Television commercial breaks are the bane of every N.F.L. fan. They interrupt a game already riddled with stoppages, bombard viewers with come-ons and force fans and players in the stadium to stand around for about two and a half minutes, sometimes in the freezing cold.

Yet commercials are the lifeblood of the N.F.L. Without them, broadcasters could not afford to pay the league billions of dollars for rights fees, money that goes to paying players’ salaries and much more.

Most games have 18 commercial breaks. A few timeouts, like at the end of the first and third quarters and at the two-minute warnings, are fixed. The league and networks avoid taking breaks if a team’s opening drive of the game ends quickly, because they want fans to settle into the broadcast. If all goes well, the last commercials run at the two-minute warning in the fourth quarter.

Most commercial breaks, though, are chosen in real time as league executives, network producers and officials on the field look for natural breaks in the action. Finding them is more art than science because every game unfolds differently, with long drives, three-and-outs, injury timeouts and coaches’ challenges.

“Our fans know that the commercial breaks are coming,” said Mike North, vice president of broadcast planning and scheduling at the N.F.L. “The whole idea from where we sit is to try to use those breaks to cover downtime: resetting the field after a score; if there happens to be an injury, hopefully a minor one; or an instant replay review when the referee goes to the sideline.”