People under stress stick to habits, good or bad
It seems old habits really do die hard - whether they are good or bad.
The study in the Journal of Personality and Social Psychology, contradicts the idea that being under pressure leads to bad habits like over-eating or shopping sprees.
Instead, people are just as likely to maintain gym habits or eat healthily.
In short, they say stress does lead to relapses into bad behaviour - but that it can reinforce good habits too.
Doughnuts v oatmeal
The University of Southern California team looked at the behaviour of 65 students over a 10-week term.
They wanted to study how much willpower someone had in a time of stress - in this case, during exams.
They found that during testing periods, when students were stressed and sleep-deprived, they were even more likely to stick to old habits - as if they didn't have the energy to do something new,
So those who ate pastries or doughnuts for breakfast during the term ate even more junk food during exams.
But the same was true of the healthy "oatmeal eaters" - they too were likely to stick to their routine and habits.
Those who read the editorial pages of the newspaper every day continued even when they were short of time.
And regular gym-goers were even more likely to go to the gym even more when stressed.
Prof Wendy Wood, who led the study, said: "When we try to change our behaviour, we strategise about our motivation and self-control. But what we should be thinking about instead is how to set up new habits.
"Habits persist even when we're tired and don't have the energy to exert self-control."
She added: "Everybody gets stressed. The whole focus on controlling your behaviour may not actually be the best way to get people to meet goals.
"If you are somebody who doesn't have a lot of willpower, our study showed that habits are much more important."
Prof Wood said the findings had implications for those seeking to affect people's behaviour.
"The central question for behaviour-change efforts should be, how can you form healthy, productive habits?
"What we know about habit formation is that you want to make the behaviour easy to perform, so that people repeat it often and it becomes part of their daily routine."