An alarm clock is a clock that is designed to wake a person at a specific time. The primary use of these clocks is to awaken people from their night's sleep or short naps; they are sometimes used for other reminders as well. Some use sound, some use light, and some use sensors to identify when a person is in a light stage of sleep, in order to avoid waking someone when they're deeply asleep, which causes tiredness, even if the person has had adequate sleep. To stop the sound or light, a button or handle on the clock is pressed; but most clocks automatically stop the alarm if left unattended long enough. A classic analog alarm clock has an extra hand or inset dial that is used to specify the time at which to activate the alarm.
Traditional mechanical alarm clocks have one or two bells that ring by means of a mainspring that drives a gear that propels a hammer back and forth between the two bells or between the interior sides of a single bell. In some models, the back encasement of the clock itself acts as the bell. In an electric bell-style alarm clock, the bell is rung by an electromagnetic circuit and armature that turns the circuit on and off repeatedly.