Best Light Schedule for Autoflowers: 18/6 vs 20/4 vs 24/0

18, 20 or 24 hours of light for your autoflowers? We searched the research and found no trials comparing these schedules in autos. But three tested factors explain most of it. More light means more flower: in a 2021 study, yield rose in a straight line with light up to the brightest level tested, and autos can take a longer day without a brighter lamp. Light spread over more hours is used more efficiently: lettuce given the same total light over 24 hours instead of 18 grew 6–23% more. But steady round-the-clock light stressed tomatoes, and in a lab study of autoflower plantlets, 20 hours gave the best all-round growth, while 24 hours with bright light caused light damage. Plus the power bill: 20/4 uses 11% more electricity than 18/6 with the same lamp, and 24/0 a third more. 0:00 Intro 0:25 The short answer 0:38 More light, more flower 1:24 The catch with 24/0 2:11 The verdict 2:23 Recap 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 Seed Science is the https://seedscanner.com series on the genetics, germination and growing science of seeds. Educational content for adults. Laws vary by country: check yours. #SeedScience #Autoflowers #GrowTips #LightSchedule #PlantScience