For most indoor cannabis grows, 18 hours of light and 6 hours of dark is the schedule to start with. It gives photoperiod plants a long day that keeps them in vegetative growth, leaves a dark window for respiration and root activity, and trims roughly 25 percent off the electricity a 24/0 room would burn. We ranked the common vegetative schedules on four things: total light delivered per day (daily light integral, or DLI), power and heat load, stress on the plant, and how quickly the canopy fills the space before you flip to flowering.

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How These Schedules Were Judged

Hours of light are only half the story. DLI is the metric that ties it together: DLI (mol/m²/day) equals PPFD (µmol/m²/s) multiplied by photoperiod in hours, multiplied by 0.0036. A 400 µmol/m²/s fixture running 18 hours delivers about 25.9 mol/m²/day. The same fixture at 24/0 delivers about 34.6. That gap matters, because past a certain point extra photons stop converting into extra growth and start converting into heat, stretch, and nutrient demand you have to keep up with.

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The other criteria are practical. Electricity is billed per kilowatt hour, so every extra light hour is a real line item. Dark periods let the plant move sugars, take up water, and build root mass. And room temperature typically falls when lights go off, which some cultivars handle better than others.

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18/6: The Default Pick

Eighteen on, six off is what most photoperiod growers run from seedling through the flip. It sits in the middle of the range on DLI, keeps plants comfortably in veg, and pairs well with a dark period placed during the hottest part of the afternoon in summer or overnight in winter to cut cooling costs.

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Pros

  • Solid DLI at moderate PPFD, so you do not need to push fixture intensity to the limit.
  • Dark window supports respiration, root development, and calcium movement.
  • Lower power draw than 20/4 or 24/0, which matters over a 4 to 8 week veg.
  • Works for photoperiod and autoflower plants without adjustment.

Cons

  • Slower canopy fill than longer schedules in a commercial veg room where turnaround is the priority.
  • The 6 hour dark block can swing temperature and humidity in poorly sealed rooms.

Best for: home growers, first-time veg runs, and anyone paying retail electricity rates.

20/4: The Middle Ground

Twenty hours on, four off adds about 11 percent more DLI than 18/6 at the same intensity. It is a common choice for growers who want faster vegetative growth but still want the plant to get a genuine dark period.

Pros

  • More photons per day without raising PPFD, which reduces light stress risk.
  • Still leaves a dark window for nightly processes and equipment cooldown.
  • Fits autoflower plants that ignore photoperiod entirely.

Cons

  • Roughly 11 percent higher lighting cost than 18/6.
  • The short dark period is easy to disrupt with a light leak or a late-night room entry.
  • Little benefit if your PPFD is already near the top of the useful range.

Best for: growers running a tight veg cycle who want more growth without maxing out fixture intensity.

24/0: Maximum Light, Maximum Cost

Continuous light removes the dark period completely. It delivers the highest DLI of any schedule, which can shorten veg time. It also runs fixtures, fans, and dehumidifiers around the clock.

Pros

  • Fastest vegetative growth in most side-by-side setups.
  • Stable temperature and humidity, since there is no lights-off swing.
  • Simple to automate, no timer logic to get wrong.

Cons

  • Highest electricity and heat load, and 33 percent more light hours than 18/6.
  • Plants get no dark period for respiration and root activity.
  • Raises the odds of light burn, bleaching, and magnesium deficiency symptoms when PPFD is pushed high.
  • Autoflower plants sometimes stall or throw odd growth under continuous light.

Best for: commercial veg rooms with cheap power, strong climate control, and a hard deadline.

16/8: The Low-Power Option

Sixteen on, eight off is the floor for keeping photoperiod cannabis in vegetative growth. It is legal in the sense that most photoperiod plants will not flip at 16 hours, though some fast-flowering cultivars get twitchy below 16.

Pros

  • Lowest lighting cost of the four schedules.
  • Longer dark period supports root growth and recovery from training.
  • Useful in summer when heat is the limiting factor, not light.

Cons

  • Slowest canopy fill of the group.
  • Risky near the flowering threshold, since any timer error can trigger early flowering.
  • Requires higher PPFD to match the DLI of 18/6, which can stress plants.

Best for: mothers kept small, low-power home setups, and hot climates where heat is the bottleneck.

Matching the Schedule to Your Room

  1. Home grow, retail power, photoperiod seed: run 18/6.
  2. Autoflower seed, any setup: 18/6 or 20/4 both work; avoid 24/0.
  3. Commercial veg with climate control and cheap power: 20/4 is the practical ceiling, 24/0 only if you need the speed.
  4. Hot climate or undersized exhaust: 16/8 with dark hours placed during peak afternoon heat.

Whichever schedule you pick, change it gradually rather than jumping from 16 to 24 hours overnight, and step down to 12/12 for flowering only when the canopy is where you want it.