In a TV season when NBC is starved for good news at night, Jay Leno (search) is providing some.

His "Tonight" show has widen its lead over chief rival David Letterman (search), a noteworthy achievement given CBS' prime-time dominance and NBC's fading fortunes. The typical "Tonight" audience has increased by 4 percent this year to 5.6 million viewers, while the "Late Show" is down 5 percent, according to Nielsen Media Research.

NBC believes Leno — who's planning to turn "Tonight" over to Conan O'Brien in 2009 — has more nimbly responded to the changing late-night landscape, where viewers have many more choices than Jay, Dave and the soon-to-be-gone Ted Koppel.

"Jay's show is different every night," said Mark Evanier, a veteran TV series writer who has his own entertainment Web log. "I find it a better party to be at."

Leno has won consistently for a decade now, but during that time Letterman's camp has complained it wasn't a fair fight. For years, NBC was the stronger prime-time network, and CBS said that meant more passive viewers kept sets tuned to NBC. CBS' loss of pro football during the 1990s (the National Football League is back now) also meant fewer opportunities to promote Letterman to young men.

To executives at NBC, those always smelled more like excuses than explanations, particularly now that those disadvantages are gone.

"Obviously, we're thrilled to see that," said Rick Ludwin, who supervises late-night programming for NBC.

CBS said it's too early to draw conclusions, that this year's numbers are skewed by Letterman's odd decision to take off the second week of the TV season. Letterman is averaging 4.1 million viewers this season; ABC's "Nightline" and Jimmy Kimmel are also down in the ratings this season.

While NBC is having a bad year in prime time, it's still strong at the 10 o'clock hour that leads into local news and the late-night comics, said David Poltrack, CBS' chief researcher. The move of "Medium" to 10 p.m. on Mondays has improved NBC's competitive position, he said.

But the ratings can't be a heartening development for the people at "Late Show," who have never quite believed that more Americans consciously choose Leno.

"Dave is definitely not the essential viewing that he was even two, three years ago," said Aaron Barnhart, critic for the Kansas City Star and longtime expert on late-night TV.

Letterman is now a traditional, old-school television choice, said Barnhart, who, like many critics, is no fan of Leno's.

Hipsters have turned elsewhere; the late-night show to get the most attention this season is Stephen Colbert's send-up of a clueless cable television host on Comedy Central.

"I watch (Letterman), but I don't TiVo it," he said. "I don't feel a sense of urgency around the show — that has shifted now to Jon Stewart's show. Whether that's fair or not, the fact is that it's happened."

Leno has tried to freshen his show by adding more comic contributors. Howie Mandel's hidden camera hijinks are a regular feature, Steve Bridges is a resident Bush impersonator and Fred Willard, Gilbert Gottfried and Gary Busey have made frequent appearances. Leno has given TV debuts to cartoons from JibJab.

"You have to continually say to viewers, `You haven't seen this yet, you have to come back,'" Ludwin said. "You have to give even your regular viewers new reasons to come back."

As for Letterman, Evanier said he reaches for the remote when he sees another installment of the "Will It Float?" segment. Another strange favorite of Letterman's this year involves continually pushing a man in a bear suit into a closet. He also — possibly as a genuflection to idol Johnny Carson — does a "Stump the Band" segment, which dates back four decades.

"I think that Mr. Letterman's greatness is an established fact," he said. "But he's also kind of mined his bag of tricks as far as they can go. It's repetitive. I like to watch them, but I can't watch them every night."

The workaholic Leno takes infrequent breaks and makes new shows five days a week. Letterman frequently tapes his Friday night show a day early. He was on vacation again last week, although he taped two new shows in advance for Thursday and Friday, the start of November ratings sweeps.

The late-night institutions have gradually evolved to become less talk shows, and more comedies. Way back when the shows were 90 minutes, serious authors and scientists were guests, too. Leno's monologue has grown from five minutes to 12. The first guest never appears until after midnight.

Research reveals that viewers are more likely to stay up if they're promised comedy is on the way, Ludwin said.

"Late Show" executive producer Rob Burnett isn't so sure. He finds that Letterman's audience is loyal, that it sticks with the show throughout.

"Obviously, Dave isn't new and yet at the same time I constantly find college kids and high school kids who are discovering Dave and adding to his fan base," he said.

He cautions against counting Letterman out in this eternal competition and is expecting a strong November, including a guest appearance by Jennifer Aniston.

"If CBS and NBC continue in their current models, Dave will beat Jay," Burnett said, "and I also don't think it matters. Dave's place in talk show history is secure no matter what happens."