Among its many features, the iPhone provides a configurable alarm clock. You can set an alarm to ring every day at a specific time or only once and choose which ring tone it should alert you with. While the alarm clock doesn't work if your iPhone is completely switched off, it will still function if your iPhone is locked.
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Locking Your IPhone
To lock your iPhone, press the sleep/wake button on the top of the iPhone once, briefly. Your iPhone will also be automatically locked if you don't interact with the screen for a minute. When your iPhone is locked, the screen is switched off, and the iPhone will respond only if you press the home button or the sleep/wake button. However, you can still listen to music and receive calls, text messages, alarms and other alerts.
Setting an Alarm
Tap the clock icon on your iPhone's screen, and then tap "Alarm." Tap the "+" button to display the "Add Alarm" screen. Select the time when you want the alarm to sound, as well as whether the alarm should repeat, which ring tone it should play, whether you'll be able to hit snooze and an optional label for the alarm. Then tap "Save."
Editing an Alarm
Tap the clock icon on your iPhone's screen, and tap "Alarm." Tap the "Edit" button to display a list of all alarms you have previously set, and then tap an alarm to display the "Edit Alarm" screen. You can change the time when you want the alarm to sound, as well as whether the alarm should repeat, whether you will be able to hit snooze, which ring tone the alarm should play and an optional label for the alarm. Tap "Save" to save your changes when you're finished.
The ring tone you've chosen for the alarm always plays through the iPhone's speaker, regardless of whether you have headphones plugged in or not and of whether the iPhone is set to the silent mode. The only way to prevent an alarm from going off at the specified time other than canceling the alarm is to switch the iPhone off entirely by pressing and holding the sleep/wake button then sliding the red "Slide to power off" slider.