However, the study, led by Professor Peter Rogers and Dr Lindsay St. Claire at the University of Bristol, also found that men working in teams could feel less stressed after enjoying a sharp intake of caffeine.
Existing theories about stress management suggest that caffeine consumption can trigger stress. However, while studies into the effects of caffeine show that it can sometimes worsen anxiety, there is also evidence that it boosts confidence, alertness and sociability as well as making us better able to perform various tasks.
The Bristol research, which tried to reconcile these different perspectives, was sparked by an anecdote from a man taking part in a stress management workshop, describing a trip to the United States with his small, cohesive business team.
During the trip, he said, coffee was freely available, and the team over-indulged. Within days, stress levels had escalated and they believed that the extra caffeine had disrupted emotional feelings and relationships, and impaired their ability to perform normally.
Aiming to test this theory, the researchers found that caffeine did indeed heighten feelings of stress while performing stressful tasks, but unexpectedly this happened especially in men. However, effects of caffeine on performance were likely to depend on the type of task and whether participants were working alone or in teams.
For example, when working on mental arithmetic alone and under time pressure, caffeine might help, but when working on a collaborative task, it might undermine people's ability to get along with each other.
The study also tested the impact of expectations, or whether someone who chooses to have a cup of coffee, believing it will speed reaction times, actually feels less stressed if under pressure to do something quickly.
For this reason, in one set of tests, researchers told 32 people that their coffee contained caffeine which would help their performance, another 32 that their drink did not, and a third group of 32 that they were having caffeine which causes stress-like side effects. Unknown to the participants, however, half of the drinks actually contained 200 mg of caffeine and the other half had none.
After drinking, all in the experiment did two stressful tasks and a series of other tests. Unexpectedly, men who had been told that their coffee contained caffeine to enhance performance had higher heart rates and felt more stressed. Actual caffeine consumption made people generally less confident about their ability to cope and, again surprisingly, made men feel more 'stressed'.
Videos taken during the experiments showed that caffeine tended to make men look more physically tense and sound less relaxed during a stressful public speaking task. However, it tended to reduce ratings of stress in women.
In other experiments, involving same-sex groups of five, individuals first thought out their own solutions to a problem and then agreed a set of group decisions after discussion.
This time, coffee was found to reduce some feelings of stress, particularly in men, but tended to make team-working less effective. For men, in particular, it meant that the solutions suggested after group discussion were not as good as those devised alone.
Dr St. Claire said: "Our research findings suggest that the commonplace tea or coffee break might backfire in business situations, particularly where men are concerned. Far from reducing stress, it might actually make things worse."
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