Jimmy Fallon, who is replacing fellow Saturday Night Live alum Conan O’Brien as the host of NBC’s “Late Night” talk show, says he looks forward to the looser format of late-night TV.

The most daunting competitor facing Jimmy Fallon, who takes over “Late Night” tonight with zero hosting experience and loads of public skepticism, isn’t Craig Ferguson, Jimmy Kimmel or even the ghost of Conan O’Brien.

It’s sleepiness.

Fallon will certainly enjoy a grace period in which many curious viewers will stay up past their bedtime to check out the former “SNL” star, whose greatest accomplishments to date are managing to get through a few skits without a giggle fit and convincing Drew Barrymore to play his love interest in “Fever Pitch.” But come fall, the new kid on the block will be following both Jay Leno in prime time and O’Brien’s revamped “Tonight Show,” daring even die-hard fans of his 2004 lemon, “Taxi,” to endure a third hour of topical jokes, outrageous sketches and stars pushing their latest masterpieces.

“People are always fighting sleep when they’re deciding whether to stay up for these shows or not,” said Peter Lasally, executive producer for “The Late, Late Show With Craig Ferguson” and a longtime confidant of both Johnny Carson and David Letterman. “Now we’re going to have five late-night shows in Los Angeles alone. It’s a whole new experiment, and it could shake things up tremendously.”

Just how Fallon intends to stand out remains a mystery. He’s been posting videos every weekday for months, but the bits look like outtakes from a home-improvement series, with Fallon oohing and aahing over everything from his band, The Roots, to the set’s giant screen, offering few laughs in the process. He’s been doing stand-up on the road ever since NBC tapped him, but a recent sample performance on “The Tonight Show” suggested that the monologue will be, at best, a work in progress.

“I think there’s going to be a lot of trial and error,” Fallon said. “We’re going to try stuff. That’s what’s fun about being on late. You can be looser and be more adventurous. We’ll try stuff that just hangs. And if it’s not funny, we’ll try it again. And if that’s not funny, we’ll do it one more time.”

In person, Fallon is extremely affable — strangers are addressed as “pals” — and he seems more intent on being personable than being a crackup. Great qualifications for a roommate, but it’s a suspect profile for a new talk-show host.

Of course, it’s dangerous to prejudge these kinds of shows, not to mention a tad unfair. The first to point that out is none other than Ferguson, who will be Fallon’s primary rival and has steadily climbed in the ratings since debuting in 2005 with little fanfare.

“I’ve heard some negative stuff about Jimmy, which I find a little surprising given the fact that he hasn’t done anything yet,” said Ferguson, who’s well aware that time and patience are key ingredients to late-night success. “During the writers’ strike, we played some really old shows on the air, and I watched a couple of them,” Ferguson said. “I was like, ‘That’s a very uncomfortable man.’”

The most famous rips-to-riches story stars O’Brien. When he debuted 16 years ago, the reviews were devastating, and many predicted he wouldn’t last a month.

“I hate to sound like the preacher of common sense, but I’ve told Jimmy that nobody who hasn’t done one of these shows every single day can possibly imagine what it’s like,” O’Brien said. “There’s no way you can go off in a cave somewhere and completely conceive and conceptualize your show, and then go on the air and start cranking them out. The only way to learn is by doing it. There’s no college they can send you to. It’s not always pretty to watch, but it’s the only way.”