Best Light Schedule For Autoflowers Explained

If you're staring at your timer wondering whether to run 18/6, 20/4, or go full send with 24/0, breathe. Autoflowers don't need a 12/12 flip to bloom; they flower by age, so you've got options.

The trick is choosing a light schedule that fits your room, your budget, and your strain's temperament. We've trialed them all, logged the data, and grown more autos than we can count.

Here's the clear, no-drama guide we wish we had on grow #1.

Key Takeaways

• Best All-Around Schedule:18/6 (18 hours on, 6 off) is the gold standard. It provides the best balance of robust growth, energy efficiency, and heat management. Start here if you're unsure.
• For Maximum Yield:20/4 (20 hours on, 4 off) can boost growth and yield in a well-dialed environment with good genetics, but produces more heat and uses more power.
• For Cold Rooms Only:24/0 (24 hours on) is an option only if you need the light to help heat your grow space. It can stress some plants and is the least energy-efficient.
• Hours vs. Intensity: Don't just focus on the schedule. Your light's intensity (PPFD) is just as critical. Aim for 500-800 PPFD during the flowering stage for dense buds.
• Plants Need Rest: A dark period is beneficial for plant health and metabolism. This is why 18/6 and 20/4 schedules are often more stable and forgiving than 24/0.

Why the Right Light Schedule is Crucial for Your Autoflowers

Autoflowers are on a biological timer. They don't wait for shorter days to start budding like photoperiod strains do. That freedom is awesome, but it also means your "day length" is a tool you can use to shape growth, yield, and energy costs.

This unique trait is just one of several that sets them apart; understanding their entire lifecycle is the foundation for a successful grow.

Here's the core idea: more light hours = more photosynthesis. Photosynthesis is how plants turn light into sugars, the building blocks for leaves, stems, roots, and buds.

But plants also benefit from a dark period to run "maintenance" processes and avoid stress. With autos, we aim for a sweet spot between 18 and 24 hours of light per day.

The exact choice depends on your goals (max yield vs. efficiency), your environment (hot vs. cool room), and your strain's vigor.

Short answer up front: 18/6 is the safest, most balanced schedule for most growers. 20/4 can push a bit more growth if your setup is dialed.

24/0 is the aggressive play, works in cool rooms but can be wasteful (and occasionally touchy).

The Top Autoflower Light Schedules: A Head-to-Head Comparison

Below, we break down the common schedules. We'll give you the plain-English pros and cons and exactly when we'd use each one.

Autoflower Light Schedule Comparison Chart

From Our Grow Diary: We recently ran two Zkittlez Autoflowers side-by-side in identical tents. The plant on 18/6 yielded 97g, while the one on 20/4 yielded 108g. However, the 20/4 tent required an extra fan to keep temps below 82°F. For us, the extra 11g was worth it, but it highlights the importance of environmental control.

The 18/6 Light Schedule: The Gold Standard for Growers

• What it is: 18 hours on, 6 hours off, from seed to harvest.
• Why it works: It delivers ample light for strong growth while giving plants a nightly rest window to reset and manage stress.
• Pros you'll feel:
  • Lower electricity use versus longer schedules.
  • Easier heat management; those 6 dark hours can be set during the hottest part of the day.
  • Forgiving for beginners: less chance of light stress or overdriving plants.
• The trade-off: Compared to 20/4 or 24/0, you might leave a tiny bit of yield on the table, often not enough to matter for new growers.
• When we pick it: New tents, first-time runs, warmer rooms, or anytime we want the best balance of growth, cost, and plant health. If you're shopping lights on a budget, 18/6 keeps bills sensible and results high.

The 20/4 Light Schedule: For Those Pushing for Maximum Growth

• What it is: 20 hours on, 4 hours off.
• Why it works: Autos keep photosynthesizing longer each day, building more biomass (leaves and stems early, buds later) if the environment and nutrition can support it.
• Pros you'll feel:
  • Noticeably faster veg growth on vigorous genetics.
  • Potentially higher yields than 18/6 when dialed.
  • Still offers a short recovery window so plants "breathe."
• The trade-off: Slightly higher energy costs and more heat to deal with. If your light is borderline too strong or your temps creep above 82°F (28°C), you can hit diminishing returns.
• When we pick it: Cool-to-moderate rooms, solid LED with dimming, and genetics known to be hungry and hardy. If you've nailed 18/6 and want a bit more, step up to 20/4.

The 24/0 Light Schedule: Is Constant Light a Good Idea?

• What it is: Lights on 24 hours, zero dark period, from seed to chop.
• Why some love it: Maximum daily photosynthesis, especially useful in cold rooms where lights help keep temps up.
• Pros you'll feel:
  • Fastest growth potential under stable conditions.
  • No timer needed, stable temps overnight.
• The trade-off: Highest energy use. Some plants show stress signs over long runs—slight leaf tacoing, stalled prayer posture, or just not looking as "happy." Plants can still grow great under 24/0, but not all phenotypes love it.
• When we pick it: Rarely. Mostly for cold basements or winter garages where the light is your heater. If you try this, watch the plant daily and be ready to dim or drop to 20/4.

The 12/12 Light Schedule: A Niche Option for Specific Situations

• What it is: Equal parts light and dark, commonly used to trigger flowering in photoperiods.
• Does it work for autos? Yes, autos will still flower and finish. But yields are usually reduced because you're only giving half the potential light hours.
• Why use it at all:
  • If you're running autos alongside photoperiods you need to keep in flower.
  • If electricity is expensive and a smaller harvest is acceptable.
• The trade-off: Lower yields, slower growth. We don't recommend 12/12 for autos unless you have a clear reason.

The ROI: Balancing Electricity Cost and Final Yield

Is running your lights for 20 hours instead of 18 worth the extra cost? Let's break it down.

• From 18/6 to 20/4: This is an 11% increase in daily light hours and electricity usage. For this, you might see a 5-10% increase in yield if your environment is perfectly dialed in. For many, the extra heat and power cost may not justify the marginal gain.
• From 18/6 to 24/0: This is a 33% increase in electricity costs. The yield increase is rarely proportional. You are far more likely to see diminishing returns, where the extra light doesn't create a meaningful amount of extra bud, especially if heat becomes a limiting factor.

Our Verdict: For most home growers, 18/6 provides the best return on investment (ROI). The money saved on electricity can be better spent on high-quality genetics or nutrients.

Beyond the Schedule: Light Intensity and Spectrum are Key

Light hours are only half the story. Two other variables make or break your results:

1. Intensity (how strong the light is)

• Define it simply: Intensity is how many photons hit the canopy. Growers measure it as PPFD (Photosynthetic Photon Flux Density), in micromoles per square meter per second (µmol/m²/s). Think of PPFD like "sun strength" at leaf level.
• Targets for autos:
  • Seedling: 200–300 µmol/m²/s (dim the light or raise it higher).
  • Veg: 300–500 µmol/m²/s under 18–20 hours.
  • Early flower: 500–700 µmol/m²/s.
  • Mid-to-late flower: 600–800 µmol/m²/s. Autos rarely need more than ~800: above that, watch for light stress.
    
    

2. Spectrum (the color mix of light)

• Define it simply: Spectrum is the blend of wavelengths your light emits (blue, red, etc.). Full-spectrum LEDs mimic sunlight and work from seed to harvest.
• What to use: A quality full-spectrum LED (3500–4000K white with additional red diodes is great). Blue-leaning light (5000–6500K) keeps plants compact in veg; extra red (deep red/660 nm) boosts flower density.

Frequently Asked Questions and Pro Tips

We've seen the same "panic questions" a thousand times. Here are straight answers.

• What's the best overall schedule for autoflowers?
  • 18/6 is the default winner. 20/4 can outperform it in cool, well-controlled rooms. 24/0 is situational. 12/12 only if you must.
• Will my plants "sleep worse" without darkness?
  • Plants don't sleep like we do, but they do run recovery and metabolic housekeeping in the dark. That's why 18/6 and 20/4 feel so healthy. Some autos tolerate 24/0 fine; others look happier with even a short dark period.
• Did I ruin my plant by changing the schedule mid-grow?
  • Probably not. Autos are resilient. If you move from 18/6 to 20/4 (or back) to manage temps or growth, you'll be fine. Just avoid frequent flip-flopping.
• My leaves are curling or bleaching, what's wrong?
  • That's classic light stress or heat. Dim the light 10–20%, raise it a few inches, or drop temps 2–3°F. If you're on 24/0 or 20/4, you can also step down to 18/6.
    

    

• Can I keep autoflowers in the same tent as photoperiods?
  • Yes. If your photos are still in veg, run 18/6 or 20/4 and both will be happy. If your photos are in flower at 12/12, autos will still finish but yields will be lower.
• Pro tips from the lab:
  • Start 18/6 the first two weeks. If plants are thriving (tight internodes, rich green, no curl), bump to 20/4.
  • Line up dark hours with the hottest six hours in your space.
  • Keep pH in range so the extra light translates into growth: 5.8–6.2 in hydro/coco, 6.2–6.8 in soil. 
  • Don't just crank up the nutrients with the light hours. Perfecting your autoflower feeding schedule is key to avoiding burn. Watch the tips of the leaves—a slight discoloration is your upper limit.
  • Train early and gently. With autos, time is precious. LST (low-stress training) in weeks 2–4 spreads the canopy for better light use without slowing growth.
    

    

  • Seeds matter. Great light can't fix weak genetics; engines need good fuel.

Conclusion: The Best Light Schedule Depends on Your Goals

Here's the clean take: Run 18/6 if you're new, running warm, or just want a reliable, efficient path to chunky, frosty buds. Try 20/4 when your room is dialed and you want a touch more growth.

Save 24/0 for cold spaces or experiments, and keep a close eye on plant happiness. 12/12? Only when you must.

Watch the plant, not the calendar. If leaves pray gently, color stays rich, and temps stay in range, you picked the right schedule. Ready to put it into practice?

Grab proven autoflower seeds from WeedSeedsExpress, set the timer, and let those girls shine.