All indoor ficus have leaf drop in the Winter due to low light conditions. The reason is that the plant will only support enough leaves from the amount of light it collects. More light more leaves, less light less leaves. It natural and nothing to worry about, however one thing to keep in mind with ficus, is when the lighting is dimmer the plant uses less water. If you keep giving the same amount of water in low light conditions you can hurt the roots. Be patient because when the stronger lighting returns the ficus will regrow new leaves.