Human eyes can only see a small sliver of the electromagnetic spectrum (Visible Light). Just outside this range, past red, is Infrared (IR) light.
How Night Vision Works
Cameras cannot see in absolute darkness. To solve this, spy cameras act like flashlights, blasting the room with light. But instead of white light, they blast IR light.
To the camera sensor, the room is brightly lit. To your human eye, the room is pitch black. This allows a spy camera to record you while you sleep without you noticing.
How to Spot It
While your eyes can't see IR, digital camera sensors CAN. Most smartphone cameras (especially the front-facing selfie camera) do not have an IR filter.
The Remote Test
Grab your TV remote. Point it at your phone's selfie camera and press a button. You will see a purple/white flash on your screen. That is IR light.
Finding Cameras
To find a hidden night-vision camera, turn off all the lights in the room. Open the Stop Stalker camera mode and scan the room. If you see a bright purple or white dot in the darkness on your screen, that is an IR illuminator from a hidden camera.