At least 30% of viewers forLost, The Office and Heroes were procrastinators, watching either later that night or up to seven days after the programs first aired.

While "live" audiences for most shows continue to dwindle, delayed viewership from the devices, which digitally record programs and store them on a hard drive for later viewing, is filling some of that void. Nationally, 31% of homes had them in May, up from 25% a year earlier.

Many of the top shows also are the most frequently recorded: Nearly 5 million of Idol's 26 million viewers on Tuesdays delayed the show, most for just a few hours.

And while kids, teens and seniors use DVRs least often, women ages 25 to 39 are the most frequent recorders, so programs aimed at that crowd tend to get the biggest ratings bump when delayed viewing is factored in. Grey's Anatomy, House, Lost, 24 and Desperate Housewivesrank among the most time-shifted series, and a third of the 3 million viewers for CW's 90210 see the show after it first airs. Overall, 40% of the prime-time viewing in homes with DVRs is delayed, with network series far outpacing cable.

"DVRs have really changed the way people are watching television," says ABC prime-time research chief Larry Hyams. "They're recording shows they would have missed ordinarily and playing them back whenever they have a free moment." All that playback "really contributes to the rating of a show," he says.

Yet networks don't get paid for all that extra viewing. Most advertisers pay based on ratings for commercials seen within three days of airing, but networks often tout ratings for programs up to seven days after their premieres.

Among new shows, Fringe was recorded most often, by one in four of its 11.6 million viewers. CBS' The Mentalist counted 2.7 million recorders, but that was just 14% of its total.

But recording activity has as much to do with time slots as programs. DVRs are used most often at 9 p.m. ET/PT Thursdays, where CSI, Grey's and The Office are among the top 10 shows stored for later viewing. And Friday, a night out for target younger viewers, led 1.5 million fans of Fox's Dollhouse, a third of its total audience, to press the record button. (The show won a reprieve and was renewed for a second season — in the same slot.)

Fox's move of House to Mondays at 8 ET/PT, opposite the performance episodes of ABC's Dancing With the Stars, fueled an increase in DVR use for the medical drama. This season, 26% of its total audience watched later, compared with 17% who recorded the show on Tuesdays last season. In contrast, just 9% recorded Dancing's live episodes.