Oh, wow, our shortest day (sunrise-sunset, winter solstice) is a bit over 9-1/2 hours. But, at ~110 watts for the light, less hours would mean less kilowatt-hours ($$).
Anyway, thanks! - you’ve confirmed I should light-cycle lettuce too. I guess I can try 10 hours* and see how green the lettuce stays. I have enough lettuce plants, and not enough space in the basement, that I could spare one plant and keep it outside in a spot out of the afternoon sun, for a reference. When it bolts, then I’ll have seeds. :-)
*Like you, I’m not so lucky. What’s an 8 hour work day? Although these days some of mine are more like a “Mexican” schedule with a nap after a late lunch, then work late into the evening... ;-)
If ‘greening up’ your lettuce is the goal, make sure you’re fertilizing with a higher Nitrogen fertilizer. Lettuces love Nitrogen as much as my mule, Ithaca, loves carrots! :)
“Nitrogen imbalance: Lettuce requires a balanced supply of nutrients, especially nitrogen. Too much nitrogen can result in excessive leaf growth but may delay or inhibit the development of heads. On the other hand, a nitrogen deficiency can lead to stunted growth and yellowing leaves. It’s important to provide a balanced fertilizer or amend the soil with organic matter to maintain the proper nitrogen levels.”
https://thegardenmagazine.com/best-fertilizer-for-lettuce-when-to-feed-your-lettuce/