The best alternatives to a window in a bathroom depend upon the goal you are trying to achieve — natural light or ventilation.

If the goal is ventilation, you will probably want to invest in a higher-end ventilation system for the bathroom or even install two ventilation systems in a large bathroom. This ensures adequate air movement to prevent mold and mildew and to control odors and steam. A second ventilation system or vent provides both additional air flowRemodelOrMove.com: Bathroom Remodeling Workbook by Dan Fritschen and a backup in case of a malfunction.

If your goal is to admit natural light into the bathroom, there are three main options: you could install a skylight, a roof window, or a tube light. The skylight and roof window are very similar. The main difference is that a roof window could be opened for ventilation. A tube light is simply a tube that runs from the bathroom ceiling to and through the roof to admit light. With good construction and planning, you can even install several tube lights if you wish to bring natural light to different parts of the bathroom.