CINCINNATI — By the time the clubhouse doors opened Monday morning, it was difficult to discern whether the Mets were prepping for a ballgame or making cameos on “The Walking Dead.” Coffee cups were ubiquitous, as were the blank stares and deliberate gaits brought about by sleep deprivation.

Sunday night’s nationally televised game against the Nationals forced the Mets to play the Labor Day slog against the Reds on short rest, though in the end, it hardly mattered. Playing with a lineup filled with backups, a concession to fatigue, the Mets beat the Reds, 5-0, to remain within one game of the Cardinals for the second wild-card spot. They are only 1 1⁄2 games behind wild-card leader San Francisco.

“It was such a win on so many different levels,” said Kelly Johnson, who homered.

Not even a charter flight through the night could undo the Mets, who got their biggest contribution from their sleepiest player.

Matt Reynolds, called up from Triple-A to add depth, did not arrive in Cincinnati until Monday morning, about four hours behind the rest of his teammates. But he went 3-for-4 with a solo homer, an RBI single, an infield single and two runs scored.

“It was great, it was everything I could ask for,” said Reynolds, the latest bit player to come up big for the Mets. “I just want to go out there and help the team win.”

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Johnson smacked his 10th homer, extending what has been a monthlong power surge. The Mets (72-66) put the game away with a three-run seventh.

Bartolo Colon had flown to Cincinnati ahead of the team. The rest seemed to help the 43-year-old marvel, who allowed five hits and a walk in six innings. Twice he stranded runners at third base, even though they reached with nobody out.

“Getting ahead of the hitters was the key,” Colon (13-7) said through a translator.

The Mets have won 12 of their last 16 games. After stumbling out of the gates in the second half, they evened their record at 25-25 since the All-Star break.

The Mets did not start Jose Reyes, Yoenis Cespedes and Asdrubal Cabrera, who had a pinch-hit RBI single in the seventh. Cespedes and Cabrera have been given days off to rest lingering leg injuries. Reyes has been a staple in the lineup since coming off the disabled list.

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“With the turnaround the way it is last night, if I played a couple of guys today, I’m asking for them to get hurt, in my opinion,” manager Terry Collins said before the game. “I can’t have that. We’ve still got a long stretch to go here and I need those guys ready.”

As has been the case lately, everything worked out for Collins, who watched his lineup of replacements thrive although the team reached the hotel at 3:15 a.m. and reported for duty at 11 a.m.

Reynolds endured an even worse travel day to make it in time for the game. He boarded an 11 p.m. red-eye from Salt Lake City to Boston, where he changed planes to get to Cincinnati.

Kept awake by a chatty seatmate, Reynolds cobbled together barely two hours’ sleep. About a half-hour of it came from the morning nap he took in his hotel room upon arriving at about 9:15 a.m. By 11 a.m., he was sitting in a quiet corner of the Mets’ clubhouse, scrambling to get dressed and track down a cup of coffee.

Reynolds found himself in the starting lineup at shortstop and performed well, just as he did under similar circumstances when he homered in a Subway Series game against the Yankees. He had taken a red-eye then, too.

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Said Reynolds: “If I keep hitting like that, I’m going to keep taking them.”