“Citizen” app users all know that feeling — when they get a notification, alerting them to a crime or incident going down in their area, or possibly just blocks away.

Now imagine waking up at 4 a.m. to a message about a fire, which is raging inside your own building. That’s exactly what happened to city resident Dan Humphrey.

The Fordham Law School student was in bed last September catching some shut-eye when he got the nightmarish alert.

“There was a smell of smoke as I got out of bed, grabbed my cell phone and headed to the bathroom,” he recalled in a Medium blog post on Wednesday.

“The notification I saw instantly brought me out of the hazy fog of sleep: ‘NEARBY INCIDENT’ Report of Apartment Fire at 385 Fort Washington Ave,'” Humphrey said. “I took a breath and looked down at the app again to confirm: this was my address.”

Feeling “through the darkness,” Humphrey walked to his front door and opened it to see what was happening.

“No alarms ringing, no lights flashing; the only thing unusual in the hallway was a woman standing there, just as confused as I,” he said. “We looked around the corner to find a member of FDNY hurling foot after foot of fire hose up the stairwell. ‘Is there a fire?’ I asked the FDNY officer, in a state of constant motion. She either didn’t hear me, or didn’t have time to respond. It certainly didn’t seem like a drill.”

Humphrey said he checked the Citizen app again and saw that the FDNY had “escalated the fire, dispatched more trucks and was sending more firefighters.”

“At this point, I knew I had to get both my sleeping girlfriend and friend on the couch out of my apartment and evacuate the building ASAP,” he remembered. “As the three of us were walking out of the building, FDNY members were charging into it. Exiting, my girlfriend looked up and saw flames shooting out of two windows on one of the top floors. Thankfully, the members of FDNY were able to expertly contain the fire to just two units, sparing much of the building major fire and water damage.”

FDNY officials confirmed Humphrey’s account on Wednesday and said no one was hurt in the fire. Humphrey thanked both Citizen and the smoke-eaters for helping save the day.

“It’s tough to say how much worse things could have been without the professionals at FDNY and the prompt notifications from the team at Citizen,” he said.