One can argue that the main reason behind coffee’s popularity is related to its effect on alertness. Caffeine blocks the production of the brain chemical responsible for making you feel tired (adenosine) while simultaneously inducing adrenaline and cortisol release.
This combination is responsible for giving you that energy boost. However, a high dosage can make some people feel overwhelmed and exacerbate the feeling of anxiety that may already be present due to an emotional struggle or the stress of work.
This side effect of caffeine has been duly studied, and the medical community even lists it as a common caffeine-related syndrome under the name of caffeine-induced anxiety disorder. In people who are not used to ingesting caffeine, a high dosage can even trigger anxiety attacks.