Energy Usage
Equipment that is left running continuously uses a certain amount of energy, which is a concern for some people. The Energy Star program, along with Apple's energy-saving software, addresses this issue.
The United States Environmental Protection Agency (EPA) and Natural Resources Canada Energy Star® program is a voluntary partnership with office product equipment manufacturers to promote products that exceed guidelines for energy efficiency. Reducing energy consumption of office products saves money and reduces pollution by eliminating wasted energy. As an Energy Star partner, Apple is committed to the development of energy efficient products and many Apple products meet Energy Star guidelines.
In addition to products that are Energy Star compliant, Mac OS includes software that you can use to further cut down on energy usage, including the Energy Saver control panel (Mac OS 8 and 9) and the Energy Saver pane of System Preferences (Mac OS X).
Stress on Equipment
Some people believe that turning equipment on and off (for example, once each day) can, over a long period, put stress on equipment. Therefore, they argue that equipment should be left on, whether in energy-saving sleep mode or not. Others believe that leaving equipment turned on makes it more vulnerable to power fluctuations, which could put stress on equipment. Therefore, they believe that equipment should be turned off when not in use.
Lightning, Spikes, and Surges
Some people feel that the risk of damage to equipment from lightning strikes, power spikes, and power surges is great enough to warrant turning off equipment whenever it is not in use. Others, who may rarely (or never) experience such phenomena, give more weight to the benefits of leaving equipment turned on.
Conclusion
With equipment that is Energy Star compliant and in areas where there are no issues with the quality or dependability of power, you may want to consider turning off equipment if it is going to be idle for eight hours or more. For idle periods of less than eight hours, use Energy Saver software to optimize energy usage.
For more information on energy saving and the Energy Star program, refer to the following documents:
Document 12346: "Energy Star: Description"
Document 12345: "Energy Star: Registered Apple Products"
Document 18127: "Energy Saving Control Panels: Description & Compatibility"