The study, published in the Archives of Internal Medicine, found that women who drank two to three cups of caffeinated coffee a day were 15% less likely to develop depression over the 10-year study period, compared with women who consumed one cup or less per day. Women who drank four or more cups of coffee a day had a 20% lower risk. Women who drank decaf didn't show a similar reduction in depression rates.

Led by senior author Dr. Alberto Ascherio, a professor of epidemiology and nutrition at Harvard School of Public Health, the study is one of the first to investigate the long-term effects of caffeine on mood disorders such as depression. There's a lengthy history of both animal and human studies documenting the feel-good effects of caffeine - caffeinated animals have been shown to be more active in their environments, and likewise, in people, caffeine boosts alertness and energy - but most of these effects are short-lived and last only until the next cup of joe.

But Ascherio and his colleagues suggest that consistent use of caffeine may have longer-term effects on the brain. Although their findings don't show that drinking coffee directly prevents depression - the findings show only an association between coffee and mood - researchers do know that caffeine works by binding to receptors for brain chemicals associated with mood.

Earlier studies have found similar effects. People who drink caffeinated coffee have lower rates of suicide than those who don't, for example, and they have lower rates of severe depression. In animals that exhibit the tremors that characterize Parkinson's disease, which can be traced to abnormal activity of brain chemicals, caffeine has been shown to help reduce uncontrolled movement. It's an intriguing connection when it comes to understanding how caffeine may affect depression, since Parkinson's patients also tend to have higher-than-usual rates of depression both before and after their diagnosis.

Ascherio says it isn't clear yet how caffeine may exerts mood-elevating effects. "Our results suggest that caffeine may have a beneficial effect on the cellular level, and may protect neurons lost to neurodegenerative disease," says Ascherio. "We are establishing a certain degree of reasonable evidence that caffeine has a long-term effect on the risk of depression, but we cannot attribute this to any pathology."

Like other compounds that affect the body's metabolism, caffeine may reduce the risk of depression only up to certain doses, Ascherio cautions. At high doses, caffeine can increase anxiety, which can actually contribute to depression.

He also notes that caffeine content in coffee can vary greatly. For purposes of the study, his team measured one cup of coffee as containing 137 mg of caffeine. The group also looked at the effect of caffeine consumed from sources other than coffee, including tea, soft drinks and chocolate, but found no association with mood, likely because these items contain less caffeine than coffee does.

For the more than 50,000 middle-aged women in the trial, coffee was by far the major source of caffeine. Researchers tracked caffeine intake in the participants, who were all enrolled in the Nurses' Health Study, through regular detailed questionnaires. Depression was tracked through self-report of diagnoses or antidepressant use.

So can a couple of cups of java protect some women from depression? "I don't think there is sufficient evidence for a specific recommendation that people change their behavior at this stage," says Ascherio.

People drink coffee for different reasons, he notes, and react differently to its potentially stimulating effects. These are the factors that are more likely to dictate when and how much coffee people drink - not the potential longer term benefits against a condition like depression. "The amount of coffee you drink is very much determined by how you feel, and there is no guarantee in the long run that drinking coffee will be beneficial," he says. "We adjust to the level of caffeine that is optimal for us, and we cannot give a prescription for people to drink or not drink caffeine."

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny.