An alarm clock is a clock that is designed to make an alert sound at a specific date and/or time. The primary use of these clocks is to awaken people from their sleep in order to start their days in the mornings, but they are sometimes used for other reminders as well. To stop the sound, a button or handle on the clock needs to be pressed, and some stop automatically after a few minutes if left unattended. A classical analog alarm clock has an extra "hand" that is used to specify the time at which to activate the alarm.
Traditional mechanical alarm clocks have a bell on top that rings, but digital alarm clocks can make other noises. Simple battery-powered alarm clocks make a loud buzzing sound, or other similar noise to wake a sleeper, while novelty alarm clocks can speak, laugh, or sing. Some alarm clocks have radios that 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).
In a mechanical bell-style alarm clock, a mainspring drives a gear that propels a clacker back and forth between two bells or between the sides inside a single bell. In an electric bell-style alarm clock, the bell rings with an electromagnetic circuit and armature that turns the circuit on and off again repeatedly.
The first mechanical alarm clock, capable of striking an alarm at any time specified by the user, was invented by Taqi al-Din, of the Ottoman Empire, in 1559. He described the alarm clock in his book, The Brightest Stars for the Construction of Mechanical Clocks (Al-Kawākib al-durriyya fī wadh' al-bankāmat al-dawriyya), published that year. His alarm clock was capable of sounding at a specified time, which was achieved by means of placing a peg on the dial wheel to when one wants the alarm heard and by producing an automated ringing device at the specified time.
Another mechanical alarm clock was created by Levi Hutchins, of New Hampshire in the United States, in 1787. This device he made only for himself however, and it only rang at 4 AM, in order to wake him for his job. The French inventor Antoine Redier was the first to patent an adjustable mechanical alarm clock, in 1847.
Alarm clocks, like almost all other consumer goods in the United States of America, ceased production in the spring of 1942, as the factories which made them were converted over to war work during World War II, but they were one of the first consumer items to resume manufacture for civilian use, in November 1944. By that time, a critical shortage of alarm clocks had developed due to older clocks wearing out or breaking down. Workers were late for, or missed completely, their scheduled shifts in jobs critical to the war effort because "my alarm clock is broken". In a pooling arrangement overseen by the Office of Price Administration, several clock companies were allowed to start producing new clocks, some of which were continuations of pre-war designs, and some of which were new designs, thus becoming among the first "postwar" consumer goods to be made, before the war had even ended. The first radio alarm clock was also invented by James F. Reynolds, in the 1940s. The price of these "emergency" clocks was, however, still strictly regulated by the Office of Price Administration.
Modern digital alarm clocks typically feature a radio alarm function and/or beeping or buzzing alarm, allowing a sleeper to awaken to music or news radio rather than harsh noise. Most also offer a "snooze button", a large button on the top that stops the alarm and sets it to ring again at a short time later, most commonly nine minutes. Some alarm clocks also have a "sleep" button, which turns the radio on for a set amount of time (usually around one hour). This is useful for people who like to fall asleep with the radio on.
Digital clock radios often use a battery backup to maintain the time in the event of a power outage. Without this feature, digital clocks will reset themselves incorrectly when power is restored, usually starting at midnight, causing a failure to trigger the alarm.
Alarm clock software programs have been developed for personal computers. A computer acting as an alarm clock may allow a virtually unlimited number of alarm times and personalized tones.
Some modern cell phones feature built-in alarms that do not require the phone to be switched on for the alarm to go off. Some of these cell phones feature the ability for the user to set the tone of the alarm, music can be downloaded to the phone and then chosen to play for waking.
Among annoyances caused by alarm clocks is sleep inertia, a feeling of grogginess that results from abrupt awakening. Progressive alarm clocks claim to solve this issue. They include sunrise alarm clocks, dawn simulators and progressive auditory alarm clocks.
Some scientists believe that the human mind may develop a tendency to adapt to alarm sounds so that they no longer disturb sleep. This way, the alarm clock loses effectiveness. The next generation progressive alarm clock also claims to solve this issue.
Scientific studies on sleep having shown that sleep stage at awakening is an important factor in amplifying sleep inertia. Alarm clocks involving sleep stage monitoring have appeared on the market in 2005 . Using sensing technologies such as EEG electrodes or accelerometers, these alarm clocks are supposed to wake people only from light sleep.