Autoflower Light Schedules: 18/6 vs 20/4 vs 24/0
Comparing the three most common photoperiods for autoflowering cannabis, what the evidence actually says, and how to choose.
Autoflowers will finish on almost any light schedule from 18/6 to 24/0 because they flower on age, not photoperiod. The 'best schedule' debate is largely folklore — there is no published, peer-reviewed trial settling it. 18/6 saves about 25% on electricity versus 24/0 and gives plants a dark respiration period that most growers prefer. 20/4 is a reasonable middle ground. Pick based on your power bill, heat, and tent temps — not forum dogma.
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 is triggered into flowering by long nights (roughly 12 hours of darkness), autoflowers transition to flowering based on age and genetics — a trait inherited from Cannabis ruderalis populations adapted to short northern summers [1][2] Strong evidence. That means you can run autos on a single light schedule from seed to harvest without ever changing the timer.
The three schedules growers argue about:
- 18/6 — 18 hours light, 6 hours dark. The most common recommendation.
- 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 a reason: it mirrors a long northern summer day (similar to what ruderalis evolved under), gives plants a respiration/recovery window in the dark, and cuts electricity use by 25% compared to 24/0. Most autoflower breeders — including Dutch Passion and Mephisto Genetics — recommend 18/6 in their official grow guides [3][4] [evidence:practitioner].
20/4 is favored by growers who want a bit more daily light integral (DLI) — the total photons delivered per day — without going all the way to 24/0. It's a reasonable compromise if your tent stays cool and your power is cheap.
24/0 is used by some growers who believe constant light maximizes growth, and by anyone who simply doesn't want to bother with a timer. The case for it is weak: plants perform photorespiration and translocate sugars during dark periods, and continuous light has been shown to cause physiological stress and chlorosis in some species (the so-called 'continuous light injury') [5] Weak / limited. Whether cannabis specifically suffers from 24/0 has not been rigorously tested in peer-reviewed work — claims either way are anecdotal.
When to start (and stop)
Pick your schedule before the seed cracks and run it the entire grow. Autoflowers do not need — and do not benefit from — a 'flip' to 12/12 the way photoperiod plants do. Changing schedules mid-grow can stress the plant but will not delay or accelerate flowering meaningfully, because the flowering trigger is genetic age, not photoperiod [1][2] Strong evidence.
Run the same schedule from day 1 through harvest. Some growers drop to 12/12 in the final 1–2 weeks hoping to boost trichome production or color — there is no published evidence this works on autos, and it cuts your DLI right when the plant is filling out buds Anecdote.
How to do it: step-by-step
- Choose your schedule. For most home growers in a tent, 18/6 is the safest default. Choose 20/4 if your environment is cool and electricity is cheap. Choose 24/0 only if you've decided the savings on a timer outweigh the (unproven) risks.
- Set a mechanical or digital timer. Even for 24/0, use a smart plug — power outages or future changes are easier to handle.
- Align the dark period with your hottest hours. If your room peaks at 2 PM, run lights off from roughly noon to 6 PM on 18/6. This reduces heat load and stabilizes VPD.
- Match light intensity to stage, not schedule. PPFD targets for cannabis are roughly 200–400 µmol/m²/s for seedlings, 400–600 for veg, and 600–900 for flower [6] Strong evidence. A longer photoperiod does not replace adequate intensity.
- Track daily light integral (DLI). DLI = PPFD × seconds of light ÷ 1,000,000. Cannabis tolerates 40–65 mol/m²/day in flower [6] Weak / limited. 18/6 at 700 PPFD ≈ 45 mol — already in the sweet spot.
- Monitor leaf temperature and tip burn. If you see clawing, taco-ing, or bleached tops, you have too much light, not too little — schedule changes won't fix this. Lower the fixture.
- Don't switch mid-grow. Pick one and stick with it for the whole run so you can actually compare results next time.
Common mistakes
- Treating schedule as a yield knob. No controlled study shows 24/0 outyields 18/6 in cannabis. Genetics, PPFD, VPD, and nutrients dominate yield. Folklore No data.
- Flipping autos to 12/12. Wastes light hours, doesn't speed up flowering. The plant's clock is internal.
- Running 24/0 in a hot tent. Without a dark period, ambient temps climb and stay high, raising VPD stress and transpiration demand.
- Interrupting the dark period. Brief light leaks won't re-veg an autoflower (unlike photoperiods), but constant disturbance during the 'night' on 18/6 or 20/4 disrupts the plant's circadian-regulated processes Weak / limited.
- Comparing your friend's 24/0 grow to your 18/6 grow. Different genetics, light, medium — the schedule is the least controlled variable in that comparison.
The electricity math
For a 200 W LED running 75 days:
- 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 $0.20/kWh, that's $54 vs $60 vs $72 — a $derable real cost difference with no demonstrated yield benefit at the top end. For most growers, 18/6 wins on economics alone.
Related techniques
- Autoflowering Cannabis Basics — genetics and lifecycle.
- Daily Light Integral (DLI) for Cannabis — measuring what your plant actually receives.
- VPD Management — why dark-period temperature matters.
- LST on Autoflowers — training a plant on a fixed timeline.
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.
- Practitioner Mephisto Genetics. Autoflower FAQ and grow recommendations.
- 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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