Alarm Clock

An alarm clock is a clock designed to alert a person or group of people at a specified time, most often to wake them from sleep, and most alarm clocks signal through sound, though some instead use light or vibration, with some models using sensors to avoid waking a sleeper who is in a deep stage of sleep. Many alarm clocks include radio receivers that can be set to start playing at a chosen time, known as clock radios, and modern televisions, computers, mobile phones and digital watches now commonly include their own alarm functions as well.