The authors used data from a study of nurses in the United States. Participants also completed surveys to measure levels of coffee consumption, psychological well-being, happiness and optimism over time.
The results showed little association between minimal coffee consumption and long-term well-being. Moderate coffee consumption was also not significantly associated with happiness, while drinking more than four cups of the drink a day was associated with lower levels of sustained happiness.
The bidirectional analysis found that the effect of well-being on coffee consumption was also weak and inconsistent.