Does the Alarm Still Work If Your iPhone Is Turned Off?by John Papiewski
The iPhone's clock has a handy alarm feature you can use to start your morning when you're away on business. However, if you set the alarm and then turn the iPhone completely off, the alarm will not sound. In all other cases, such as when the iPhone is in sleep mode, you'll hear the alarm at the correct time. Spending a few minutes familiarizing yourself with the iPhone's various alert modes will help you get the best use of the alarm feature.
When you're actively using the iPhone's apps, the phone is awake; its screen is on and responds to your touch commands. In this mode, the iPhone can sound any of its alerts, including the clock alarm. The iPhone remains awake according to the time you set in Auto-Lock in the Settings app. Unless you specifically set the phone not to auto-lock, it automatically goes into sleep mode after a minute or two, saving battery power.
In sleep mode, the iPhone's screen is dark and inactive. During the course of a normal day, your phone spends most of its time in this mode. Although it appears off, the iPhone's is still working at a reduced level. Any important event, such as receiving a call, turns the screen on momentarily. The phone's calendar, reminders and timers also continue to run during sleep mode, so the alarm will sound in this state.
You can power the iPhone off by pressing the "Sleep/Wake" button for a few seconds and then sliding the slide control that appears. In this mode, the iPhone is completely inactive; it does not receive calls, sound alarms or perform any other action until you turn the phone back on. When you set the alarm time before retiring for the evening, do not turn it off; simply set the iPhone down. It will automatically go into sleep mode and wake you at the right moment.
When you don't want ring tones or most other alerts to disturb a quiet setting, slide the iPhone's "Ring/Silent" switch on the side of the phone to the silent position. This does not silence the clock alarm; it will still go off. The Do Not Disturb setting works in a similar way. When enabled, it blocks calls and normal alerts, but the clock alarm still sounds.
Information in this article applies to an iPhone running iOS 6. It may vary slightly or significantly with other versions.