Several studies have looked at caffeine intake and depression. For example, one study found that, among healthy college students, moderate and high coffee drinkers scored higher on a depression scale than did low users. Interestingly, the moderate and high coffee drinkers also tended to have significantly lower academic performance. There is a reason for this effect and it relates to the chronic effects of caffeine on the brain.

Other studies have implicated that depressed patients tend to consume fairly high amounts of caffeine (e.g., greater than 700 mg/day, or about 7 cups of coffee). In addition, caffeine intake has been positively correlated with the severity of symptoms in psychiatric patients, especially related to panic disorders and depression.

Caffeine affects cortisol levels (the stress hormone). In one study, researchers gave healthy men and women different doses of caffeine throughout the day and found that the caffeine spiked their cortisol levels.