One 13-year-old said she was "going to die" if her mobile phone was taken away.

Another admitted checking Facebook and other apps 100 times a day - when she was in school.

Indeed, a CNN study of more than 200 eighth graders at eight different schools in six states across the United States found many were virtually addicted to their smartphones.

"I would rather not eat for a week than get my phone taken away. It's really bad," CNN reported 13-year-old Gia as saying. "I literally feel like I'm going to die."

The study - "#Being13: Inside the Secret World of Teens" - done in partnership with child development experts, is the first large scale study to analyse what kids actually say to each other on social media and why it matters so deeply to them, CNN said.

Students participated willingly with the permission of their parents. They registered Facebook, Twitter and Instagram accounts through a secure server created by Smarsh, an electronic archiving company hired by CNN.

The US-based cable news network analysed some 150,000 social media posts collected over six months with its partners. In addition, the teens responded to survey questions about their use of social media, CNN reported.

The study found that the more teens look at social media, the more distressed they can become. Some of the kids admitted to checking their Facebook and other social media accounts more than 100 times a day, sometimes even during school hours.

The study also found that teens no longer see a distinction between their lives in the real world versus the online world, CNN reported, yet they still post online what they admit they would never say in person.

Most shocking to analysts in the study and parents of the students was the sheer amount of profanity and explicit sexual language - especially given the age of the participants.

"Go die. Stop trying to be popular. Holy s--t your (sic) ugly," read one social media post sent to a girl in the study, CNN noted.

Why do the kids do it? The cable news network found that 61 per cent of the 13-year-olds said they wanted to see if their online posts are getting likes and comments, 36 per cent said they wanted to see if their friends are doing things without them, and 21 per cent said they wanted to make sure no one was saying mean things about them.

Sociologist Robert Farris, who was a co-author of the study said he saw a lot of evidence of "if not outright addiction to social media, a heavy dependence on it".

"There's a lot of anxiety about what's going on online, when they're not actually online, so that leads to compulsive checking," CNN reported him as saying.