Look at it this way while I am searching for the thread - outside after the daylight hours hit 12 and begin shortening light is the trigger for pot to flower and finish! As the days get shorter the pot finishes quicker. We leave the lights on to keep the THC production up.
"Lowering the humidity in the room on that last night before harvest morning will ensure increased resin production, without having to let the medium go bone-dry first. Additionally, some growers like to subject their gardens to prolonged dark periods of up to 24 hours just before cutting, claiming they notice spikes in resin production. This is all right as the low humidity will cut down on light uptake anyway, plus it helps to make sure liquid foods within the plants drain down to the root zone." from High Times and the link is below.
http://www.hightimes.com/read/key-points-harvest-time
