If you are among those who spend hours on twitting or interacting with Facebook friends, please note: Repeated exposure to social networking sites is causing an "identity crisis" among the users, a leading scientist has warned.

Baroness Greenfield, a professor of pharmacology at Oxford University, said Facebook and Twitter have created a generation obsessed with themselves, who have short attention spans and a childlike desire for constant feedback on their lives.

Professor Greenfield believes the growth of Internet "friendships" as well as greater use of computer games could effectively "rewire" the brain.

This can result in reduced concentration, a need for instant gratification and poor non-verbal skills, such as the ability to make eye contact during conversations, she said.

"What concerns me is the banality of so much that goes out on Twitter. Why should someone be interested in what someone else has had for breakfast?" Professor Greenfield was quoted as saying by the Daily Mail.

"It reminds me of a small child (saying): 'Look at me Mummy, I'm doing this', 'Look at me Mummy I'm doing that'. It is almost as if they are in some kind of identity crisis. In a sense it's keeping the brain in a sort of time warp."

The academic suggested that some Facebook users feel the need to become "mini celebrities" who are watched and admired by others on a daily basis.

They do things that are "Facebook worthy" because the only way they can define themselves is by "people knowing about them".

More than 750 million people in the world use Facebook to share photographs and videos and post regular updates of their movements and thoughts.

Millions have also signed up to the micro-blogging site Twitter which lets members circulate short text and picture messages about themselves.

Professor Greenfield, former director of research body the Royal Institution, said: "It's almost as if people are living in a world that's not a real world, but a world where what counts is what people think of you or (if they) can click on you," she said.

"Think of the implications for society if people worry more about what other people think about them than what they think about themselves."

Her views were echoed by Sue Palmer, a literacy expert and author, who said girls in particular believe they are a "commodity they must sell to other people" on Facebook.

She said: "People used to have a portrait painted but now we can more or less design our own picture online. It's like being the star of your own reality TV show that you create and put out to the world."