Filling the holes in NBC's fall primetime calendar just got a lot easier. When Jay Leno hands over the Tonight Show reins to Conan O'Brien next year, he won't be riding off into the retirement sunset like Johnny Carson.

Instead, Leno will simply start 90 minutes earlier, on an hourlong show that will air five nights a week at 10.

Leno's new timeslot is a brilliant move. NBC Universal, majority-owned by General Electric (NYSE:GE), won't have to worry about Leno defecting to a rival network, like David Letterman did when he vaulted to CBS (NYSE:CBS). The new Leno show also helps plug NBC's programming gaps at a time when networks are trying to cut costs.

It's far cheaper to produce a variety show than one of the slick dramas or cop shows that typically air in that slot. Celebrity guests and musical acts are honored to be invited to appear on talk shows for promotional purposes, while successful scripted shows often involve escalating salaries and egos. With only two hours of programming to fill every night instead of three, NBC can also be more selective. This may explain why NBC was quick to push a few electorally themed Saturday Night Live installments into primetime two months ago, gauging its audience's appetite for late-night programming earlier in the evening.

So why isn't CBS moving Letterman to 10 p.m.? Why isn't Disney's (NYSE:DIS) ABC giving Jimmy Kimmel free rein before the 11 p.m. news? Well, history isn't entirely kind to such monolithic programming blocks.

Anyone remember Who Wants to Be a Millionaire? ABC's hit primetime game show was so popular -- and ABC's programming so uninspiring -- that it eventually aired for as many as four nights a week. The overexposure killed the show as a primetime staple.

Still, couch potatoes don't seem to tire of watching the same hosts at 11:30 every night. Why should 90 minutes make a difference? Advertisers will appreciate the consistency. Audiences won't have to check the TV listings to know what goes on at that slot. Everybody wins, right?

We'll have to see. Network television is an industry of copycats. How many singing-and-dancing talent competitions are on the air? How many spinoffs have fertile franchises like CSI and Law & Order birthed? If this works, you know that CBS, Disney, and News Corp.'s (NYSE:NWS) FOX will undoubtedly follow suit.

The losers, naturally, will be the writers and actors of half-hour sitcoms and hour-long dramas, who'll have less leverage in angling for fewer gigs. Fans of such scripted dramatic series also might be irked. However, at a time when advertisers are scaling back, and broadcasters like CBS and Time Warner (NYSE:TWX) are trading in the single digits, slotting Leno in at 10 p.m. is the right move at the right time.

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Longtime Fool contributor Rick Munarriz hasn't watched Leno in ages, because his wife has Comedy Central locked on the set at that time. Rick is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. He does own shares in Disney. The Fool's disclosure policy is more of a Jon Stewart fan.