It makes sense. Harvard School of Public Health researchers found drinking 4 cups of regular coffee a day can reduce our risk of depression by 20%. That’s compared to people consuming caffeinated tea, soft drinks, energy drinks, or chocolate. Researchers aren’t exactly sure why caffeinated coffee is the best mood booster. But the Mayo Clinic says 400 milligrams of caffeine – about what’s in 4 cups of brewed coffee – is considered safe for most adults.