autoflower breeders—including Dutch Passion and Mephisto Genetics—recommend 18/6 in official grow guides [3] Weak / limited.
20/4 appeals to growers seeking higher daily light integral (DLI—the total photons delivered per day) without committing to 24-hour lighting. It is a reasonable compromise if your tent stays cool and electricity is cheap.
24/0 is used by some growers who believe constant light accelerates growth, and by anyone avoiding the cost or complexity of a timer. The evidence for yield advantage is absent. Plants perform photosynthesis at lower rates in darkness, carry out photorespiration, and translocate sugars to storage organs during dark periods—all essential metabolic processes. Continuous light in other plants has been shown to cause physiological stress [4] Weak / limited; whether cannabis specifically suffers from continuous light has not been tested in controlled peer-reviewed studies No data. Claims of 24/0 superiority are anecdotal.
When to Start and Stop
Choose your schedule before the seed germinates and run it for the entire grow. Autoflowers do not require—and do not benefit from—a photoperiod 'flip' to 12/12 the way photoperiod plants do. Flowering is triggered by age and genetics, not day length [1][2] Strong evidence. Changing schedules mid-grow can create stress but will not meaningfully delay or accelerate the flowering transition.
Run the same schedule from day 1 through harvest. Some growers reduce to 12/12 in the final 1–2 weeks hoping to boost trichome density or color; there is no published evidence this practice works on autoflowers, and it cuts daily light integral precisely when the plant is filling out buds Anecdote.
How to Do It: Step-by-Step
- Choose your schedule. For most indoor growers, 18/6 is the safest default. Choose 20/4 if your environment stays cool and power costs are low. Choose 24/0 only if you accept that the yield benefit is unproven and you value timer simplicity over a small electricity saving.
- Set a mechanical or digital timer. Even for 24/0, use a smart plug or timer. Power outages or future changes are easier to manage if a timer is in place.
- Align the dark period with your hottest hours (if using 18/6 or 20/4). If your room peaks at 2 PM, run lights off from roughly noon to 6 PM. This reduces heat load during the hottest part of the day and stabilizes VPD (vapor pressure deficit).
- Match light intensity to plant stage, not schedule. Target PPFD (photosynthetic photon flux density) values for cannabis are roughly 200–400 µmol/m²/s for seedlings, 400–600 for vegetative growth, and 600–900 for flowering [5] Strong evidence. A longer photoperiod does not replace adequate light intensity.
- Calculate and track daily light integral (DLI). DLI (mol/m²/day) = PPFD × seconds of light per day ÷ 1,000,000. Cannabis generally tolerates 40–65 mol/m²/day in flowering [5] Weak / limited. An example: 18/6 at 700 PPFD ≈ 45 mol/m²/day—already within the acceptable range.
- Monitor leaf temperature and visual stress. If you see leaf clawing, taco-ing, or bleached leaf tips, you likely have excessive light intensity, not insufficient photons. Lower the light fixture; schedule changes will not solve this problem.
- Don't switch schedules mid-grow. Stick with your chosen schedule for the entire run so you can make meaningful comparisons in your next grow.
Common Mistakes
- Treating schedule as a yield lever. No controlled peer-reviewed study demonstrates that 24/0 outyields 18/6 or 20/4 on cannabis No data. Genetics, PPFD, VPD, substrate, and nutrient timing dominate yield far more than photoperiod. Claims otherwise are folklore.
- Flipping autoflowers to 12/12. This wastes light hours and does not accelerate flowering because autoflower flowering is internally timed, not photoperiod-dependent. It is a wasted electricity cost.
- Running 24/0 in a warm tent. Without a dark period, ambient temperature climbs and stabilizes at a higher baseline, raising VPD and transpiration demand. This increases heat stress risk, especially late in flower.
- Interrupting the dark period on 18/6 or 20/4. Brief light leaks will not re-veg an autoflower (unlike photoperiod strains), but constant disturbance of the dark period may disrupt circadian-regulated processes Weak / limited.
- Comparing your grow to someone else's without controlling other variables. Your friend's 24/0 yield may beat your 18/6 yield, but they may also have better genetics, a more powerful light, better humidity control, or more experience. Schedule is rarely the limiting factor.
Electricity Cost Comparison
For a 200 W LED fixture over a typical 70–80 day autoflower cycle (using 75 days as a reference):
- 18/6: 200 W × 18 h × 75 d = 270 kWh
- 20/4: 200 W × 20 h × 75 d = 300 kWh
- 24/0: 200 W × 24 h × 75 d = 360 kWh
At a typical U.S. residential rate of $0.20/kWh, that is:
- 18/6: $54
- 20/4: $60
- 24/0: $72
The difference between 18/6 and 24/0 is $18 per grow—or about 33% higher energy cost with no demonstrated yield benefit. For most home growers, 18/6 wins on economics alone.
The Bottom Line
All three schedules will produce mature autoflowers. The choice should depend on your heat management, electricity cost, and personal preference rather than yield expectations. The most reliable path to higher yields is maximizing light intensity (PPFD), maintaining stable VPD, and feeding according to plant demand—not manipulating the photoperiod. If you are new to autoflowers, start with 18/6: it is the most forgiving default, balances electricity use with plant recovery time, and lets you focus on the factors that actually drive yield.
Sources
- Peer-reviewed Small, E., & Cronquist, A. (1976). A practical and natural taxonomy for Cannabis. Taxon, 25(4), 405–435.
- Peer-reviewed Spitzer-Rimon, B., Duchin, S., Bernstein, N., & Kamenetsky, R. (2019). Architecture and Florogenesis in Female Cannabis sativa Plants. Frontiers in Plant Science, 10, 350.
- Practitioner Dutch Passion. Autoflower Grow Guide — recommended light schedule.
- Peer-reviewed Velez-Ramirez, A. I., van Ieperen, W., Vreugdenhil, D., & Millenaar, F. F. (2011). Plants under continuous light. Trends in Plant Science, 16(6), 310–318.
- Peer-reviewed Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment. Frontiers in Plant Science, 12, 646020.
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