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Alarm Clock
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.
