Whenever you leave a room even for a few minutes, turn the lights off. Leaving on lights in classrooms, conference rooms, and offices when empty increases energy use for lighting as well as to cool the building, since lighting produces additional heat. Whenever possible, take advantage of natural lighting. Many offices, labs, classrooms, and hallways have exterior windows that provide ample light during daylight hours. Turning the lights off during the day helps keep electric use down during critical summer days.