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Dark period and watering is what it looks like to me. I'd venture to say you have nothing to worry about. The leaves aren't clawing so it's not quite root problems or overwatering but it still could end up that way. Just make sure the medium dries out a bit before next watering. Looking good!

Oh and heads up if a person called Chow answers this question I would not take his answer seriously he has been reported by several growers for cheating in the contest so please take this in mind when selecting an answer nobody likes a cheater :)

Way overwatered my friend only water plants when the medium is dry if using soil coco users have a bigger bufferzone as its inert but soil mixes are designed to make the most out of what water you put in with perlite and stuff like that which is designed to hold onto water for periods of time they leaves will droop naturally when they are in dark periods if you look at a grow time lapse we call it breathing cause thats what it looks like but this is due to over watering in my opinion. personally I wouldnt flower this plant yet I would wait for a bit more veg growth some more side branching ect that way you going the guarentee so good bud sites :) Best of luck anyway my friend :) 👊