Many people reach for a cup of coffee when they are tired, but depending on what type of worker you are, that may not be such a good idea. Drinking coffee may turn hard workers into slackers, according to a new study.

Researchers analyzed the effects stimulants such as caffeine and amphetamine had on rats and found that worker rats, rats that typically favored high-difficulty/high-reward tasks, became slackers when given stimulants. But natural slacker rats, rats that went for low-difficulty/low-reward tasks, became hard workers when given amphetamines.

Every day, millions of people use stimulants to wake up, stay alert and increase their productivity -- from truckers driving all night to students cramming for exams, Jay Hosking, lead researcher and a PhD candidate at the University of British Columbia, said in a statement. These findings suggest that some stimulants may actually have an opposite effect for people who naturally favour the difficult tasks of life that come with greater rewards.

Caffeine works by interfering with adenosine, a chemical in the brain associated with sleep. Adenosine slows down activity in nerve cells, causing tiredness. Caffeine binds to adenosine receptors but speeds the cells up instead of slowing them down. Researchers are not sure why hard-working rats became lazy after being given other stimulants.

Researchers presented 20 rats with five food dispensers that rewarded them with either one or two sugar pellets based on how hard they concentrated. Some rats naturally concentrated harder to get the second pellet, while others were content with just the one, according to the study. When given a stimulant, the roles reversed.

I think this is already somewhat understood in everyday life; for some of us, coffee really does the trick for those long hours in the middle of the day, but for other people it makes them too jittery or aroused to concentrate on their work, Hosking told Fox News.

Researchers hope that this finding will pave the way for more personalized treatment of conditions such as ADHD, which is treated with amphetamines. By understanding patients underlying behavior, doctors could better tailor a treatment plan, according to the study.

We need to look at individual differences when we design therapies, rather than just looking at the main effect of a drug on an entire group, Hosking told the Vancouver Sun. If these results extend to humans, then maybe a cure isn't a one-size-fits-all kind of thing.

Researcher said the most important thing to take away from the study is that everybody does not make decisions the same way.

Some of us are happy to put in the extra effort in hopes of a promotion, while others may be satisfied to earn their wages and do the bare minimum at work, Hosking told Fox News

The journal Neuropsychopharmacology published the study on Wednesday.