I do it to avoid light fatigue giving a high DLI without supplemental CO2. Hard to explain right now in my present state so I asked Gemini for help...
1. The "Wake Up" Period (Photomorphogenesis)
Plants don’t have an instant "on" switch for 100% efficiency. When the lights come on, two main things need to happen:
Stomatal Opening: The "pores" on the leaves must open to allow CO_2 in. This typically takes 15 to 30 minutes, depending on the intensity of the light and the humidity (VPD).
Enzyme Activation: Key enzymes for photosynthesis, such as Rubisco, are light-activated. It takes roughly 10 to 20 minutes for the Calvin Cycle (the "dark reactions" of photosynthesis) to reach full speed.
Total "Startup" Cost: Roughly 30–45 minutes per cycle. In a 9/3 split, you pay this tax twice (1.5 hours total), whereas in 18/6, you only pay it once (45 minutes).
2. The "Go to Bed" Period (Starch Export)
Plants don’t just stop working when the lights go out; they shift gears.
During the day, plants store energy as starch in their leaves.
During the night, they break that starch down into sucrose to transport it to the roots and growth tips.
The Benefit: By giving the plant a "nap" (the 3-hour break), you initiate starch export earlier. Instead of the leaves being "clogged" with sugar for 18 hours, you clear the pipes twice. This often accelerates development rather than slowing it down, because the growth tips get a mid-day delivery of fuel.
3. Addressing the "Slowing Down" Concern
It feels intuitive that more "disruptions" would slow growth, but here is why the 9/3 cycle often wins at high DLI:
Efficiency vs. Duration: Photosynthesis follows a curve of diminishing returns. The first 6 hours of light are incredibly efficient. By hour 15, the plant is often just "treading water" to deal with the heat and light stress.
The "Net Gain"
Even if you lose 45 minutes of peak production to a second "wake-up" period, you likely gain it back (and then some) by avoiding the 3 to 4 hours of sluggish performance that happens at the end of a grueling 18-hour day.
Root Growth: Most root expansion happens in the dark. Breaking up the light cycle provides two distinct windows for root development and nutrient uptake.
The Verdict
The plants do take time to wake up, but the "slow down" is usually an illusion. Think of it like a human worker: you could work one 16-hour shift, or two 8-hour shifts with a long nap in between. You'll likely be much more productive overall with the nap, even though you had to "get ready for work" twice.
