NEW YORK (Reuters Life!) - Surfing the net has become an obsession for many Americans with the majority of U.S. adults feeling they cannot go for a week without going online and one in three giving up friends and sex for the Web.

A survey asked 1,011 American adults how long they would feel OK without going on the Web, to which 15 percent said a just a day or less, 21 percent said a couple of days and another 19 percent said a few days.

Only a fifth of those who took part in an online survey conducted by advertising agency JWT between Sept 7 and 11 said they could go for a week.

"People told us how anxious, isolated and bored they felt when they are forced off line," said Ann Mack, director of trend spotting at JWT, which conducted the survey to see how technology was changing people's behavior.

"They felt disconnected from the world, from their friends and family," she told Reuters.

The poll, released on Wednesday, found the use of cell phones and the Internet were becoming more and more an essential part of life with 48 percent of respondents agreeing they felt something important was missing without Internet access.

More than a quarter of respondents -- or 28 percent -- admitted spending less time socializing face-to-face with peers because of the amount of time they spend online.

It also found that 20 percent said they spend less time having sex because they are online.

Cell phones won out over television in a question asking which device people couldn't go without but the Internet trumped all, regarded as the most necessary.

"It is taking away from offline activities, among them having sex, socializing face-to-face, watching TV and reading newspapers and magazines. It cuts into that share," said Mack.

"I don't suppose their partners are too pleased about it."

Mack said a clear trend to emerge from the survey was the increasing need for mobility with people no longer satisfied with just broadband access from home and wanting hand-held devices like iPhones and BlackBerrys.

JWT, whose parent company is WPP, has come up with a new advertising category for people whose lives are so tied up with new technology.

"We are calling them 'digitivity denizens,' those who see their cell phones as an extension of themselves, whose online and offline lives are co-mingled and who would chose a Wi-Fi connection over TV any day," said Mack.

"This is how they communicate, entertain and live." - Reuters.