Do consider that the natural source of many sativa were in the tropics. In the tropics the length of day and night are very smell compared to artic zones. On the equator 11.5 hours of light may be winter and 12.5 hours for summer ( I guess, or something like this). So a hybrid with traits of the topical gene source may like 12/12 hours right on through. And a little bit of extra dark to flower 'em.