Updated
You had the same alarm app for months. Rock solid. Then one morning it just did not go off, and nothing you did the night before looked any different.
On Samsung, Xiaomi, and most other Android phones, this is almost always battery management. It shows up most often right after a system update or a reboot.
What is actually happening
Android already puts unused apps to sleep to save power. On top of that, each manufacturer adds its own layer. Samsung has One UI battery limits, Xiaomi has MIUI, and others do the same. These layers can decide overnight that your alarm app does not need to run, and stop it. The app still looks fine. The alarm is still set. The system is quietly not letting it fire.
The reason it often starts after an update is simple. A system update can reset your battery settings back to the defaults, which are aggressive, and the exception you set earlier is gone.
How to fix it on Samsung and Xiaomi
Open Settings, then Battery, then Background usage limits. If your alarm app is listed under Restricted or Sleeping, move it to Unrestricted. Then, in Settings, Battery, look for the option that automatically puts unused apps to sleep and add your alarm app as an exception.
On Xiaomi, open Settings, Apps, Manage apps, pick your alarm app, and set Battery saver to No restrictions. Turn on Autostart for the same app.
Every manufacturer buries this in a slightly different place, and that is half the problem. The goal is always the same one thing. Take your alarm app out of battery optimization.
Check the volume too
While you are in there, confirm the alarm volume. Android keeps media volume and alarm volume separate. If the alarm volume is down, the app can be running perfectly and still play too quietly to wake you. Settings, Sounds and vibration, Volume, and raise the alarm slider.
The part you cannot see
None of these settings show any sign the night before. The alarm reads on. It looks fine. You only find out that battery management stopped the app on the morning it fails.
That is why we built Snora. Snora is an alarm you set like any other, and before you sleep it checks the permissions and settings your alarm depends on, then shows one status for whether tomorrow's alarm is set up to go off: 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. Device coverage is still expanding, since every manufacturer behaves differently.
Whatever alarm app you use, the fix is the same. Take it out of battery optimization, and raise the alarm volume. Do it tonight, and check it again after the next system update.
FAQ
Why did my Android alarm stop going off after a system update?
A system update often restores battery settings to their aggressive defaults, so the exception you added earlier is wiped and your alarm app gets put to sleep again. Re-add it under Settings, Battery, Background usage limits and set it to Unrestricted. On Xiaomi, updates can also clear Autostart and Battery saver, so turn Autostart back on and set Battery saver to No restrictions.
How do I stop my Android phone from killing my alarm app in the background?
Take the alarm app out of battery optimization so the system stops putting it to sleep overnight. On Samsung, go to Settings, Battery, Background usage limits and set the app to Unrestricted. On Xiaomi, open Settings, Apps, Manage apps, pick the app, set Battery saver to No restrictions, and turn on Autostart. Because these settings give no sign the night before, a pre sleep check like Snora can flag when battery optimization is still limiting your alarm.
Does Do Not Disturb stop an Android alarm from going off?
Do Not Disturb normally lets alarms ring, so it is usually not the reason an alarm stays silent. The exception is a total silence mode or a custom rule that mutes everything, which you can review under Settings, Notifications, Do Not Disturb. If alarms are already allowed there, the cause is more likely battery optimization or a low alarm volume.
Why does my Android alarm ring but play no sound?
Android keeps alarm volume separate from media and ring volume, so an alarm can run on time and still be too quiet to wake you. Raise the alarm slider under Settings, Sounds and vibration, Volume. If it still sounds wrong, check whether audio is routing to a Bluetooth speaker or earbuds you left paired overnight.