Cannabis plants need different amounts of light depending on their growth stage. The two key schedules are 18 hours of light and 6 hours of dark (18/6) during vegetative growth, then 12 hours of light and 12 hours of dark (12/12) to trigger flowering. Getting these cycles right is the single most important factor in controlling when your plant grows leaves versus when it produces buds.
Why Light Cycles Matter for Cannabis
Cannabis is a photoperiod-sensitive plant, meaning it uses the length of night to determine what season it is. Long days with short nights signal summer, telling the plant to focus on growing stems and leaves. When nights get longer, the plant interprets this as autumn approaching and shifts its energy into producing flowers. Indoor growers exploit this biology by controlling exactly how many hours of light and darkness the plant receives each day.
The plant tracks darkness through a light-sensitive protein called phytochrome. During uninterrupted dark periods, phytochrome accumulates and eventually triggers the hormonal cascade that starts flowering. This is why the dark period matters just as much as the light period, and why even tiny disruptions to darkness can cause problems during bloom.
Seedling Stage: 18 to 24 Hours of Light
For the first few days after seeds sprout, 16 to 18 hours of light is enough. Once the first true leaves develop (the serrated ones, not the round seed leaves), you can increase to 18 to 24 hours per day. Seedlings need continuous bright light to prevent them from stretching tall and thin as they search for a light source. Keep intensity moderate at this stage, roughly a third to half the brightness you’d use for mature plants. If you’re using an LED panel, raising it higher above the canopy reduces intensity without changing the schedule.
Vegetative Stage: The 18/6 Schedule
The standard vegetative light cycle is 18 hours on, 6 hours off. This simulates long summer days and keeps the plant focused on building size, branches, and leaf mass. Cannabis will stay in the vegetative stage indefinitely as long as it receives more than about 13 hours of light per day, though most growers stick with 18 hours for strong, compact growth.
Some growers run lights 24 hours a day with no dark period at all (24/0). This does push slightly faster growth, but the gains are modest and your electricity bill goes up by a third compared to 18/6. A 300-watt LED running 18 hours a day uses roughly 162 kWh per month. Bump that to 24 hours and you’re at 216 kWh. Whether that extra cost is worth a small speed boost depends on your priorities.
The vegetative stage lasts as long as you want it to. Most indoor growers veg for 4 to 8 weeks, using this time to train plants to the desired shape and size before flipping to flower.
Flowering Stage: The 12/12 Switch
To trigger flowering, you abruptly switch the light schedule to 12 hours on and 12 hours off. This equal split of light and dark mimics the shortening days of late summer and early fall. Most cannabis genetics begin showing flowers within one to two weeks of the switch, and the flowering period typically runs 8 to 10 weeks depending on the strain.
The 12/12 schedule is not flexible the way vegetative lighting is. The plant needs a full, uninterrupted 12 hours of darkness every single night to maintain flowering. Even small amounts of stray light during the dark period can cause serious problems.
Why Dark Period Interruptions Are Dangerous
Cannabis plants are highly sensitive to light interruptions during their dark cycle in flower. When light breaks the dark period, the phytochrome balance resets, and the plant gets conflicting signals about what season it is. The consequences range from annoying to crop-ruining:
- Delayed or reversed flowering. Even brief light exposure can slow bud development or cause the plant to revert back to vegetative growth, producing single-bladed leaves from the middle of forming buds.
- Hermaphroditism. Stressed plants may develop both male and female reproductive organs, pollinating themselves and filling buds with seeds instead of producing the seedless flowers growers want.
The sources of these leaks are often subtle: an indicator LED on a power strip, light creeping through a tent zipper, or a door opened briefly during the dark period. If you’re growing in a tent or room, check for light leaks by sitting inside with the lights off and the door closed. If you can see any light after your eyes adjust, your plants can detect it too.
Autoflowering Strains: A Different System
Autoflowering cannabis varieties don’t rely on light cycles to start flowering. These strains carry genetics from Cannabis ruderalis, a subspecies that evolved in northern climates with very long summer days. Instead of waiting for shorter nights, autoflowers begin flowering based on age, typically around 3 to 4 weeks after sprouting.
Because they don’t need a dark period trigger, most growers run autoflowers on 18/6 or 20/4 from seed to harvest. Some use 24/0, though giving plants at least a few hours of darkness each day saves energy without meaningfully slowing growth. The entire life cycle of an autoflower, from seed to harvest, usually takes 8 to 12 weeks regardless of the light schedule.
Light Intensity by Growth Stage
The number of hours your lights run is only half the equation. How bright those lights are matters just as much. Plants that get the right number of hours but at the wrong intensity will underperform. Light intensity for growing is measured in PPFD (the amount of usable light energy hitting your canopy per second), and each stage has a target range.
During the seedling stage, aim for 100 to 300 PPFD. This is gentle enough to avoid burning delicate young leaves. In the vegetative stage, increase to 400 to 600 PPFD to fuel rapid leaf and branch growth. During flowering, plants can handle and benefit from 600 to 900 PPFD, with commercial operations pushing up to 1,000 or higher. Going above 800 to 900 PPFD only helps if you’re supplementing with extra CO2 in a sealed growing space; without it, the plant can’t use the additional light efficiently.
If your grow light doesn’t have a PPFD reading, the general rule is to start with the light farther from the canopy and lower it gradually over days, watching for signs of stress like curling or bleaching leaf tips.
Estimating Your Electricity Costs
Light cycles directly affect your power bill since grow lights are the biggest energy draw in an indoor setup. The math is straightforward: multiply your light’s wattage by the hours it runs per day, then by 30 days, and divide by 1,000 to get kilowatt-hours per month.
A 200-watt LED running 18 hours a day during veg uses about 108 kWh per month. Switch that same light to 12/12 for flowering and consumption drops to 72 kWh. At the U.S. average electricity rate of roughly 16 cents per kWh, that’s about $17 per month in veg and $11.50 in flower for a single light. Smaller setups with a 50-watt panel running 12 hours a day cost only about 18 kWh per month, or $2 to $5 depending on your local rate.
Quick Reference by Stage
- Seedlings (week 1-2): 18/6 light cycle, low to moderate intensity
- Vegetative (weeks 3-8+): 18/6 or 20/4, moderate to high intensity
- Flowering (weeks 8-18+): 12/12, high intensity, zero light leaks during dark period
- Autoflowers (entire life): 18/6 or 20/4 from seed to harvest, no schedule change needed