18/6 vs 20/4 vs 24/0: The Best Veg Light Schedule
More light hours means more photosynthesis, but also more electricity and less recovery time. For most home growers the answer is 18/6, and here is why.
18/6 — the default
Eighteen hours on, six off gives near-maximum growth with a real dark period. Plants perform root work and respiration in the dark, and the six hours off drops your power bill by a quarter versus running around the clock.
20/4 — for autoflowers in a hurry
Some auto growers see slightly faster growth on 20/4. The gain over 18/6 is small and the extra heat load in a small tent can cancel it out.
24/0 — rarely worth it
Constant light works but returns diminish sharply, heat builds, and the plant never rests. It also means a dehumidifier and fans running 24 hours with no cool-down window.
Flower is not negotiable
Photoperiod plants flower on 12/12 with a completely uninterrupted dark period. A light leak during those 12 hours can revert plants to veg or trigger hermaphrodite flowers.
Frequently asked questions
- Do I need a timer?
- Yes. Manual switching is the single most common cause of light-schedule stress. A dual-outlet digital timer costs about the price of a coffee run.