PEOPLE who smile a lot and say "have a nice day" are headed to an early grave while the grumpy stay fit.

Researchers at a German university - a land where surliness is an Olympic discipline -say enforced jolliness on the job is much more likely to make people fall ill.

They cite flight attendants, sales personnel, call centre operators, waiters and others in contact with the public for extended periods of time as being at risk of seriously harming their health.

Psychologists at Frankfurt University said the fake friendliness led to depression, stress and a lowering of the immune system itself, which in turn can trigger more serious ailments.

"Every time a person is forced to repress his true feelings there are negative consequences for his health," said Professor Dieter Zapf, a researcher into human emotions.

The study tested students working in an imaginary call centre who were subject to abuse from clients.

Some of the participants were allowed to answer back, while others had to be polite and friendly all the time.

Those who stood up to clients had a rapid heartbeat for a brief period, but for those who had to remain friendly their heart was still racing long after the client had hung up.

The conclusion reached by the psychologists was that "being friendly against one's will causes nothing but stress".