Watch the episode · 2:55This article contains paid links to GrowAce, a US shop. We may earn a commission at no cost to you.
18, 20 or 24 hours of light for your autos? Nobody has tested it head to head. Autoflowers don't count the dark, so growers argue over how long to leave the lights on. We searched the research and found no trial comparing these schedules in autos. But three things that have been tested settle most of it.
The short answer ▶ 0:25
| 18/6 or 20/4 | 24/0 | |
|---|---|---|
| Who it suits | most grows | moderate lights only |
| Power bill | baseline, or +11% | +33% vs 18/6 |
18 or 20 hours of light are both safe bets. 24 hours can work, but only with moderate light, and it costs a third more electricity than 18.
More light, more flower ▶ 0:38
First, more light means more flower. In a study we met in why some autoflowers stay tiny, yield rose in a straight line with light, right up to the brightest level tested. Photoperiod plants are stuck near 12 hours of light in flower. Autos aren't, so a longer day adds light without a brighter lamp.
Source · Rodriguez-Morrison et al., Front. Plant Sci. (2021)
total daily light from a lamp giving 600 µmol per m² per second
Under the same lamp, 18 hours gives about 39 units of light a day, 20 hours 43, and 24 hours 52.
Second, leaves use gentle light more efficiently than blazing light. Nobody has tested this in cannabis yet, so here is lettuce. In a 2026 Dutch study, lettuce given the same total light, spread over 24 hours instead of 18, grew 6 to 23% more.
Source · Núñez Ocaña et al., Front. Plant Sci. (2026)
The catch with 24/0 ▶ 1:24
So why doesn't everyone just leave the lights on? Because some plants can't take it. Again this is a different crop, because it is the clearest test: in a 2026 study, tomatoes under steady light around the clock had stressed leaves and lower yields than tomatoes on 16 hours, with the same total light.
Source · Lanoue et al., Front. Plant Sci. (2026)
And in cannabis? One lab study grew tiny plantlets of an autoflower under 16, 20 and 24 hours. 20 hours gave the best all-round growth. 24 hours added weight, but with bright light around the clock, the plantlets showed light damage. These were lab plantlets, not plants in a tent, but the pattern fits.
Source · Liang et al., Biology (2025)
Then there is the bill. With the same lamp, 20 hours uses 11% more electricity than 18, and 24 hours uses a third more.
The verdict ▶ 2:11
- 0118/6
the safe default
- 0220/4
more light, same lamp
- 0324/0
moderate light only
18 hours on a timer is the safe default. 20 hours squeezes more light out of the same lamp. And if you run lights round the clock, keep them moderate and watch the leaves for stress.
There is no head-to-head trial in autos yet. Longer days add light, round-the-clock light can stress plants, and 24 hours costs a third more power.
Follow us on Google
Add Seedscanner as a preferred source and Google shows you more Seed Science in Top Stories.
Paid links. Links to GrowAce, a US grow shop, are paid links. If you buy through them we may earn a commission, at no cost to you. It never changes what the research says or what we recommend. Check shipping and the rules where you live.
Sources
- Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. (2021). "Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment." Frontiers in Plant Science 12: 646020. doi:10.3389/fpls.2021.646020
- Núñez Ocaña, D., Esmaeli Sooderjani, H., Kusuma, P., Marcelis, L.F.M. & Heuvelink, E. (2026). "Continuous light at fixed daily light integral enhances lettuce growth due to improved light interception and light use efficiency." Frontiers in Plant Science 17: 1756524. doi:10.3389/fpls.2026.1756524
- Elkins, C. & van Iersel, M.W. (2020). "Longer Photoperiods with the Same Daily Light Integral Increase Daily Electron Transport through Photosystem II in Lettuce." Plants 9(9): 1172. doi:10.3390/plants9091172 (background)
- Lanoue, J., St Louis, S., Little, C., Soltaninejad, S., Seguin, R. & Hao, X. (2026). "Dynamic lighting mitigates photoperiodic injury in greenhouse tomatoes." Frontiers in Plant Science. doi:10.3389/fpls.2026.1731972
- Liang, J., Ji, F., Zhou, Q. & He, D. (2025). "Optimizing LED Light Intensity and Photoperiod to Promote Growth and Rooting of Medicinal Cannabis in Photoautotrophic Micropropagation." Biology 14(6): 706. doi:10.3390/biology14060706
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.
