A light that turns itself off while someone is sitting still in the room is not malfunctioning. Most household sensors detect movement, and stillness is indistinguishable from absence to them.
Infrared sensors watch for changing heat
The common passive infrared sensor contains a segmented lens that divides its view into zones, and a detector that reports when the amount of infrared energy in one zone changes relative to another.
A warm body crossing between zones produces exactly that change, which is what triggers the output. A warm body sitting still produces a steady reading that looks identical to an empty room.
This is why these sensors are accurate for corridors and entrances and unreliable for rooms where people read, watch or work. The physics reports motion, and the label on the box says occupancy.
Timeouts paper over the gap
Installers compensate by holding the output on for a period after the last detection. A longer hold reduces false switch-offs but wastes energy and delays the response when a room genuinely empties.
Because the correct hold time depends on the room, a single default suits none of them well. A hallway wants seconds and a home office wants many minutes.
Extending the timeout also masks a poorly aimed sensor, so a system that appears tuned may simply be waiting long enough that nobody notices the detection gaps.
Microwave and radar sensors see differently
Radar-based sensors emit a signal and measure how the reflection changes, which lets them detect movement as small as breathing. That makes genuine presence detection possible rather than motion detection alone.
Their sensitivity is also their weakness. They see through thin partitions and detect movement in adjacent rooms, so a sensor in one room can be triggered by activity elsewhere.
Tuning them therefore involves reducing sensitivity until neighbouring rooms stop registering, which pulls the detection performance back toward what a simpler sensor would have achieved.
Placement matters more than specification
Sensors mounted where movement approaches them head-on perform worse than those mounted where movement crosses the field of view, because crossing produces the zone-to-zone change the detector needs.
Heat sources also interfere. Sunlight moving across a floor, a heating vent or a radiator warming up all change infrared readings without a person being involved.
Combining signals is what makes it reliable
Systems that fuse several inputs, such as a door contact, a motion sensor and a device that indicates activity, can distinguish entering, remaining and leaving in a way that no single sensor can.
The logic is simple: a door opening followed by motion suggests arrival, while a door opening followed by silence suggests departure. State is inferred from sequence rather than from any one reading.
That approach costs more devices and more configuration, which is why most installations settle for a motion sensor and a generous timeout instead.