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Florida officials have apologised for giving residents in the sunshine state an "unexpected" and "frustrating" pre-dawn wakeup.
Many residents were jolted awake by an blaring emergency alert on their phones at 04:45 local time (08:45 GMT).
But there was no real emergency on Thursday - a state agency made a mistake while testing its alert system.
The Florida Division of Emergency Management later admitted that a "04:45 wake up call isn't ideal".
The agency said each month it tests the alerts on a variety of platforms and "this alert was supposed to be on TV, and not disturb anyone already sleeping".
On Twitter, it vowed that it was "taking the appropriate action to ensure this will never happen again and that only true emergencies are sent as alerts in the middle of the night".
The message to residents read: "TEST - This is a TEST of the Emergency Alert System. No action is required."
According to St Lucie County, the alert was sent to every wireless subscriber in the state of Florida.
In a statement on Twitter, Governor Ron DeSantis called the early test alert " a completely inappropriate use of this system".
"I've ordered FL Division of Emergency Management Director Kevin Guthrie to bring swift accountability for the test of the emergency alert system in the wee hours of the morning," he said.
Angry Floridians took to social media to complain about being woken up before dawn, with one writing on Twitter: "I need to fight whoever decided to test Florida's emergency alert system."
With people sharing tips on how to turn the notifications off on their phones, the emergency management agency urged residents to keep the function on for public safety .
"We want to stress that while this wake-up call was unwarranted, disasters can happen at any time and having a way to receive emergency alerts can save lives," Alecia Collins, the agency spokesperson, said.
The UK is also preparing for the first nationwide test of its emergency alert system, though that test is scheduled for the midafternoon on Sunday.