Spyder7answered grow question 4 years ago There's no consensus here with autos. They're grown where the light really does shine for 24 hours a day for several months a year - I've lived there, and you would have shade your windows at night or sleeping at midnight is like sleeping at high noon in terms of light.
Since these autos have those ruderalis genes that are native that far north baked in, they don't need *any* darkness at any point in the grow - though through experimentation, growing 18/6 or 20/4. starting at any point in the grow if you begin 24/0 as I do, growers have found that using 20/4 or 18/6 instead of 24/0 doesn't seem to impact grow time much, or yield at all, and since it doesn't, 18/6 or 20/4 are viable alternatives since you're consuming less electricity and generating less heat.
I ran my last grow 24/0 and only switched to 18/6 for the last couple weeks because heat was getting a bit too high with an early spring, and since I'm not venting it outside, I adopted an 18/6 to lower temps since I could time the dark period for the hot part of the day.
If you're having temps that are too low which 24/0 is solving, then the time to go back to 20/0 is after you solve your low temp issue. The plant is content to wait as the Ruderalis genes means its quite content with 24/0 if that's what it needs to keep the temp high enough.
This grow is 24/0 right now since I don't think seedlings need darkness and I just planted last night (after planting, I turn the light on and leave it on); but since some darkness *may benefit* them after they hit veg, I'm going to switch this grow to 18/6 after week 1. This is not something you have to do and 20/4 serves the same purpose. I'm doing it this grow because I think some darkness, once veg starts, helps (4 hours or 6 doesn't matter I don't think) and it helps manage heat which can get a little too warm, especially in late flower when the light is close and in summer.