Good topic to bump, and I appreciate your objectivity! I have a few thoughts on the matter and a question of my own.
I have seen a pretty convincing research paper about 24 hrs of light vs a 4 to 6 hour “resting” period (sorry I did not save the link, but the research was as credible as you could hope for). They saw a significant increase in dry mass with the 24 hrs of light over the course of weeks. (Whether or not there are diminishing returns per watt of light beyond a certain point is a really interesting question.) This makes sense physiologically via how we currently understand photosynthesis to function - purely based on inputs being present for the reaction to take place (mainly light and CO2); these reactions have been shown to take place in the light, no matter how long it’s been on. As mentioned before, if your plants continue to grow without damage, that’s a good indicator that darkness is unnecessary. And I haven’t seen any scientific data to support the “needs rest” theory, just a lot of people with feelings… so please post something if you’ve seen anything. I’d love to read it!
My other thought on the subject is that if it indeed made a significant difference in dry mass, more people would have noticed and would be doing it… I think that it might be a good idea to run lights 24/7 if you are limited in veg space/time and need maximum growth per day. On the other hand, I find that a few hours of darkness comes in handy for any foliar sprays (mainly oils) or sulfur burning. Do note also that various insect species need particular light requirements to complete their life cycles, which has some interesting implications.
What I think can be argued in favor of some dark time would be that it naturally helps the temperature and RH swing a little bit, which is a good thing for building a bit of tolerance in your plants to various conditions they might later encounter (on purpose or not). Good research is out there on secondary metabolite and phytochemical production in various crops in response to abiotic stress (to include temperature and RH swings). As a living soil grower, I also must take into account what the microbes might like, which is obviously hard to pinpoint. I would think more varying conditions would encourage diversity. I do feel strongly that if your lights and environment are constantly the same without much fluctuation, you’re asking for a bit of vulnerability in your crop, so that’s something to look out for.
My question is a little off topic, but fits well in this thread: Have any of you ever tried a 24 to 48 hour period of uninterrupted darkness at the beginning of flower? My thought is that it might help convert more phytochrome to its red (Pr) form and initiate a stronger hormonal response to help speed things along, (but if all the phytochrome is already converted within little more than 12 hours, we would just be losing out on photosynthesis time…) Photoperiodism is very complex with multiple steps within the process. I was mostly wondering if anyone has tried it and noted a significant difference…?
One more light question: What ppfd or ppf have you all found to work best for rooting cuttings? What about specific spectrum? Feel free to include temperatures as well!