The only place I've seen recommend reducing light levels is at growlightmeter.com. When I tested Korona/Photone and corresponded with the programmer (2021), I asked him if they had any citations for the light levels that were recommended and he stated that they were on the bottom of each web page.
The reason I asked is that I can't find any literature re. decreasing light levels and, since light is the only way that plants generate food, which means the only way they can create energy, reducing light levels didn't make sense to me. I can't think of a living organism that benefits from having a reduced energy level.
In the three years since then, I still haven't found anything that supports the idea of reducing light levels. In contrast, research has shown that yields and crop quality (the ratio of flower to total above ground mass) increases in an almost linear manner to light levels. Those light levels were 1800µmol in one expirement and, per Bugbee and Westmoreland, yield increased even when they "ran out of light" at 2000µmol.
The recent video by Mitch Westmoreland that he posted on YouTube is a goldmine. It sums up a lot of the bits and pieces that I've found I wondering around the internet over the past few years. He posted it a few months ago and it's essentially a summary of the topics that he defended in his PhD dissertation. It's only 50± minutes and it substantiates a lot of the things that Bugbee has said, as well as topics that Bugbee has hinted at over the past three years.
Back to reducing light levels - if anyone has any research to show that reducing light levels improves yield, quality, or enhances secondary metabolites, I'd love to read it. Along with that would be why that happens because, as I wrote a few paras up, reducing light levels means that there's less glucose generated with means that plant metabolism slows.
The big reveal in his video is something that he mentioned in his video on hemp back in 2022(±)* and that Bugbee first hinted at the beginning of the year — run your temps up to 85°± in veg and early flower but drop temperatures into the 70's or high 60's as flower progresses. That's the best approach to preserving secondary metabolites. Now that I think of this, Shane @ Migro dropped a YTY vide on this in the past week or so and DeBacco will probably repeat it in the next few weeks so there's lot of outlets for the info.
In short, don't drop light levels but do drop temperature levels (starting in about the third week of flower - Shane's video makes is crystal clear, Westmoreland isn't specific on when to start dropping temp).
*That video is about "hemp" but that's just the appellation he was using at the time for cannabis that was low enough THC to fit under the then-limit for THC levels for research at federally-funded institutions.
