When motion is observed, a motion sensor light responds. They can be mounted both inside, on walls, floors, and in doorways, and outside, on the exteriors of buildings and homes. Some motion sensor lights, known as occupancy sensors, work by shutting off lights in empty rooms and spaces. When motion is detected, the sensor activates the light; when motion is no longer detected, the sensor switches the light off. Occupancy sensors are one low-maintenance tool for reducing energy bill charges caused by lights that are left on while no one is at home or in a bed.