climatological parameters, heat radiation, air temperature, air humidity and wind speed affect evaporation. in a room where the lights are off, air temp, low humidity and air circulation all assists in evaporation.
Except that lights off means a drop in temperature which spikes humidity. I run my grow area, the room with my grow tents in it, between 45% and 50%. Lights off inside the tents will spike RH as much as 5% with my setup. It would spike even more if a grower is not making an attempt to keep humidity down.
Now, during a Michigan winter, controlling the humidity in the grow room is a relatively easy task. The dehumidifier cycles and the dry air from the furnace helps keep it all down. There's days when RH is in the upper 30's/low 40's naturally this time of year.
However ... come Spring or Fall, that changes. I can be in the 70% humidity range without a dehumidifier... which is way too high indoors. The dehuey runs almost constantly and I'm frequently using mostly dehumidifier water to water my plants.