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Lighting · 8 minute read · Updated October 20, 2026

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

Growers argue about hours. Plants count something else: the total light that lands on the leaves in a day. Once you see that number, the 18/6 versus 20/4 question gets much simpler.

What do 18/6, 20/4 and 24/0 mean?

The first number is hours of light, the second is hours of dark, in a 24 hour day. 18/6 means 18 hours on and 6 off.

For photoperiod cannabis, the veg schedule has one job: keep the nights short enough that the plant does not flower. Researchers keep plants vegetative under long days, commonly 18 hours, then switch to 12 hours of light to start flowering. A 2022 hemp paper notes that under 18 or 24 hours of light, plants stay vegetative. So all three veg schedules hold a photoperiod plant in veg. The real question is what the extra hours buy you, and what they cost.

Daily light integral: the number that actually matters

Purdue Extension describes daily light integral (DLI) as the total photosynthetic light a crop receives in a day, measured in moles of light per square meter per day (mol/m²/d). Their comparison: DLI is like a rain gauge. It adds up the whole day, not one moment.

Two things make DLI: how bright the light is at the canopy (PPFD, in µmol per square meter per second) and how many hours it is on. The math is simple:

DLI = PPFD x hours x 0.0036

That 0.0036 is just unit conversion: 3,600 seconds in an hour, divided by a million to turn micromoles into moles. Purdue's 24 hour version of the same factor is 0.0864.

Here is a canopy at 400 µmol under each schedule:

For a sense of scale, the University of Guelph light study gave its cuttings about 425 µmol for 18 hours in veg, which works out to a DLI of about 27.5. Do you know the PPFD at your canopy, or only the wattage on the box? Your light maker's PPFD chart at your hanging height is the place to start.

More hours or more intensity?

You can raise DLI two ways: leave the light on longer, or make it brighter at the canopy by dimming up or lowering it within the maker's distance. Both add light. Only one of them costs the plant its dark period.

Brighter light does pay. In the Guelph study, dry flower yield rose in a straight line as canopy light went from 120 to 1,800 µmol, with no leveling off. That was in flower, on one cultivar, but it shows how strongly cannabis responds to intensity. Light level did not change cannabinoid concentration in that study.

So before you move to 20/4, ask: is your light already at the maker's veg setting, at the right height? If it is dimmed down or hung high, turning it up may give the same extra light without the extra hours. Our guide to LED light height by stage has the starting points.

Is 24/0 bad for cannabis?

Continuous light keeps a photoperiod plant in veg, and many growers run it without obvious damage. The research on cannabis specifically is thin, and what exists points toward caution.

A 2022 study on two hemp cultivars compared 16, 20 and 24 hours of light. Continuous light raised a marker of leaf membrane damage in both cultivars, and the authors found 16/8 the most favorable of the schedules they tested. It measured leaf stress, not growth or yield, so it does not prove that 24/0 grows a smaller plant. It does say the dark period is not wasted time for the leaf.

We found no peer-reviewed study that directly compares the growth of cannabis on 18/6, 20/4 and 24/0 in veg. Anyone who quotes you a yield gain for 24/0 is quoting a belief, not a trial.

What do extra hours cost in electricity and heat?

Electricity use is watts times hours. A light that draws 300 watts at the wall uses:

Multiply by the rate on your power bill to get your own cost, or use our grow tent electricity cost guide, which adds fans and other gear. Your light's real draw is on the maker's spec sheet or a plug-in meter.

Heat follows the same line. Every lit hour adds heat to the tent, and the dark hours are when it cools. On 24/0 there is no cool period at all, which matters in a hot room in summer. If your tent already runs warm, see grow tent too hot before adding hours. Never plug a bigger load into an outlet or power strip than it is rated for.

What light schedule is best for autoflowers?

Autoflowers do not wait for long nights to flower. A 2022 genetics study describes them as photoperiod insensitive, or day neutral, and found that most high-cannabinoid cultivars did not flower under continuous light, except plants carrying two copies of the trait the authors named Autoflower1.

Because an auto flowers on its own clock, growers commonly keep it on one schedule from seed to harvest, usually 18/6 or 20/4, and some use 24/0. We found no controlled trial comparing those schedules for autoflowers, so pick the one your heat and power bill can live with and keep it steady. More in autoflower vs photoperiod.

Can you add hours in flower too?

Not with photoperiod plants. Flowering needs long, unbroken nights. A 2026 Utah State dissertation found that longer photoperiods favored vegetative growth over flower yield, and that yield did not rise even when the longer days gave more total light. It also found that light during the dark period disrupts the plant's timekeeping and weakens flowering. Researchers are testing schedules a little past 12 hours, with mixed results by strain, but for a home grow 12/12 is the safe choice. See when to switch to 12/12 and light leaks and light schedules.

Which schedule should you pick?

Whatever you choose, write it down with your light height and dimmer setting, and note how fast the plants put on new nodes. Two grows of notes in a journal tell you more about your tent than any forum thread.

Questions growers ask

Read next

• How much does a grow tent cost in electricity?
• How high should an LED grow light be at each stage?
• Autoflower vs photoperiod: which should you grow?
• How many watts of LED does a grow tent need?

Education only, for adults 21+ where home cultivation is legal. Updated October 20, 2026.