A "dark side" to the internet suggests a strong link between time spent surfing the web and depression, say psychologists.
British scientists found that the longer people spent online, the less likely they were to be happy.
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A small group of the worst affected individuals were both depressed and addicted.
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But it was not clear whether using the internet causes mental health problems, or whether people with mental health problems are drawn to the internet.
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More work is needed to answer this "chicken and egg" question, say the researchers.
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Study leader Dr Catriona Morrison, from the Institute of Psychological Sciences at the University of Leeds, said: "The internet now plays a huge part in modern life, but its benefits are accompanied by a darker side."
The scientists employed the internet to carry out their research.
An online questionnaire was used to assess levels of internet dependency and depression in 1,319 individuals ranging in age from 16 to 51.
In general, the longer people spent online the more depressed they tended to be, the scientists found.
"There was a high correspondence between the amount of time spent on the internet and levels of depression," said Dr Morrison. "If you look at how dependent people feel they are on the internet that is likely to correspond with how happy or sad they feel."
Her team identified a small group of 18 hard-core internet users who spent many hours online each day and were classed as "internet addicted".
Their average depression score was more than five times higher than that of non-addicted users, and they were more likely to be moderately or severely depressed.
The addicts spent proportionately more time browsing porn sites, gaming sites and online communities. They also tended to be young, having an average age of 21.
Although they only made up 1.2% of the total number of participants, this was a higher fraction than the 0.6% of people in the general population who are addicted to gambling.
"While many of us use the internet to pay bills, shop and send emails, there is a small subset of the population who find it hard to control how much time they spend online, to the point where it interferes with their daily activities," said Dr Morrison.
"Our research indicates that excessive internet use is associated with depression, but what we don't know is which comes first - are depressed people drawn to the internet or does the internet cause depression?
"Now we need to investigate the nature of that relationship and consider the issue of causation."
Incidents such as the spate of suicides among teenagers in the Welsh town of Bridgend in 2008 have led to questions about the psychological dangers of social networking sites. Some experts are worried about their potential for fuelling depressive thoughts in vulnerable teenagers.
Dr Morrison, whose research appears today in the journal Psychopathology, added: "This study reinforces the public speculation that over-engaging in websites that serve to replace normal social function might be linked to psychological disorders like depression and addiction.
"We now need to consider the wider societal implications of this relationship and establish clearly the effects of excessive internet use on mental health."