Sunlight, especially the ultra-violet ray, affects the growth of trees. Therefore, except in special cases such as immediately after repotting, extensive trimming, etc, bonsai should be placed in a sunny location. Bright light will also work well but the tree should not be placed more than 12" away from the direct light source.
An east, west or southern exposure works best. A northern exposure will require the use of "grow lights" which should remain on up to 16 hours each day and the lamp should not be more than 2 inches from the top of the tree. Incandescent light is too hot and will not provide the various spectrum of light that is required to maintain your bonsai tree.
Bonsai need about 5 hours of direct or indirect sunlight per day. Certain species of bonsai do best in the winter if they receive most of their light from indirect sources. Bonsai trees can burn if taken from a shady location to a sunny location so care should be taken to gradually increase their light exposure.
If you do not have a window or light source that provides an east, west or southern exposure, be sure to select a bonsai tree that does well in lower lighting conditions.