The small hours of the morning — when the galleries were empty, hushed and dim — were Greg Kwiatek’s favorite part of his 25 years as a night guard at the Metropolitan Museum of Art, when he could spend hours looking at a single painting like El Greco’s “Christ Carrying the Cross,” Turner’s “Whalers” or ​Vermeer’s “A Maid Asleep.”

Then, shortly after sunrise, Kwiatek, now 74, went home to his rent-controlled railroad flat apartment in Hoboken ($557 a month) to work on his own paintings, which were often inspired by those he’d guarded at the museum.

Now Kwiatek’s work is on view, through May 14, in a small group show at Fierman Gallery on the Lower East Side.

“He developed a very intimate relationship with much of the collection and a lot of that has really permeated his practice,” said Alissa Friedman, who organized the show, “Everybody Knows This Is Nowhere,” which includes work by the artists Chonon Bensho and Amy Bessone. “Some of his works are direct homages.”

Working at the Met taught Kwiatek how to look. When the Met had a Francis Bacon retrospective in 2009, for example, Kwiatek said he logged about 70 hours.

“You get an hour to do a route,” said Kwiatek in a recent interview at his cramped garment district studio, referring to one of the museum’s seven sections. “I would do a route in maybe 40 minutes, and then I would have 20 minutes to focus on one piece. I got to know some paintings pretty well by doing that.”

Kwiatek is emblematic of a large but little-known swath of the art world — those who have never been famous and likely never will be but doggedly, passionately keep at it anyway.

His paintings are quiet and understated. He often makes versions of the same image repeatedly — in particular a series inspired by a 1906 photograph of Cézanne carrying his paintings. The small ones go for about $5,000; the larger ones for about $20,000. He also painstakingly sews needlepoint images, many of which echo his paintings of the moon and sun.

A tall, solid man from a Polish family in Pittsburgh, Kwiatek radiates the taciturn intensity of an introvert who would much rather be communing with paintings than human beings.

Indeed, this is what made Kwiatek gravitate toward the overnight shift at the Met in 1987, where he worked until retirement in 2011. “I’m not a people person,” Kwiatek said. “I figured by working at night, I wouldn’t have to deal with the public much.”

The schedule wasn’t easy — working from 12:15 a.m. to 8:20 a.m. and then going home to paint meant he was always tired. But the lifestyle suited him. And he took pride in the work.

“My job was to walk at least four hours a night,” Kwiatek said. “You know every square inch of this building — you’re doing surveillance. You cover every gallery, every catwalk, every roof, cellar, offices, bathroom. You’re looking for fire and water and so on.”

It’s been over a decade since Kwiatek last walked these routes, yet the Met’s physical plant remains in his bones. “Route Three includes European painting, painting conservation, Japanese art, musical instruments, Arms and Armor,” he said. “The Rockefeller wing, that would be Route Six. You’re looking at all the cases. You’re looking at that boat that’s hanging from the ceiling.

“We’re drinking 20 cups of coffee a day. I would sleep an hour on my lunch break at 4 o’clock in the morning,” he continued. “You’re living with works of genius. And I’m not a genius. But I knew that what I had the privilege of guarding — it was otherworldly.”

Kwiatek was featured in Alexandra M. Isles’s 2011 documentary “Hidden Treasures: Stories from a Great Museum” talking about the layers in El Greco’s “View of Toledo.”

“From this point of view this modest size painting looks like a very large painting, the details are not clear — is it a landscape? Is it an abstraction? Maybe it’s a mirage,“ Kwiatek says in the film. “The hidden subtleties will not reveal themselves unless one is willing to come back time and time again and live with this work in an extended relationship.”

Growing up near Polish Hill in Pittsburgh — where his father worked in the steel mill, and his mother crocheted in front of soap operas — Kwiatek got his first exposure to art at the historic Immaculate Heart of Mary Church, which was influenced by Baroque and Renaissance architecture.

“The entire church was a work of art,” he recalled.

After high school, he went to commercial art school and came to New York at 21, where he lived in a YMCA, got a job doing paste-ups for magazines and worked on his own illustrations at night.

In 1969, he landed an appointment with a successful illustrator for Gordon’s gin who looked at his portfolio and told him, “‘You’re no illustrator, you’re a painter,’” Kwiatek recalled. “That changed my life.”

He spent two years at the Art Institute of Pittsburgh, then went to work for the American Can Company in Easton, Pa. His landlady was a Sunday painter who suggested he visit the van Gogh exhibit at the Philadelphia Museum of Art.

“I knew that I was seeing something,” Kwiatek said, “but I really didn’t fully understand it.”

In 1975, he traveled to Amsterdam for two months, where Kwiatek went to the Rijksmuseum or the Van Gogh Museum every day. In 1977, he worked for a year as a merchant marine on oil tankers.

After five semesters at Carlow College in Pittsburgh, Kwiatek applied to Carnegie Mellon, where he earned his bachelor’s degree in fine arts in 1979.

He met the gallerist Alfred Kren, who decided to represent Kwiatek in his Cologne gallery and to set him up with an apartment and studio there, though the artist moved back to the U.S. when the two fell out.

Dirk Schroeder, a lawyer in Cologne, has more than 60 of Kwiatek’s paintings and drawings; his first purchase holds pride of place over his dining table, Schroeder said in an email, “so that I can see it every day.”

Kwiatek’s “paintings are ample proof of how much he absorbed from the masters he studied at night at the museum,” he added. “His paintings grow on the viewer not only in the short term, but over the years.”

He has along the way won several grants, including ones from the John Simon Guggenheim Memorial Fellowship and the Pollock-Krasner Foundation.

His work has been exhibited periodically, mostly in group shows, including one last year at the Elizabeth Foundation for the Arts — where his studio is located — and one at Zwirner in 2008.

“He was an extremely nuanced colorist in his abstractions — they were very mysterious paintings,” said the dealer David Zwirner, who owns several Kwiatek pieces. “There was such maturity to the work and this kind of intensity. There is probably not another person who’s looked at art longer than Greg.”

Kwiatek never married or had children. His whole life has been art, aside from a preoccupation with classic films (he has watched about 650 in just the last three years — “The Third Man,” “Casablanca,” “Roman Holiday”). His needs remain few; his ambitions modest.

“I’d like to make enough money so that I can afford to keep myself off the street,” Kwiatek said. “Maybe buy a studio apartment.”

Everybody Knows This Is Nowhere

Through May 14, Fierman Gallery, 19 Pike Street, Lower East SIde/Chinatown, 917-593-4086; fierman.nyc.