Cannabis Light Schedule: The Complete Guide to 18/6, 12/12, and Autos

Light is the single biggest lever you control indoors. Not nutrients, not fancy additives, not the price of your tent. The number of hours your plants spend in light versus dark decides whether they stay in growth mode or start stacking buds. Get the schedule right and everything downstream gets easier. Get it wrong and you’ll chase problems you can’t diagnose.

I’ve been in the seed business 16 years, and light schedule confusion is one of the most common things new growers write me about. So here’s the whole thing, plain and honest, with the mistakes that actually cost people harvests.

The Quick Answer

If you just want the numbers:

• Photoperiod plants, vegetative stage: 18 hours light / 6 hours dark (18/6).
• Photoperiod plants, flowering stage: 12 hours light / 12 hours dark (12/12).
• Autoflowers, whole life: 18/6 or 20/4, start to finish, no change needed.

Everything below is the why, plus the details that separate a clean grow from a stressed, hermie-prone mess.

Why Photoperiod Plants Flower on a Timer

Cannabis is what botanists call a short-day plant. In nature, it grows all through the long days of summer and starts flowering as the nights get longer heading into fall. The plant is literally reading the season through the length of the dark period.

Here’s the part most people get backwards: it’s the darkness that triggers flowering, not the light. The plant measures how long the uninterrupted night is. When nights get long enough, a pigment system in the leaves flips a hormonal switch and the plant shifts from making leaves to making flowers. Indoors, you fake this transition by dropping from long days (18/6) to equal day and night (12/12). Twelve hours of solid, unbroken darkness is the signal.

This is why the dark period is sacred during flower, and why light leaks are such a big deal. More on that below.

Vegetative Stage: 18/6

While your photoperiod plants are in veg, you want them growing hard. Long days keep them in vegetative mode, building the stems, branches, and root system that will support your buds later.

The standard is 18/6. It gives plants plenty of light energy to grow fast while still handing them a 6-hour dark period. Could you run 24/0? Some people do, and plants will grow under constant light. But I don’t recommend it and never have. That dark period is when plants do a chunk of their metabolic housekeeping, and running lights around the clock just burns extra electricity for little to no gain. The extra 6 hours of darkness costs you almost nothing in growth and gives the plant a rest. Save the money, run 18/6.

How long you veg is up to you. It’s driven by the space you have and the size you want, not the calendar. A common rule of thumb: plants roughly double in size after you flip to flower, so switch when they’re about half your target final height. In a typical tent, that’s often 4 to 8 weeks of veg, but a small tent might mean flipping earlier to keep plants from swallowing the space.

Flowering Stage: 12/12

When you’re ready to flower, you drop your photoperiod plants to 12/12. Twelve hours of light, twelve hours of unbroken dark. That long night is the trigger. Within a week or two you’ll see the plant stop stretching upward as aggressively and start forming flower sites. Full flowering typically runs 8 to 10 weeks depending on genetics, sometimes longer for sativa-leaning strains.

Two rules during flower:

1. Keep the light hours consistent. Same on-time and off-time every day. A timer is non-negotiable. Don’t eyeball it.
2. Keep the dark period truly dark. This is where grows go sideways.

Light Leaks: The Silent Grow Killer

During flower, “dark” means dark. If light sneaks into your tent during the 12-hour night, you can confuse the plant’s internal clock. The consequences range from delayed or stalled flowering to the worst outcome: stress-induced hermaphroditism, where your plant grows male pollen sacs and seeds your entire crop. One good light leak can turn beautiful flower into a bag of seeds.

Common culprits I see:

• The little green and red LED status lights on fans, timers, and power strips inside the tent. Cover them with electrical tape.
• Tent zippers that don’t fully close, or seams and passthrough ports that glow.
• A phone screen or room light flicking on during the dark cycle when you peek in.
• Streetlights or hallway light bleeding through a closet or window.

The test is simple: turn everything off during the dark period, get inside or right up against the tent, let your eyes adjust for a minute, and look for any glow. If you can see light, the plant can too. Seal it. Photoperiod plants are unforgiving about this, so treat the dark period like it matters, because it does.

Autoflowers: Skip the Whole Song and Dance

Autoflowering genetics changed the game for a lot of growers, and light schedule is where they shine. Autos flower based on age, not light. They carry ruderalis heritage, so they start flowering after a few weeks regardless of the light cycle. That means no flip, no counting hours to a transition, no light-leak panic during a strict 12-hour night.

You run autos on the same schedule their entire life. The two common choices:

• 18/6: The efficient pick. Plenty of light to grow and finish, with a 6-hour rest and lower power bills.
• 20/4: A bit more light for slightly faster growth, if your electricity is cheap and your tent stays cool.

Both work. I lean 18/6 for most people because the energy savings are real and the yield difference is marginal. Autos typically go seed to harvest in about 10 to 12 weeks. They’re the easy button for beginners, small spaces, and anyone who doesn’t want to babysit a photoperiod transition. Since they don’t respond to the dark period the way photoperiods do, a minor light leak won’t hermie an auto the way it can wreck a 12/12 plant, though I still keep things tidy out of habit.

Seedlings and the Transition

Fresh seedlings and clones are delicate. Give them gentle light on an 18/6 or 20/4 schedule and keep the intensity modest, dimming an LED or raising it higher until they establish. Blasting a seedling with full flower-stage intensity just bleaches and stresses it.

When you make the veg-to-flower flip on photoperiods, do it in one clean move: end a day, then start your 12/12 the next cycle. Don’t ramp gradually, and don’t flip-flop back and forth. Waffling on the schedule sends mixed signals and can stress plants into problems. Decide, flip, and hold it steady to the finish.

Light Schedule Mistakes to Avoid

A schedule is only as good as your discipline running it. The errors that cost people harvests:

Running lights by hand instead of a timer. If you’re flipping the switch manually, you will eventually forget, oversleep, or leave for the weekend and blow your dark period. A cheap outlet timer removes human error entirely. Buy one before you buy anything else fancy.

Treating the dark period as optional. In flower, a photoperiod plant’s 12 hours of darkness is a signal, not a suggestion. Peeking in with the room light on, or a phone flashlight, or leaving a tent zipper cracked, all count as interruptions. Do your tent checks during the light period.

Confusing intensity with duration. Light schedule (hours) and light intensity (how bright, how close) are two different knobs. A seedling on the correct 18/6 schedule can still get bleached if you park a powerful LED six inches over it. Match your intensity to the stage, then keep the hours consistent. Getting the schedule right doesn’t excuse cooking your plants with a light hung too low.

Flip-flopping the schedule. Changing veg times, bouncing between 18/6 and 20/4, or reverting a flowering plant back to long days (“revegging” by accident) stresses plants and can trigger deformed growth or intersex traits. Pick a schedule for the stage and hold it steady.

Assuming autos need a flip. New growers sometimes switch their autoflowers to 12/12 partway through out of habit. Don’t. Autos flower on age, and cutting their light hours in flower just costs you yield for no reason. Leave them on 18/6 or 20/4 the whole way.

FAQ

Can I grow cannabis on 24 hours of light?
You can, but you shouldn’t. Plants tolerate constant light in veg, but the dark period costs you almost no growth and gives them a rest while saving electricity. Run 18/6 in veg. And never run 24/0 in flower for photoperiods, they need the dark to bud.

What light schedule do autoflowers need?
The same one their whole life: 18/6 or 20/4. Autos flower by age, not light cycle, so you never switch. No flip, no strict dark period required to trigger flowering.

Will a light leak really ruin my grow?
It can. During a photoperiod plant’s flowering dark cycle, stray light can delay flowering or trigger hermaphroditism, which seeds your buds. Seal your tent, cover LED indicators, and verify true darkness. Autos are far more forgiving.

When should I switch from 18/6 to 12/12?
When your plants are about half their target final height, since they roughly double after the flip. That’s often 4 to 8 weeks of veg, but it depends on your space and the size you want, not a fixed date.

Start With Genetics That Match Your Setup

Your light schedule only matters if you’ve got seeds worth running under it. Photoperiod strains give you full control over veg time and size; autoflowers hand you a simpler, forgiving grow. At Seeds Here Now we’ve spent 16 years curating both from breeders we stand behind. Browse feminized and autoflower seeds and pick the genetics that fit how you want to grow.