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 (usually to midnight) when power is restored, causing a failure to trigger the alarm
To solve this issue, some digital clock radios trigger the alarm at e.g
00:01 after a reset to midnight, so that at least the user is able to correct the clock and alarm time
Some other radio clocks (not to be confused with clocks with AM/FM radios) have a feature which sets the time automatically using signals from atomic clock-synced time signal radio stations such as WWV, making the clock ready for use right out of the box.
Other alarm signals
The deaf and hard of hearing are often unable to perceive auditory alarms when asleep
They may use specialized alarms, including alarms with flashing lights instead of or in addition to noise
Alarms which can connect to vibrating devices (small ones inserted into pillows, or larger ones placed under bedposts to shake the bed) also exist.
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.
Online services like alarmrock.com and onlineclock.net offer free web-based alarm clocks.
Cell phone alarms
Many modern cell phones feature built-in alarms that do not require the phone to be powered 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, and in some cases music can be downloaded to the phone and then chosen to play for waking.