Incandescent bulbs (including halogens) rely on a wire filament held inside a vacuum. When turned on, the filament heats and emits light. Both regular incandescent bulbs and halogen lights are great at producing instantaneous light at the flick of a switch. 

Their main problem, though, is that light is a byproduct of their filament wire heating up, so their main energy output is 90 percent heat. Not only do they consume a substantial amount of electricity, they have shorter lifespans than other types of bulbs.

The lifespan of incandescent lights is not affected by rapid switching, so there is no compelling reason not to switch off the lights when not in use. Leaving incandescent on for too long can harm your efficiency in another way, too, since their heat output adds to your home’s cooling load during the summer.

LED bulbs use semiconductors, are low wattage, put out nominal amounts of heat and resist vibration. Since LEDs produce light through electro-luminescence, frequently switching on and off has no effect on them.

Even though they only use a fraction of the energy of an incandescent bulb, you’re still saving energy by turning off the lights, so there’s no reason to leave them on when you leave the room.

The simple rule for using these in an energy-efficient home is to turn them off when you don’t need them.