An exemplary embodiment of notebook computer includes a main body, a display, and a proximity sensor. The display is pivotably connected to the main body. The proximity sensor is installed in the main body and includes an emitter and a receiver. The emitter is configured for emitting an emission. The receiver is configure for receiving the emission, determining if an included angle between the main body and the display is in a predetermined range based upon the received emission, and, if yes, generating a signal indicative of placing the notebook computer in the sleep mode.