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.
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.
Sleepers can become accustomed to the sound of their alarm clock if it has been used for a period of time, making it less effective
Because progressive alarm clocks have a complex waking procedure, they can deter this adaptation due to the body needing to adapt to more stimuli than just a simple sound alert.
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 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.
A digital clock is a type of clock that displays the time digitally, i.e
in cyphers, as opposed to an analog clock, where the time is displayed by hands
Usually, digital clocks are associated with electronic drives, but the "digital" description refers only to the display, not to the drive mechanism
(Analog clocks are driven mechanically or electronically.) The biggest digital clock is the Lichtzeitpegel ("Light Time Level") on the television tower Rheinturm Düsseldorf, Germany.
Digital clocks typically use the 50 or 60 hertz oscillation of AC power or a 32,768 hertz crystal oscillator as in a quartz clock to keep time
Most digital clocks display the hour of the day in 24 hour format; in the United States and a few other countries, a more commonly used hour sequence is 12 hour format (with some indication of AM or PM)
Some clocks can display either time mode according to the owner's preference
Emulations of analog-style faces often use an LCD screen, and these are also sometimes described as "digital".
To represent the time, most digital clocks use a seven-segment LED, VFD, or LCD display for each of four digits
They generally also include other elements to indicate whether the time is AM or PM, whether or not an alarm is set, and so on.