The early start of daylight-saving time hit the broadcast nets hard last week, as virtually every show — even “American Idol” — seemed affected.

Fox again dominated the weekly ratings races, prevailing in all key demo categories for the week of March 12-18, with “Idol” claiming the top two spots in the rankings.

Daylight-saving time began three weeks early this year, conspiring with the start of spring break in some colleges across the country to reduce the number of young adults sitting in front of their television sets. According to Nielsen data, 35.6% of adults 18-49 were watching television in primetime during any given minute, down 6% from 37.9% the same week a year ago.

As a result, every net was down vs. the same frame a year ago, with the major broadcasters down a collective 16% in adults 18-49.

“American Idol” came in more than 10% below the corresponding week’s score of a year ago. Still, its performance edition on Tuesday (11.7 rating/31 share in adults 18-49, 29.82 million viewers overall) was easily the week’s No. 1 show, and its 30-minute results show a night later ranked second (10.1/26 in 18-49, 26.18m).

Meanwhile, net has to be encouraged by the return of first-year laffer “‘Til Death” (5.8/15, 14.07m), whose relaunch behind “American Idol” produced series-best scores — and matched a February episode of CBS’ “Two and a Half Men” as the season’s highest-rated half-hour comedy perf in 18-49.

Also, recent addition “Are You Smarter Than a 5th Grader” (4.6/13, 12.56m) passed its first test in its regular Thursday 8 o’clock time period, giving Fox its first in-season 18-49 victory in the hour with a regular program since April 1993 (“The Simpsons” and “Martin”).

Viewers don’t appear to be joined at the hip, though, with the net’s new drama “The Wedding Bells” (1.8/5 in 18-49, 5.75m), which struggled in its first regular-slot try on Friday.

Runner-up CBS had a broader audience than usual thanks to the NCAA men’s college basketball tournament, ranking second to Fox in not only adults 25-54 and 18-49 but also among those 12-34.

Hoops action dominated Friday and Saturday’s primetime and also won Sunday’s opening hour, lifting the net to victory on that night as well. “The Amazing Race: All-Stars” hit a high (3.9/10 in 18-49, 12.00m), and “Cold Case” stood as the night’s top program (4.2/10, 15.09m).

Surrounded by repeats and basketball on the net’s schedule, Wednesday drama “Jericho” (2.3/7 in 18-49, 7.76m) again failed to show any spark — even with “American Idol” moved out if its way.

Over at ABC, “Grey’s Anatomy” was back in originals and stood as the week’s No. 3 program (9.0/23 in 18-49, 22.68m), leading into new drama “October Road” (5.6/15, 13.93m), which led its time period (Daily Variety, March 19). Also winning its slot for ABC was Wednesday drama “Lost” (5.4/15, 12.48m).

Net also did well Sunday with an encore “Extreme Makeover: Home Edition” (3.8/10, 10.71m) but the repeat dramas that followed it, “Desperate Housewives” (2.4/6, 7.12m) and “Brothers & Sisters” (1.9/5, 5.21m), were ineffective.

Highlights for NBC included Monday’s two-hour “Deal or No Deal” (4.8/13 in 18-49, 16.77m) and Friday’s return of gameshow “Identity” (2.8/8, 9.05m), which finished second in its hour to basketball on CBS.

Thursday saw a nothing-special bow for laffer “Andy Barker, P.I.” at 9:30 (2.4/6 in 18-49, 5.96m), while lead-out drama “Raines” did a bit better in its special preview (2.9/8, 10.46m); show shifts to Fridays next week.

CW was led by Wednesday’s “America’s Next Top Model” (2.4/7 in 18-49, 5.16m) and a repeat of “Pussycat Dolls Present: The Search for the Next Doll” (1.6/4, 3.36m), which fared better than its initial airing the previous night (1.1/3, 2.52m).

Also, opposite basketball on CBS, the net’s Thursday tandem of “Smallville” and “Supernatural” hit a high in females 12-34 (2.3/6).

On the cable side, Monday saw a nice debut Monday for FX’s “The Riches” (1.9/5 in 18-49, 3.84m). Outside USA’s two hours of “WWE Raw” (2.4/6, 5.77m), it was cable’s top draw in 18-49. Also excelling Monday were VH1’s “I Love New York” (1.7/4 in 18-49, 3.70m) and MTV’s “The Hills” (1.6/4, 2.81m).

It was a big week for “Family Guy,” as five airings of the toon vet on latenight Adult Swim ranked among the frame’s top 15 total-day cable programs in adults 18-49. “Family Guy” was also TBS’ top show in the demo with a pair of primetime episodes on Monday, which helped the Turner net rank third among cable nets in primetime for the week in 18-49.