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Your plant doesn't count the hours of light. It counts the dark. Here is the proof: give cannabis 12 hours of light, the classic flowering schedule, then switch a lamp on for just one hour in the middle of the night. In a 2025 study, plants grown like this stayed in veg for weeks. Same hours of light. No flowers.
Source · Grover et al., Plants (2025)
The short answer ▶ 0:27
| One long night | Broken night | |
|---|---|---|
| The plant reads | a long night | two short nights |
| So it | starts to flower | keeps growing leaves |
A plant flowers when its nights get long enough, and it times the longest unbroken stretch of dark. Switch a light on in the middle, and one long night reads as two short ones. Short nights mean: keep growing leaves.
How plants count the dark ▶ 0:43
Inside the leaves is a light sensor called phytochrome. Daylight, especially red light, switches it on, and while it is on, the flowering signal stays blocked. Through the night, the plant's internal clock keeps time. Only after a long enough stretch of unbroken dark does the flowering signal get through. A light in the middle switches the sensor back on, and the night is broken.
It doesn't even have to be light we can see well. In a 2021 study, strong near-infrared lamps, almost invisible to our eyes, shone on cannabis through the night and delayed flowering by 12 days.
Source · Kusuma et al., PLoS ONE (2021)
How dim is too dim? ▶ 1:22
light intensity, µmol per m² per second
Not much light is needed to break the night. A grow light in flower typically gives around 900 µmol. In a 2021 hemp study, the dim light of dusk, about 2 µmol, still counted towards the day. And in a 2023 study, lamps giving just 1 µmol, stretching the day, stopped plants from flowering properly.
Source · Zhang et al. (2021); Park et al. (2023)
So think about a gap in a tent zip, a street lamp at the window, a glowing power light. Many growers blame leaks like these for herms. A 2026 review points to one small 2024 student study that found no link between dim light leaks and male flowers, so the herm claim isn't proven. What is proven is that light in the night can delay or stall your flowers.
Source · Richards et al., Plants (2026)
Is 12/12 the magic number? ▶ 2:07
Not exactly. In a 2023 study, 10 cultivars all started to flower with up to 14 hours of light, some up to about 4 days later. At 15 hours, some started, then stalled.
Source · Ahrens et al., Plants (2023)
The edge can be sharp, too. In hemp, just 15 minutes more daylight delayed flowering by up to 13 days in some cultivars.
Source · Zhang et al., Front. Plant Sci. (2021)
So why not run longer days for more light? Research at Utah State found longer days pushed plants towards leaves rather than buds, and yield didn't rise. 12/12 isn't magic. It is a safe margin.
Source · Eckels, Utah State University (2026)
What to do in your grow ▶ 2:45
- 01Seal leaks
tape gaps, cover LEDs
- 02Same timer
every single day
- 03Night checks
rare and brief
- 04Stick to 12/12
the safe margin
Seal the leaks: tape the gaps and cover glowing lights. Keep the same timer every single day. Keep night checks rare and brief. And stick to 12/12, because it is the safe margin.
Plants time the dark, not the light, and even dim light can break the count. Autoflowers don't count the dark at all, which raises a different question: the best light schedule for autoflowers.
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Sources
- Grover, E.F., Haiden, S.R. & Berkowitz, G.A. (2025). "Examining the 'Night Break' Method in Cannabis sativa Horticulture: Vegetative Daily Light Integral Affects Yield of Extractable Biomass in C. sativa." Plants 14(19): 3095. doi:10.3390/plants14193095
- Kusuma, P., Westmoreland, F.M., Zhen, S. & Bugbee, B. (2021). "Photons from NIR LEDs can delay flowering in short-day soybean and Cannabis: Implications for phytochrome activity." PLoS ONE 16(7): e0255232. doi:10.1371/journal.pone.0255232
- Zhang, M., Anderson, S.L., Brym, Z.T. & Pearson, B.J. (2021). "Photoperiodic Flowering Response of Essential Oil, Grain, and Fiber Hemp (Cannabis sativa L.) Cultivars." Frontiers in Plant Science 12: 694153. doi:10.3389/fpls.2021.694153
- Park, J., Collado, C.E., Lam, V.P. & Hernández, R. (2023). "Flowering Response of Cannabis sativa L. 'Suver Haze' under Varying Daylength-Extension Light Intensities and Durations." Horticulturae 9(5): 526. doi:10.3390/horticulturae9050526
- Richards, C.D., Ryu, B.-R., Gim, G.-J. & Park, S.-H. (2026). "Hermaphroditism in Cannabis sativa L.: Impacts, Inducers, and Industry Implications." Plants 15(11): 1643. doi:10.3390/plants15111643 (citing Oliver, W. 2024, SURG 15(1), for light leaks and male flowers)
- Ahrens, A., Llewellyn, D. & Zheng, Y. (2023). "Is Twelve Hours Really the Optimum Photoperiod for Promoting Flowering in Indoor-Grown Cultivars of Cannabis sativa?" Plants 12(14): 2605. doi:10.3390/plants12142605
- Eckels, M.J.H. (2026). "Cannabis: A Model Plant to Study Light-Mediated Reproductive Development." PhD dissertation, Utah State University.
- Gendron, J.M. & Staiger, D. (2023). "New Horizons in Plant Photoperiodism." Annual Review of Plant Biology 74. doi:10.1146/annurev-arplant-070522-055628 (background)
Every claim in this article comes from the episode script, which is checked against these papers. Prices on Seedscanner are dated observations; this page is not growing or legal advice.
