Using the sun's light instead of electricity to light a space is known as daylighting. Because it is free, using it makes sense when possible.  Of course, it's only available during the day, and if you are air conditioning your home, it may cost more to let the sun's heat in since your air conditioner will have to remove it.  Much of a home's potential to use daylighting is determined when it is designed and constructed.  Homes and commercial buildings constructed prior to having electricity have tall windows to allow sunlight to pour in since lighting otherwise meant burning oil. 

Today, you can paint interior walls with light colors to reflect sunlight deeper into rooms, and careful selection of drapes and furnishings can make daylighting more useful.  If you are making changes to your home, you might consider how sky lights and other window configurations can bring in reflected sunlight to make the space more attractive and lower lighting needs.  But keep in mind what a small portion of the home's total energy costs are from lighting.  In most homes, lighting accounts for only 6 to 7 percent of the total energy cost, so it's not enough to justify a big expense.  The bigger advantage of daylighting is it's often more aesthetically pleasing than other forms of lighting.