Hereā??s an excerpt from an article over at CannaStats

Because the flowering response of cannabis is triggered by the duration of the dark phase, it will flower when it receives 12 hours of uninterrupted darkness, but it will not flower with 12 hours of interrupted darkness. Manipulating light timer settings in such a way as to provide 12 hours of light over a 24 hour period, but not permitting 12 hours of uninterrupted darkness to occur, can reduce growth by 25% when compared to the traditional 16 hour vegetative photoperiod without triggering the flowering response. A timer capable of 4 on/off cycles per day, using the settings in the following table, will produce such results.

The whole article here: CannaStats - Low Maintenance Mothering

It makes me wonder how the guys in the extremely northern climates with very long days get their mota to flower?

HMR