Updated
Short answer. On a stock iPhone, a normal alarm from the Clock app will still ring with Do Not Disturb or a Sleep Focus turned on. Apple designed alarms to break through. So most nights, Focus is not your problem.
The trouble starts at the edges, and those edges are exactly where people get caught.
When a Focus does mute your alarm
A custom Focus with tight settings. Do Not Disturb lets alarms through by default. A Focus you built yourself does not always inherit that. If you set up a Work or a custom Focus once and forgot about it, it can be running on a schedule tonight with rules you no longer remember.
A third party alarm app. The Clock app is trusted to break through. A separate alarm app can play its sound like a normal notification, unless it is allowed to bypass Focus, and a notification is exactly what Focus is built to silence. If your reliable alarm lives in another app, Focus can mute it.
Scheduled Focus you forgot. This is the most common one. A Sleep Focus tied to your bedtime, or a schedule you set months ago, turns on every night without you thinking about it. The alarm still shows on. You just cannot see that a rule is waiting to quiet it.
How to check tonight
Open Settings, then Focus. Read what is actually scheduled to run tonight. For each Focus, check whether time sensitive notifications and your alarm app are allowed through. If your wake up alarm lives in an app other than Clock, that is the allowance to confirm: add the app to the Focus allow list, and check it against every Focus that runs at night.
While you are there, check one more thing that gets blamed on Focus but is really separate. Your ringer volume. Settings, Sounds and Haptics, and make sure the Ringtone and Alerts slider is up. A muted alarm often has nothing to do with Focus and everything to do with a slider sitting at the bottom.
The catch
You have to remember to check, every night, and none of it is visible when you set the alarm. The Focus is scheduled quietly. The alarm looks on.
That is what Snora does before you sleep. Set your alarm in Snora, and before you sleep it checks what can keep your alarm quiet. It reads the permissions it can see and walks you through the settings it cannot, like your Focus schedule, then shows one status for whether it will actually ring: Safe, Caution, or Risk. It is a status, not a score to chase, and it is not a promise about your morning. The alarm and the check are free, and no account is needed.
If you have ever slept through an alarm that was on, a forgotten Focus is worth ruling out tonight.
FAQ
Will an alarm go off on Do Not Disturb on Android?
Yes, on most Android phones the default Clock alarm still rings when Do Not Disturb is on, because alarms are treated as an exception. You can confirm this under Settings, then Sound and vibration, then Do Not Disturb, where Alarms should be allowed. The exact wording varies by manufacturer, and a separate alarm app may still need its own permission to sound.
Does silent mode turn off your alarm?
No, flipping an iPhone to silent does not stop a standard Clock alarm, which is built to play even when the ringer switch is off. Silent mode can still quiet a third party alarm app that sounds through notifications. Because Do Not Disturb and the mute switch are two different controls, it helps to rule out both when an alarm stays quiet.
What is the difference between Do Not Disturb and a Sleep Focus?
Do Not Disturb is a built in Focus that lets alarms through by default, while a Sleep Focus is a separate mode tied to your bedtime that can carry its own notification rules. Both can run on a schedule at night, so the mode quietly muting an app based alarm is not always the one you switched on yourself. Reviewing each under Settings, then Focus shows which is active and what it lets through.
How can I be sure my alarm will ring before I fall asleep?
Check every Focus scheduled to run overnight and make sure your alarm app sits on its allow list before you fall asleep. In Settings, then Focus, look over tonight's schedule and confirm that time sensitive notifications and your wake up app are permitted. Snora runs this kind of pre sleep check and shows one status, Safe, Caution, or Risk, while the final settings stay yours to confirm.