How Long Should Grow Lights Be On?
Most plants under grow lights do best with 12 to 18 hours of light and at least 6 hours of darkness. Seedlings and vegetative plants sit at the long end of that range. Flowering photoperiod cannabis switches to about 12 hours of light once you flip. That range covers houseplants, vegetables, herbs and cannabis, though the exact number of hours changes with what you are growing and why.
The hours get most of the attention, but they are only half of what a plant receives. Intensity decides the other half, and the two combine into a single number: the daily light integral. Get the hours right and the intensity wrong and the plant still underperforms. I've watched growers add hours to fix a weak light when the real problem was a fixture too dim for the canopy.
Grow Light Hours by Growth Stage
The table below gives the schedule most growers run for each stage or plant type. Where a university extension source publishes a specific photoperiod, it is cited. Where no source publishes one, the row says "grower convention" because that is what it is: a widely used practice, not a measured optimum.
The seedling and transplant rows come straight from University of Maryland Extension, which specifies 14 to 16 hours a day on an outlet timer for starting vegetable seeds indoors (UME). If you are raising a flat of seedlings, follow that window, and the seedling grow light range covers fixtures sized for that job. Hanging height matters as much as hours at that stage, because a light left too high produces the leggy stretch people blame on the timer. Our guide to seedling lighting walks through the distances.
Hours for Houseplants
Houseplants around 10 to 14 hours are matching indoor daylight more than they are feeding a crop. In summer a plant near a bright window already gets close to that. In winter, when natural light falls to a fraction of its summer strength, a supplemental fixture running 10 to 12 hours keeps a foliage plant from stalling. Push a low-light houseplant past 14 hours of stronger artificial light and you shorten its dark period.
The reason a houseplant does not want a cannabis light schedule is that it is not trying to yield anything. You are keeping it healthy, not pushing mass. Aim for the middle of the range and watch the plant, not the clock.
Vegetables and Herbs
Leafy greens, herbs and fruiting vegetables want a longer day than most houseplants but less than a flowering cannabis plant. Lettuce, basil and cilantro run happily on 14 to 18 hours and grow faster in the longer part of that window, provided the light is bright enough to matter. Push them too long and faster-bolting herbs like cilantro and dill can go to seed early. As transplants, tomatoes and peppers get 14 to 16 hours of light.
The whole seedling-to-transplant window runs on the UMD figure of 14 to 16 hours, and the same timer that runs the seedling shelf can run the vegetable tent. Full spectrum matters here too. A bar built for flowering cannabis works fine for basil, and a full spectrum LED grow light gives greens a balanced spectrum rather than the red-heavy output some cheap fixtures push. If you are still sorting spectrum from hours, what full spectrum actually means covers it.
Cannabis Light Schedules: Veg
Cannabis in the vegetative stage is conventionally run at 18 hours of light and 6 of dark. The reason is biological. Cannabis is a short-day plant that starts to flower when day and night lengths are approximately equal (USU Extension). At 18/6, the dark period stays at 6 hours, well short of an equal day and night, so the plant keeps building leaves and stems instead of switching to flower. A 16/8 or 20/4 schedule is also common in the veg tent. The dark period just needs to stay short enough to keep the plant in veg.
More hours is not automatically better in veg. A plant given 18 hours of weak light gets less usable energy than one given 14 hours of strong light, because daily photons are the product of intensity and time. Where a plant is receiving the right intensity, 18/6 in veg is a sensible, proven schedule. Where it is not, adding hours is a slow way to fix a light that is too dim.
Flower and Autoflowers
For photoperiod cannabis the standard flowering schedule is 12 hours of light and 12 of dark, and that is the right place to start. It is not the only place to finish, though. Work published in Plants tested flowering cannabis lines at longer photoperiods and concluded the assumption that 12/12 is optimal for every line is incorrect because some lines can yield considerably more with a longer light period during flowering (Peterswald et al., 2023). Research at Utah State University across diverse cannabis cultivars found that longer photoperiods favored vegetative growth over flower yield, and yield did not increase even when the total light sum went up (Eckels, 2026). The catch is that every line responds differently, and longer photoperiods favor vegetative growth over flower yield. My read is that 12/12 stays the default for a first run, and extending to 13 or 14 hours is worth trying only once you know how a specific cultivar flowers.
Autoflowers are the simpler case. They flower on a fixed clock rather than on dark-period length, so you can give them 18 to 20 hours of light throughout and let them take all the daily photons they can use. A 20/4 autoflower run is grower convention, not a published optimum, but it is a common choice because more light hours at the same intensity means more growth.
When you want more light, the better move is usually a brighter fixture rather than more hours, and the higher-output LED grow lights are where that gain lives. A high-output fixture like the TotalGrow Solark KC or the AC Infinity Ionframe EVO10 delivers enough photons that 12 hours can do the work 20 hours of a weak light cannot. Once you are squeezing a large canopy, the 1000W LED grow lights give you the punch that makes shorter schedules viable, and the related 500W LED grow lights cover a mid-size tent.
Why the Dark Period Matters
Plants use light as a resource for photosynthesis and as information about the time of day. For cannabis specifically, human-caused light during the dark period disrupts the plant's timekeeping and reduces the strength of flowering, and in some cases can prevent flowering entirely (Eckels, 2026). That is why a light leak into a flower tent is a real problem and not a cosmetic one. A zipper left open a few inches, a glowing power strip inside the tent, or a green LED on a dehumidifier can each put light into the dark period, and the intensity, color, timing and total photons of that light all affect flowering.
A night break is the same mechanism used deliberately. Interrupting the dark period with even a short burst of light can change flowering response, which is why greenhouse growers use night-break lighting to hold short-day crops in vegetative growth. In a home flower tent you want the opposite, so the practical rule is simple: the dark period should be genuinely dark. That means no inspection visits with a phone flashlight mid-flower, and any light-emitting device inside the tent either switched off or taped over.
Hours Are Only Half the Dose: Daily Light Integral
Daily light integral, or DLI, is the total photosynthetically active light a plant receives across a day, combining intensity and duration into one number. Purdue Extension defines it as the amount of PAR received each day as a function of light intensity and duration (Purdue HO-238). That definition is why hours alone never tell the full story. A dim light run for 20 hours can deliver less usable energy than a bright light run for 12.
You can estimate DLI yourself. Multiply PPFD (in micromoles per square meter per second) by the number of light hours, then by 0.0036, and you get moles per square meter per day. A canopy receiving PPFD of 600 for 18 hours comes out to 600 x 18 x 0.0036 = about 38.9 mol/m²/day. Run the same 600 PPFD for 12 hours and the DLI drops to about 25.9 mol/m²/day. Same light, same plant, 33 percent less daily energy. That difference is why flowering schedules that shorten the light period often call for a brighter fixture to keep DLI up. If the PPFD concept is fuzzy, what PPFD actually measures covers the inputs you need for the formula.
Is 24 Hours of Light Ever OK?
Continuous light is a grower caution, not a universal rule. For most plants a genuine dark period is part of healthy growth, and cannabis in particular needs an uninterrupted dark stretch long enough to flower. Running a flowering photoperiod plant under 24 hours of light will stall or prevent flowering rather than speed it up. Where 24 hours can be defensible is a narrow set of vegetative cases, such as rooting cuttings under a constant low-intensity light or keeping a mother plant in perpetual veg, and even then the plant benefits from some darkness.
The bigger risk with 24/0 is heat and electricity. A light left on constantly pushes tent temperatures up around the clock, and in a sealed room that can force a dehumidifier or air conditioner to work harder. A dehumidifier for grow tents and grow rooms running against a light that never shuts off is a losing battle on humidity. I'd keep a dark period on almost everything, and give the odd exception a real reason.
Setting a Timer or Light Controller
Hours only stay consistent if something switches the light for you. A dual-outlet digital timer is the simplest tool for the job. The Autopilot TM01715D runs on 120V, is rated 15A/1725W, controls two outlets simultaneously, and holds up to 8 on/off cycles per day with 1-second interval resolution (Hydrofarm spec). Both outlets switch on the same schedule, so two lights plugged into it share one program. The Grow1 dual-outlet 120V digital timer carries a 15 amp max load, 8 programmable options, and 1-minute on/off increments, and can be bypassed to run 24 hours when you need it.
Check the light against the clock now and then so a failed timer does not go unnoticed. For rooms with several lights on different schedules, a grow light controller coordinates them from one point instead of a wall of plug-in timers. If your lights also run through an environmental system, the timers and controllers pair to keep light hours and climate aligned.
Frequently Asked Questions
- Is 12 hours of grow light too much?
- No. Twelve hours is fine for most plants and is the standard flowering schedule for photoperiod cannabis. Houseplants can run up to 14 hours, and vegetables and herbs up to 18. Twelve hours is only too short for a plant that wants more daily light, such as an autoflower or a fruiting vegetable crop.
- Is it bad to leave a grow light on all the time?
- Yes for most plants. Cannabis needs an uninterrupted dark period to flower, and continuous light stalls or prevents it. A light with no off period also raises tent temperatures and humidity load around the clock. A consistent daily dark period is the safer default.
- Should I run my grow lights at night or day?
- Either works as long as the schedule stays consistent. Running lights at night is common in warm rooms, because it shifts heat into cooler hours and keeps the plant cooler in the day. The critical point is that the dark period stays dark, and that the light and dark windows stay the same length every day.
- How long to leave grow lights on for seedlings?
- Fourteen to 16 hours a day, matching University of Maryland Extension's guidance for starting seeds indoors. Keep the fixture close enough that the seedlings stay stocky rather than stretching. Run the whole cycle on an outlet timer so the hours are identical every day.
- What light schedule do autoflowers need?
- Eighteen to 20 hours of light with 4 to 6 hours of dark is the common schedule. Autoflowers flower on age rather than photoperiod, so more light hours at the same intensity means more daily energy. The exact split is a grower convention, not a published optimum.
- Can I run my grow lights less in winter?
- Yes, for some plants. A houseplant can drop to 10 to 12 hours. Cannabis photoperiod plants should keep their 12/12 or 18/6 schedule year-round; only the low-intensity plant types tolerate a seasonal reduction.
- How many hours of light do plants need?
- Most plants need 12 to 18 hours of light and at least 6 hours of dark. Seedlings and vegetative plants sit at the top of that range, flowering cannabis at around 12, and houseplants at the bottom. The exact number depends on the plant type and how bright the fixture is.
The Bottom Line
Pick a schedule by what you are growing: 14 to 16 hours for seedlings and transplants, 18 hours for vegetative cannabis, 12 for the flower flip, 18 to 20 for autoflowers, and 10 to 14 for houseplants. Then check whether the light is actually strong enough for the hours you are running. Hours are the easy half. If you want the full picture on choosing a fixture before you set a schedule, our guide to the best grow lights covers output, coverage and efficiency together. To see how hours, photon measure and distance work in a specific room, photoperiod and plant light cycles explained and how far to hang your LED grow lights go deeper. Once the schedule is set, timers and grow light controllers keep it locked in.
