Interestingly, a number of studies have found a connection between caffeine consumption and a reduced risk of depression— especially when it's consumed in the form of coffee. Research has even suggested caffeine lowers the risk of suicide, at least for men. However, at least one of these studies only found this connection with caffeinated coffee, not tea, though others found the same effect for tea as well.
However, caffeine also pumps up our adrenaline levels, which can leave us irritable.
Caffeine excites our brain cells, which tells our hormone control center the pituitary gland that there’s an emergency. The pituitary tells the adrenal glands (located above the kidneys) to flood the body with adrenaline.
That's the hormone behind the “fight or flight” response. Adrenaline prompts us to either stay and face a threatening situation or flee a scene. In this excited state, we tend to be more irritable, anxious, and far more emotionally-charged.
While that can be helpful for running from someone or defending ourselves in a fight, the aggressive hormone does little good in more delicate situations like negotiating in a meeting or responding to a text.
It improves our memory.
Caffeine has been shown to improve certain types of memory — especially the ability to remember lists of words and straightforward information — in some (but not all) studies. Some research shows that it helps those memories “stick” in the brain as well, making it easier to recall that information later.
This enhancement, however, seems to be strongest for people who aren’t already hooked on caffeine in the first place.
One recent study also indicates that extroverts get more of a working-memory boost from caffeine than introverts. This may explain why some studies have observed a more significant effect than others. Stephen Braun, the author of “Buzz: The Science and Lore of Alcohol and Caffeine,” explains that individual’s reactions to caffeine vary greatly — while one person might thrive on a high level of caffeine, it’ll make another person unable to get anything done.
Too much caffeine can lead to a decrease in performance across the board, however.
Caffeine can increase our attention span.
One of the common reasons people drink caffeinated coffee or tea is to help them focus on a task, and no wonder: one of the clearest mental effects of caffeine is a boost in the ability to focus, especially for someone who is fatigued.
Research shows that commercial drivers who cover long distances are significantly less likely to crash if they’ve consumed caffeine in any form — coffee, tea, pills, or energy drinks.
However, most people are familiar with caffeine jitters too, and know it can get hard to focus on anything after consuming too much.