Hayden Panettiere

Heroes' life span will be a little shorter next season.

NBC plans to cut the number of episodes of its superhero drama from 25 this season to 18 to 20 in its fourth season, Ad Age reports.

The reasons for the cutback are legion: Though the show's latest episodes have reinvigorated many fans, a series of complicated and wide-ranging storylines cost the show ratings in Season 2 and earlier this season.  

Also, NBC will have five fewer primetime hours to fill in 2009-10 season, thanks to Jay Leno's new hour-long program at 10 pm weeknights. And other networks are also experimenting with condensed seasons: ABC's Lost has kept to 16 or 17 episodes in recent seasons, and CBS's Harper's Island has a 13-episode format that could be repeated (with a new setting and cast) if the murder mystery succeeds with viewers.

Though Heroes debuted to strong ratings in 2006, since the 2007-08 writers strike its viewership has slipped to about 7.7 million viewers on average, four million fewer than it earned before the work stoppage, according to Nielsen.

The show's writers have already started planning for the show's fifth volume, which will involve a series of mysterious deaths. Or could they be murders?