Empires rose and empires fell, but if there was one thing the world could depend on, it was the 8 p.m. curtain time for theater, opera, and classical music performances. That, however, was back in the 20th century.

In the 21st century, if you don't check daily listings, you may be late for Act One. Though the Philadelphia Orchestra holds steady with music at 8, a weeknight performance at the Wilma Theater, the Arden Theatre, Philadelphia Theatre Company, or 1812 Productions may start as early as 6:30. For Sunday-night touring Broadway shows at the Academy of Music, all traditional bets are off.

"Maybe we're becoming more health-conscious and realizing that sleep is important," said Matt Glandorf, director of Choral Arts Philadelphia, which launched a concert series this season called "Bach at 7." "People are putting more boundaries on their free time."

"The world has changed," agreed Matt Wolf, director of the Kimmel Center's Broadway Philadelphia programming, "and time is so precious now."

Theories are everywhere. We're getting older and want to go to bed early. We're getting younger and want to stay out later after the theater. For the family shows, children may aspire to be night owls, but the parents who pay for the tickets know better. With a 6:30 p.m. Sunday Lion King curtain, nobody stays up past bedtime.

Everybody likes seamlessness. At Philadelphia Theatre Company's 6:30 p.m. Tuesday show, Ann and Pete Certo, a pair of Center City professionals who live in Marlton, said 8 p.m. is awkward. "Do you drive home and stay there for a little bit before coming back?" said Pete. Or go to a 6:30 show and get the babysitter home early?

"Since we do plays that encourage dialogue," says Wilma managing director James Haskins, "people enjoy going out afterward and discussing it among themselves."

Once one of the slower-selling nights of the week, Tuesday is now up there with Saturday.

Working-all-hours medical professionals and working-any-hours techies need flexible curtain times. And Sara Garonzik, executive producing director of the Philadelphia Theatre Company, is happy to do what she can. "We've done everything but midnight," she says, "and maybe that might work at summer comedy shows."

When performances are taken out of the middle of the evening, spontaneity seems more possible. In October, on only a few hour's notice, the Philadelphia Orchestra was able to fill Verizon Hall for a free 6:30 pop-up concert, announced via social media after a Carnegie Hall date went south due to a stagehand strike.

"We deliberately timed it as a test to see what effect that might have," said orchestra spokeswoman Katherine Blodgett. "We had a lot of families show up. Several tell us that it was a good time for them to bring children."

It also worked for retired bartender Annette Lombardo, who lives a few blocks from the Kimmel Center and had never heard the orchestra. "I was getting my mail downstairs when a neighbor of mine said, 'Annette, are you going to the concert? Tickets are free.' It was about 5 p.m. and I ate real fast and got there by 6 to get a nice seat," she says. "I was so happy."

Of course, change in Philadelphia tends to generate resistance, often habitual. "At the beginning, people missed curtain times," said Haskins at the Wilma, "even though the curtain times have been stated time and again."

Some of the resistance is practical. When the Mendelssohn Club of Philadelphia timed its first collaboration with the Kimmel Center organ for 7:30 p.m. on a Friday this fall - director Alan Harler aimed to attract young professionals who want to do several things on an evening out - one Quakertown organ devotee barely got there due to rush-hour traffic.

The once-golden Friday night slot now is more like silver. The lush life of decades past when people had two-martini lunches and considered Friday an obligatory party night is less evident.

"People who attend performances generally work very hard," says Wolf, "and the reward for them is to spend time at home on Friday with the wife, husband, or children."

"Friday is the new Tuesday," says Philadelphia Theatre Company marketing and communications director Amy Lebo. "It's not horrible. But some of the restaurants tell us that the weekend starts early, on Thursday" - another reason Friday audiences have been diluted.

Numbers don't always tell the tale because long-term trends may be offset by what Wolf calls "the personality of the show."

"For our one-week engagements, it can vary by title. Friday evenings can sell anywhere from a few hundred tickets above Thursday evening to a few hundred below Thursday."

The Philadelphia Theatre Company charts trends by the percentage of total sales. When Tuesdays were still at 8 p.m. in 2008-09, the slot accounted for 9 percent of the total sales. Friday was 14 percent. Last season, Tuesday and Friday were tied for 13 percent.

While Friday has waned, the so-called Sunday-matinee ladies now have company. "Instead of bike riding or going to the movies, we have twenty- and thirtysomethings who live in Center City walking over to the theater," says Lebo. "We also see girlfriends who have been out shopping and fathers who have their kids for the weekend." That slot claimed 18 percent of total ticket sales in the 2008-09 season but 20 percent last season (2012-13).

Not to be overlooked is the changing function of theater in society. In the 18th century, operas were like modern sporting events: Audiences listened, talked, and ate. Mini-operas or organ concertos were performed at intermission like halftime shows. Since then, theater performances have shrunk from five acts (19th century) to three (20th century) to the current trend of an intense, intermission-free 90 minutes that lends itself to more flexible start times.

Still, more attentive modern audiences still happily immerse themselves in extended epics: The Wilma's Angels in America, a major event of 2013 whose two sections run longer than three hours each, sold better than many shorter plays.

Marketing considerations aside, a decisive modern wrinkle can be technology. The Metropolitan Opera's live movie-theater simulcasts start at 12:55 p.m. East Coast time to avoid backstage gridlock at Lincoln Center between afternoon and evening performances of different productions.

That means West Coast audiences have to face Wagner at 9:55 a.m. Yet they seem undeterred. Why else would a smaller city like Sacramento have Met simulcasts in four theaters? In other words, if audiences want the show badly enough, any time may be a good time.