"The difference between day- and nighttime temperatures is referred to as the DIF and it is widely agreed that increasing the DIF has a positive effect in steering most plants towards flowering. When growing indoors it is pretty straightforward to record day and night temperatures using a simple max./ min. thermometer. It is also straightforward to drop the night temperature, because the biggest source of heat - the lamp - is off during the night period.
During the veg stage, you will achieve optimum growth by keeping the DIF constant. Use a fan heater, or similar, to raise the night temperature once the lamp is off. Measure your minimum night temperatures and set the heater on a timer to warm the grow room throughout the lights off' period.
For the flowering stage, the optimum DIF is around 46.4º - 50º F (8º - 10º C), so if your maximum day temp is around 82.4º F (28º C), aim for a night temperature of around 64.4º F (18º C).
This drop in temperature forces sugars held in the plant towards the buds. This is because as the air temperature cools, the plant will cool with it. The leaves hold less mass than the buds and therefore cool at a quicker pace, causing the sugars held in them (which they have been producing all day) to be moved to the flowering sites, where they advance bud growth."
Quoted from
http://www.cannabis.info/us/abc/10008151-now-for-something-completely-different