New research has found too much caffeine can make men grumpy, but women don't seem to suffer the same side-effects.
Findings reported by New Scientist magazine show caffeine made men's stress worse and reduced teamwork.
Public speaking in particular was harmed by caffeine, although mathematical performance was not affected. But the most interesting finding was that personal interactions became more aggressive and intense.
Behavioural science professor John Raeburn says a lot of people think having a cup of coffee is going to reduce their stress.
"But the latest research, for instance from Duke University and other research in Bristol University, shows that probably it may have the opposite effect."
Interestingly, women's coffee drinking didn't make such a difference to their work behavour.
The gender difference can't yet be explained, but it could come down to physiological differences.
"Men do tend to be more violent than women, and it could be an interaction of the stress and coffee effects and men's propensity to be more violent than women," Raeburn says.
