"After many years of first-hand experience breeding herb indoors as well as outdoors, I am of the opinion that the two most influential factors involving phenotypic variation and expression among current indoor herb breeding projects are the photoperiod (hours of light per day) and the angle of light in relationship to the growing plant.
Specifically, I find the single most powerful influence to the Indica dominant phenotype is the traditional 18/6 veggie cycle and 12/12 flowering cycle. The 18/6 veggie and 12/12 flower cycle is an attempt, however poor, to mimic the Indica-producing photoperiod. It is my belief that this light cycle strongly influences for Indica phenotypic expression.
Sativa phenotype characteristics will manifest under a more equatorial photoperiod, closer to a 13/11 veggie cycle and an 11/13 flower cycle. This is the light timing range to use to elicit more Sativa dominant expression from your plants.
As for the exact photoperiod formula that I incorporate into my growing/breeding regime, this will presently remain a trade secret. My advice is to experiment with different photoperiods, keep good notes and pay attention. Avoid the 18/6 and 12/12 photoperiods, while tweaking the times a bit differently with each breeding cycle until more desirable results in the finished product and their offspring are noted. Here's a hint: work in half-hour increments or a little less, and good luck" - Dj Short
Link to entire article:
Breeding tips