^Agreed. I would say that if heat was an issue, they'd be drooping at the end of the cycle, after being hit by the heat, not when the heat is least, right? Could be them simply being satisfied, not desiring any moisture, then raising up when the lights come on, for maximum moisture wicking. Basically, the soil be a tad wetter than they want, during the dark period. If that's the case, it should clear up once they've gone unwatered for another day or two, and nothing to worry about. Mine have drooped for alll those reasons, and none of them were real critical, of course, unless it's a case of really high heat, really low humidity, etc....