Coffee has many wonderful properties. It can make you more alert, and there are plenty of studies that have shown that caffeine can help you to concentrate better. More recently, some researchers have found that coffee can help boost your dopamine transmission which makes your process positive words and rewards much faster. Picking up on positive words and actions more quickly makes everything seem a little brighter – in other words, it makes you a bit more optimistic and a bit happier overall.
A little bit of caffeine isn’t going to completely turn a frown upside down (and I think if you were truly having a bad day and tried to counteract it with coffee alone you would get jittery faster than you would be gleeful), but it is going to help take the edge off. I’ve experienced this myself on a rough day – but usually gave credit to just relaxing in a comfy chair at a coffee shop, rather than to the coffee itself!