Adult Swim via New York Times News ServiceAdult Swim recently added the show “Rick and Morty,” an animated series about an alcoholic scientist and his bad influence on his grandson. The network is expanding again by moving up an hour and starting at 8 p.m.

The big developments in late-night television are not all happening on NBC this week. Adult Swim, the network that is, in many ways, the new force in late night, has some significant news, too: It is expanding again.

Though its programming usually draws far less media attention than, say, the changing of the guard on NBC’s “Tonight” show, Adult Swim has amassed enviable viewership numbers. Among the audience groups many advertisers want most to reach, Adult Swim beats just about everything else in late night.

The network is adding another hour of programming to its schedule, moving further backward into prime time. Starting March 31, Adult Swim — a mix of animated and live-action shows, replete with the kind of irreverent humor that has been the stock-in-trade of late-night television for half a century — will offer programming from 8 p.m. to 6 a.m. every day.

In explaining the move, Stuart Snyder, the president of Animation, Young Adults and Kids Media for Turner Broadcasting, said the time was right for the additional hour “coming off a record-setting ratings year in 2013.” Not to mention the “increasing demand we are receiving from advertisers,” he said.

Adult Swim is the top-rated cable network in total day ratings among viewers in the two younger-adult categories, viewers between the ages of 18 and 34 and 18-49.

In the late-night period, from 11 p.m. to 1 a.m., the network dominates in the youngest end of that competition. With the exception of recent viewership numbers for Jay Leno, who has been racking up steady wins as he prepares to end his tenure as “Tonight” show host this week, Adult Swim has beaten all late-night competitors in the 18-49 category. During the last 12 months, it has been No. 1 with that group, averaging 1.15 million viewers a night to 1.05 million for Leno.

All the other late-night hosts, including David Letterman, Jimmy Kimmel, Jimmy Fallon, Jon Stewart and Stephen Colbert, are behind the Adult Swim lineup, which consists of a hodgepodge of shows with titles like “Childrens Hospital,” “Aqua Teen Hunger Force” and “Your Pretty Face Is Going to Hell.”

Perhaps most impressive of all, the network is especially strong in drawing young male viewers, which generally attract a premium price from advertisers because they are so difficult to reach. Adult Swim’s audience is about 60 percent male, and it is spectacularly young-skewing. The median age for an Adult Swim viewer is 22.9. By comparison, Leno’s show has a median age of 57.8. The only late-night host with a median age under 40 is Conan O’Brien at 39.6.

Since its start about a decade ago as a two-hour block starting at midnight, Adult Swim has steadily built a following with young male viewers, starting with a lineup of acquired animated shows like “Family Guy” (which is still a bulwark in its late-night lineup). It began adding original shows (like “Space Ghost Coast to Coast” and “Robot Chicken”) and expanding, first to more nights in late night, and then backward into prime-time.

The original idea for Adult Swim sprang from the need to find an audience of nonchildren for a cable channel that was the Cartoon Network during the daytime hours, Snyder said. It broke ground in offering program creators different formats (like welcoming shows only 15 minutes long) and by importing shows that had been first created as series on the web (like “Childrens Hospital”).

Now, Snyder said, the 10:30 time period is being used to introduce new programs that will migrate to other areas of the schedule.

This season the channel added the show “Rick and Morty,” an animated series about an alcoholic scientist and his bad influence on his grandson, co-created by Dan Harmon of “Community” on NBC. On since December, it looks like a hit for Adult Swim, scoring better 18-34 and 18-49 ratings than “Community.”

The plan for the 8 p.m. hour is to begin with some repeat programs (Adult Swim has a library of animated series like “King of the Hill,” and “American Dad”) and gradually add in original series.

“We have a lot of shows coming down the pike,” Snyder said.

Many of these are typified by the raucous, unapologetically irreverent style of “Your Pretty Face Is Going to Hell,” which features the daily foibles of the horned denizens of Hades, who are depicted as harried office workers under Satan’s thumb. (And that one is not a cartoon.)

Another thing that Adult Swim has going for it, Adgate said, is that its young viewers tend to watch a lot of the shows live on the air, rather than delaying them for later viewing. Adgate suggested the network may be fitting into a pattern of viewing behavior for younger men.

“They watch the Adult Swim” shows live and then the next day they catch up on the clips of the best from Jimmy Kimmel, Jimmy Fallon, Jon Stewart and Stephen Colbert,” Adgate said. “It’s the new way of consuming late-night television.”