Hi Melissa,
It is hard to give you specific advise without knowing what type of tree you have. *If it is a broadleaf plant, it is probably a serissa or ficus. *Both of these plants are prone to dropping leaves if they become stressed. *Too little light or too much water and they do the same thing.
You are correct that the indoor environment is very dark and dry for plants. *Your bonsai could benefit from a grow light; however, watering is more critical. *Many "mallsai" (as mass-produced plants are sometimes called), are greenhouse grown and underpotted. *This can cause them to dry out very quickly. * Watering the tree by soaking it will ensure that the entire root ball becomes thoroughly saturated. *You can test the soil moisture level with a fingertip. *Water when the soil loses its sogginess, but is still just damp to the touch.
If the soil dries out between daily watering, the pot is too small. *You could try putting it in a humidity tray (a plastic tray for orchids that will hold a small amount of water with the plant resting on a layer of marbles or gravel) or you could repot it into a larger container without disturbing the rootball.
You may wish to read the section on mallsai on the TreeBay website
here as well as the Beginner articles located on the bottom of the same page, in particular
Watering
Happy Growing,
Dr. Bonsai