Switching from 18/6 to 20/4 or even 21/3 mid-Veg

So why is 18/6 cycle so prevalent? I'd argue that the best metric for determining a grows success would be grams/kWh. Is there any data to suggest that an 18/6 light cycle yields a higher g/kWh than 20/4? i.e. do you get diminishing returns in terms of growth once you push beyond 18 hours?

@Andrew Harper This is a controversial topic. But basically, there are plant processes that happen during lights off that are important - i.e. secondary metabolites. I've ran 18/6, 20/4, 24/0 and didn't see much difference in growth. I did see an increase in my electricity bill though which of course is no bueno.