A common side effect of excessive caffeine intake is increased anxiety and restlessness. High doses of caffeine can lead to jitteriness, heart palpitations, and disrupted sleep patterns, which can exacerbate feelings of anxiety. Individuals sensitive to caffeine may experience heightened nervousness even with moderate consumption. Additionally, excessive caffeine can lead to dependence, requiring more significant amounts to achieve the same stimulating effects. It's important to moderate caffeine consumption to minimize these side effects while reaping the benefits of improved alertness and focus.