Photoperiod cannabis runs 18 hours on during veg and 12 hours on during flower. Seedlings and clones do best at 18-20 hours. Autoflowers flower regardless of light schedule and typically run 18-20 hours throughout the entire grow. The dark period is not optional: plants process energy, complete growth cycles, and regulate hormones during lights-off. Skipping it costs you yield and plant health.
Grow Light Schedule by Stage
| Growth Stage | Lights On | Lights Off | Notes |
|---|---|---|---|
| Seedling / Clone | 18-20 hours | 4-6 hours | Keep intensity low (200-400 µmol/m²/s); humidity dome helps. See best grow lights for seedlings. |
| Vegetative | 18 hours | 6 hours | Most growers use 18/6; 20/4 also works but offers diminishing returns past 18 hours. |
| Flowering (photoperiod) | 12 hours | 12 hours | The 12-hour dark period triggers and maintains flowering. Any light leak disrupts this. |
| Late Flower / Flush | 12 hours | 12 hours | Some growers run 11/13 in final weeks to accelerate ripening; evidence is mixed. |
| Autoflower | 18-20 hours | 4-6 hours | Autoflowers flower independently of photoperiod; 18-20 hours throughout is most common. |
| Pre-Harvest Dark Period | 0 hours | 24-48 hours | Optional. Some growers run a full dark period before harvest to stress terpene production. Evidence is anecdotal. |
Why the Dark Period Matters
During the dark period, plants are doing real work. Sugars produced during photosynthesis are transported from leaves to roots and developing flower sites. Cellular processes that require darkness, including certain stages of the photoperiod hormonal cycle, complete during lights-off. For photoperiod strains specifically, the plant detects the uninterrupted 12-hour dark stretch to initiate and sustain flowering. Interrupt that dark period, even briefly, and you risk reverting the plant to veg or causing irregular development.
Running lights 24 hours a day stunts root development because roots require darkness to extend and branch properly. In photoperiod strains, 24-hour light during flower will delay or reverse bud production. Even in veg or for autoflowers where 20/4 is common, the 4-6 hour dark window gives the plant time to recover from the metabolic load of photosynthesis. The dark period is not wasted time. It is part of the growth cycle.
For a deeper look at how spectrum affects each stage, see full spectrum grow lights explained.
How to Avoid Light Leaks During Flower
Any light entering the grow space during the 12-hour dark period can disrupt hormonal signaling and cause foxtailing, irregular bud structure, or in severe cases, hermaphroditism. The sources are often subtle: a phone screen, a controller LED inside the tent, a gap around ducting, or a loose zipper seam.
Practical checks before your first flower flip:
- Stand inside your grow space with the lights off and wait 5 minutes for your eyes to adjust. Any visible light is a problem.
- Cover or relocate indicator LEDs on timers and controllers that sit inside the tent.
- Tape ducting penetrations with light-proof material.
- Run your finger along tent zippers and check seams at corners.
A reliable timer or smart controller ensures the lights-off period starts and ends consistently every day without human error. Manual light management almost always introduces variation that compounds over a 60-day flower cycle.
Using a Timer for Consistent Schedules
Consistent light schedules depend on consistent timers. A basic mechanical outlet timer works fine for single-light setups and costs under $15. For larger tents with multiple devices, a smart grow controller lets you set schedules, monitor conditions, and receive alerts if a circuit fails. Whatever you use, test the cycle for a full 24 hours before introducing plants. A timer that drifts by 15 minutes per week will shift your 12/12 schedule by more than an hour per month, which is enough to affect photoperiod stability in sensitive strains.
Frequently Asked Questions
- How many hours of light do cannabis plants need per day?
- Photoperiod strains need 18 hours during veg and 12 hours during flower. Autoflowers run best at 18-20 hours throughout the entire grow. Seedlings and clones do well at 18-20 hours with reduced intensity while roots establish.
- Can grow lights be on 24 hours a day?
- Not recommended. Plants need a dark period for sugar transport, root development, and hormonal signaling. 18/6 or 20/4 are the practical maximum for veg stages. 24-hour continuous light can stress plants, inhibit root growth, and offers minimal benefit over 18/6 in practice.
- What happens if grow lights are on too long?
- In photoperiod strains during flower, any extension of the light period beyond 12 hours can delay or reverse flowering. For seedlings, high intensity combined with extended hours can cause light bleaching and slow root development. For autoflowers, extended light past 20 hours tends to have no benefit and can cause heat stress depending on your setup.
- Can I switch from 18/6 to 12/12 gradually?
- You can, but it is not necessary. Most growers flip to 12/12 directly and plants respond within 1-2 weeks. Gradual transitions, such as dropping one hour per week, are sometimes used for stress-sensitive strains, but this is not a widely practiced technique and the evidence for a benefit over a direct flip is limited.
For options at the seedling stage, browse seedling grow lights to find fixtures sized for propagation trays and early veg.
