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 .



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 ," 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".



Flight attendants, shop assistants and carers also took part in the study of emotional behaviour, which involved 4000 people, Professor Zapf said.



"We are all able to rein in our emotions," he said. "It becomes difficult when you have to do this over a protracted period as cabin attendants are forced to on long-haul flights.



"These people need space away from the passengers where they can be on their own and let their feelings run free. We have to get away from the 'customer is king' attitude and show more respect to those working in the service industries," Professor Zapf said.