Decreasing photo-period (shorter days) triggers an increase in the growth regulator that is largely responsible for leaf abscission. Cool temperatures can intensify the effect. I would expect some leaves, primarily those on the interior of the canopy, to drop at this time of year (especially since your planting is indoors) unless you offer supplemental lighting or thin the outer canopy to allow additional light to the interior.

Over or under-watering also causes leaf loss. Over-watering rots roots or starves them of O2 which impairs metabolism. The effect is the same as not watering because the plants ability to move water to the canopy is diminished. The plant "thinks" it is experiencing a drought in either case & prepares for a drought-induced dormancy to slow water loss through leaves by shedding foliage.

Though you didn't mention species, most Ficus tolerate dry (but not totally dry) soils well. I always allow soils to become quite dry before watering thoroughly. In some plants I even withhold water until the first signs of loss of turgor (wilt) is apparent in newly emerging leaves, but this treatment could be risky for you. Try watering when soil at the drain hole shows signs of becoming a little dry.

Al