Setting a wake up alarm runs the battery down to almost nothing overnight

The Clock still sucked the battery after a reboot.

I just noticed this in the last two days.

But, I had never used the stopwatch feature of the clock before two days ago.

When I opened the clock app, the screen was sitting on the stopwatch screen (not running).

Perhaps, if the clock sits idle on the stopwatch screen, it drains the battery.

Today, I moved the clock focus off of the stopwatch screen to the world clock screen.

Hopefully, that might slow the clock sucking the battery?

Best practice is to plug the phone in at night. If the phone is plugged in, connected to WiFi and the screen is locked, it will back up to iCloud. And, it will be fully charged in the morning and ready to go,

The built in alarm clock or a 3rd party alarm clock you downloaded from the app store?

Have you tried resetting the phone? Press and hold both the Home button and Sleep button and wait 10 seconds or so until you see the Apple logo.

Turn your iOS device off and on (restart) and reset

There is probably something else that is eating up your battery, I use the same alarm clock and my battery doesn't drain.

go to settings - general - usage - battery usage - and see what app is draining yoru battery.

UUse the Settings app to determine what is using your battery.

SEttings > General > Usage > BattEry Usage

That setting shows you the last 24 hours and last 7 days. What is listed at the top of that list for 24 hours? There cannot be Nothing else in that list.

Some other process is stuck then, that is causing the device to drain. If the reset does not work, then I would suggest attempting a restore from backup first, and if necessary, as new. There is no way the alarm can drop that battery too much. How much are you loosing overnight. You never state that.

Yeah, something else is running or stuck trying to run. If a reset does not do it, you probably need a restore. Make sure all of your data is synced to a supported application as the problem could be in your backup and you would need to restore the device as new, and then sync individual content back.