Sketch framing the autoflower light schedule title

November 11, 2026

Autoflower Light Schedule: 18/6, 20/4 or 24/0?

For most indoor grows, run 18 hours of light and 6 hours of dark from seed to harvest. Pick 20/4 if your room stays cool and you want slightly faster growth. Use 24/0 only in a cold space where the extra heat helps. Whatever you choose, keep it fixed for the whole grow and adjust light intensity by stage instead.

This guide compares the common schedules, shows the energy math, gives intensity targets for each stage and explains how to spot light stress.

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TL;DR

18/6 is the best default: strong growth, a real rest period and a lower power bill.
20/4 adds 14 hours of light per week over 18/6. The gain is modest and heat and cost go up.
24/0 gives the most light hours, the most heat and the highest power use. It rarely pays off at home.
Do not change the schedule mid-grow. Autoflowers flower on age, so change PPFD (light intensity) by stage.
Watch the canopy for bleaching and upward-curling leaves. Raise the fixture or dim the light if they appear.

Why autoflowers accept different day lengths
The common schedules at a glance
18/6: the balanced default
20/4 vs 18/6: is the extra light worth it?
24/0: why most home growers skip it
What about 12/12?
Keep the schedule fixed, change the intensity
A stage-by-stage plan on 18/6
How far should the light be from the plant?
Can an autoflower get too much light?
Choosing your schedule: a quick decision guide
Lighting that fits the schedule
Our perspective
FAQ
Sources

Why autoflowers accept different day lengths

Photoperiod cannabis plants are short-day plants. Their flowering is regulated by the length of the dark period, as Greenhouse Grower explains. Growers must cut the light to about 12 hours to trigger buds.

Autoflowers work differently. They flower on age, not on a change in day length. That is why a single schedule can run from seed to harvest, as the ILGM autoflower schedule guide describes. Most growers run between 18 and 24 hours of light per day.

The hours still matter. They do not decide when the plant flowers, but they feed growth. More light hours give the plant more energy for buds. Dark hours give it time to rest and move nutrients. So the schedule you pick mostly affects growth speed, heat and your power bill.

The same Greenhouse Grower article notes that day-neutral cultivars and autoflowers can raise their daily light total under photoperiods of 20 hours or more. In a commercial setting, this can remove the need for expensive blackout systems.

The common schedules at a glance

Most home growers choose between 18/6, 20/4, 24/0 and sometimes 12/12, according to Seed Supreme. The choice affects growth speed, the electricity bill and indoor heat.

Schedule	Light / dark	Hours of light per week	Best for	Trade-off
18/6	18 h / 6 h	126	Most indoor growers	Slightly slower than 20/4
20/4	20 h / 4 h	140	Cool rooms, growers who want speed	More heat and more power
24/0	24 h / 0 h	168	Cold spaces	Highest heat and cost, no rest period
12/12	12 h / 12 h	84	Mixed grows with photoperiod plants	Lowest total light

The weekly hours are simple multiplication. They show why the schedules differ in cost.

18/6: the balanced default

18/6 gives the plant 18 hours of light and 6 hours of dark. It is the most common recommendation. Auto Seeds calls it the best all-around choice and notes that the extra 2 hours in a 20/4 schedule do not add much to yield.

Why it works for most rooms:

Plants get a full rest period each day.
Fixtures run six hours less than on 24/0, so heat stays easier to manage.
Power use is lower than on 20/4 or 24/0.
It is simple to set on any timer or app-controlled light.

If you are growing your first autoflower, start here.

20/4 vs 18/6: is the extra light worth it?

This is the comparison many growers search for. 20/4 adds roughly 11% more light time than 18/6. That is 14 more hours of light each week.

The result is usually modestly faster growth, not a dramatic change. 420 Seeds reports that a home grower on 18/6 runs the lights 14 fewer hours per week than on 20/4. It calls 18/6 the most efficient and financially practical option.

Here is an illustration of the cost. Say your fixture draws 480 W. Fourteen extra hours a week is 6.72 kWh. At an example rate of $0.20 per kWh, that is about $1.34 per week, or roughly $15 over a 11-week grow. Use your own utility rate and fixture wattage to run the same calculation. Bigger fixtures and multiple lights scale it up.

Heat matters too. The extra two hours also mean two fewer hours of cooler lights-off time. In a warm room, that can push canopy temperature higher and make light stress more likely.

Choose 20/4 when:

Your grow space runs cool.
You can hold temperature steady with good airflow.
Your power rate is low and speed matters to you.

Choose 18/6 when:

You want the best balance of growth and cost.
Your room runs warm.
You are new to autoflowers.

24/0: why most home growers skip it

Autoflowers do not need 24 hours of light. They grow well on 18 to 24 hours, and the dark hours let them rest. A 24-hour schedule can work, but it adds heat and power cost without a guaranteed gain, as ILGM notes.

Royal King Seeds describes 24/0 as uncommon among cultivators. Its main benefit is heat, which suits colder climates, and it carries a risk of light overexposure. 420 Seeds adds that autos grown under 24/0 can stay shorter, with tight internode spacing, which may reduce yields.

If your grow room is in an unheated garage or basement in winter, 24/0 may help hold temperature. In most heated indoor spaces, it only adds cost.

What about 12/12?

12/12 is the lowest-light option. Some growers use it in mixed grows, where autoflowers share a room with photoperiod plants that need 12 hours of dark to flower. It can also suit very hot climates.

The trade-off is a lower daily light total. If you run 12/12, make up for it with careful intensity. A 2023 study in a peer-reviewed journal tested photoperiods from 12 to 15 hours on ten indoor cannabis cultivars. It suggested there is potential to raise yield in some cultivars with photoperiods longer than 12 hours, because of the higher daily light integral.

Keep the schedule fixed, change the intensity

Pick a schedule on day one and keep it to harvest. Do not switch to 12/12 to “trigger” flowering. It will not do that on an autoflower, and it only cuts the light the plant uses.

What should change is PPFD, the amount of light reaching the canopy. A common guide from Blimburn Seeds suggests roughly:

Seedlings: 200 to 400 µmol/m²/s
Veg: 400 to 600 µmol/m²/s
Flower: 600 to 800 µmol/m²/s

Measure at the canopy, not at the fixture. Ramp up gradually as the plant grows.

Higher intensity can raise yield when the plant can use it. In a 2022 Frontiers in Plant Science study, cannabis grown at 1,000 µmol/m²/s produced 1.6 times more inflorescence dry weight than plants at 600 µmol/m²/s. That study used photoperiod cannabis. Autoflowers often get a lower peak. Grow Light Meter reports that autoflower growers on 20 hours of light often aim for around 700 µmol/m²/s or less during peak bloom.

The reason is daily light integral (DLI), which is intensity multiplied by hours. Weed Seeds Express gives an example: 700 PPFD on 20/4 delivers about 50.4 mol per day. The same PPFD gives about 45.4 on 18/6 and about 30.2 on 12/12. Longer days stack more light at the same intensity, so autos can reach their DLI at a lower PPFD.

Daily light integral across three schedules

For more detail on targets by stage, see our PPFD guide for cannabis. For dimming and timer setup, read our LED dimming schedule guide.

A stage-by-stage plan on 18/6

Germination and seedling (weeks 1 to 2): Run 18/6 with the light dimmed low. Hang the fixture high. Aim for the low end of the seedling range.
Early veg (weeks 3 to 4): Raise intensity in steps. Watch for leaves pointing up or pale new growth.
Late veg and transition (weeks 5 to 6): Autoflowers begin to show flowers on their own timeline. Keep the schedule the same.
Flower (weeks 7 to 10): Bring PPFD to the flower range and hold it. Keep canopy temperature in check.
Late flower: Taper intensity slightly if the plant shows stress, and keep airflow strong.

Total grow time varies by strain, so follow the breeder’s estimate rather than a fixed calendar.

How far should the light be from the plant?

Distance changes how evenly light spreads. Dimming changes how strong it is. Use both gently, as Weed Seeds Express advises.

As a general starting point, ILGM suggests 24 to 36 inches from seedlings and 12 to 24 inches above the canopy of mature, flowering plants. It also notes that the right height depends on light type, wattage, space size and plant stage. Always check the manufacturer’s guidance for your fixture and confirm with a PPFD reading.

Can an autoflower get too much light?

Yes. Light burn happens when PPFD exceeds what the plant can use, which breaks down chlorophyll. Weed Seeds Express lists these signs:

Yellowing or whitening on upper leaves
Bleached buds
Yellowing between the veins
Crispy, papery tops
Stunted vertical growth

The first fix is to raise the fixture by 6 to 12 inches or reduce the dimmer by 10 to 20%. Light stress shows on the leaves and buds closest to the light, while lower leaves stay green. If the whole plant looks off, check for nutrient problems or overwatering before blaming the light.

Longer schedules raise the risk. If you run 20/4 or 24/0, you give the plant more light every day and less recovery time. Lower the intensity if you see early signs.

Choosing your schedule: a quick decision guide

Your situation	Suggested schedule
First autoflower grow	18/6
Warm room or tent	18/6
Cool room, good airflow, want faster growth	20/4
Cold garage or basement in winter	24/0 may help hold heat
Mixed grow with photoperiod plants	12/12, with attention to intensity
Watching the power bill	18/6

Lighting that fits the schedule

A fixture with dimming control makes a fixed schedule easier to manage, because you can raise PPFD by stage without touching the timer. Our Spider Farmer G8600 full spectrum LED grow light is listed with WiFi and Bluetooth app control. You can browse our full range of top lighting for tents and rooms of different sizes.

If you want light below the upper canopy, see our under canopy grow lights. Our proximity systems are designed to improve light distribution under the canopy and, in our experience, to raise yield by over 20% and improve bud grading.

Not sure which fixture suits your tent size? Start at LED Grow Lights Depot and use our selection tools and expert advice. Most US orders ship free.

Our perspective

We see the same pattern with many growers. They spend time debating 18/6 against 20/4, then run a single fixture at one fixed intensity from seedling to harvest.

The schedule is only half of the plan. Light distribution is the other half. Upper leaves get plenty of light, but the lower canopy often gets too little. That is the problem we built our proximity systems to solve. Under-canopy light reaches the bud sites that top lighting alone does not.

Our advice is simple. Start with 18/6. Keep it constant. Dim by stage. Add under-canopy light when your plants are ready for it. Our customers tell us this works, and we rate 4.8 out of 5 across more than 5,800 reviews.

Scott

Scott

FAQ

Is 18/6 or 20/4 better for autoflowers?

18/6 is the better default for most growers. It gives strong growth and a rest period at a lower power cost. 20/4 adds 14 hours of light per week and may speed growth slightly, but it adds heat and cost.

Do autoflowers need a dark period?

No, they do not need 24 hours of light, and they do well with dark hours. The dark period lets the plant rest and move nutrients. A 24-hour schedule can work, but it adds heat and power use without a guaranteed gain.

Should I change the light schedule when autoflowers start to flower?

No. Autoflowers flower on age, not on day length, so keep the same schedule to harvest. Change light intensity by stage instead, raising PPFD from seedling to flower.

How do I know if my autoflower is getting too much light?

Look for bleached or yellowed tops, leaves curling upward and crispy tips, with green leaves lower down. Raise the fixture by 6 to 12 inches or dim the light by 10 to 20%. Rule out nutrient problems and overwatering too.