Mostly meaningless. Some claim that harvesting in the dark or right away in the morning prevents that plant from taking up more sugars and materials like it would when the lights are on causing issues with psychoactive properties when smoking it, problem with that claim is the process of which a plant begins taking up these things from the roots is mid-day at the earliest,, not as soon as daylight appears or even soon after. Some even go as far as claiming that harvesting in light at all, causes harsh bud and lowered potency, which is beyond ridiculous.
The main reason you would want to harvest before or near lights on is to get the plants out so the vegging plants ready to be flowered can move in. This was mostly a claim invented about outdoor harvesting.
Some fuckhead at HighTimes named Nico wrote an article about 5 years ago that was a repeat story from another fuckhead in Seattle that wrote a similar unproven unscientific theory about this issue in the 80s. Nico took a lot of real proven info and mixed in this bullshit and created an overall perfect storybook way to harvest in one of his High Times articles and this has become the truth for many growers ever since.
Now avoiding light and heat in general AFTER harvesting when trimming, drying and curing if of course true. Same can be said about avoiding too much handling when trimming.
If the best time for you to harvest happens to be right before or after lights on, then go for it. But you are good to go harvesting at 5-6 hours into the 12/12 cycle with no potency loss or harshness issues.
