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 gotten 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.
Digital alarm clocks can make other noises. Simple battery-powered alarm clocks make a loud buzzing or beeping sound to wake a sleeper, while novelty alarm clocks can speak, laugh, sing, or play sounds from nature.
Some alarm clocks have radios that can be set to start playing at specified times, and are known as clock radios. A progressive alarm clock, still new in the market, can have different alarms for different times (see Next-Generation Alarms). Most modern televisions and cell phones have alarm clock functions to turn on or make sounds at specified times.
Read more about Alarm Clock: History, Other Alarm Signals, Computer Alarms, Cell Phone Alarms, Next-generation Alarms
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—Henry David Thoreau (1817–1862)
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—Richard Brinsley Sheridan (1751–1816)