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Doziness, lethargy and an increasing inability to focus reached "startling" levels in the trials by 1,100 people, who also demonstrated that emails in particular have an addictive, drug-like grip.

Respondents' minds were all over the place as they faced new questions and challenges every time an email dropped into their inbox. Productivity at work was damaged and the effect on staff who could not resist trying to juggle new messages with existing work was the equivalent, over a day, to the loss of a night's sleep.

"This is a very real and widespread phenomenon," said Glenn Wilson, a psychiatrist from King's College, London University, who carried out 80 clinical trials for TNS research, commissioned by the IT firm Hewlett Packard. The average IQ loss was measured at 10 points, more than double the four point mean fall found in studies of cannabis users.

The most damage was done, according to the survey, by the almost complete lack of discipline in handling emails. Dr Wilson and his colleagues found a compulsion to reply to each new message, leading to constant changes of direction which inevitably tired and slowed down the brain.

Manners are also going by the board, with one in five of the respondents breaking off from meals or social engagements to receive and deal with messages. Although nine out of 10 agreed that answering messages during face-to-face meetings or office conferences was rude, a third nonetheless felt that this had become "acceptable and seen as a sign of diligence and efficiency".

In fact, it is a recipe for muddled thinking and poor performance, said Dr Wilson, who also called for restraint by the two-thirds of people who check work emails out of office hours and even on holiday. He said: "Companies should encourage a more balanced and appropriate way of working."