Why does caffeine make me anxious and shaky in the afternoon?

Stimulant: Caffeine acts as a stimulant. It causes the heart to race, speeds up reaction time, can cause tremor, anxiety, sweating, and sleeplessness/insomnia. It acts as a memory aid (it is the major ingredient in nodoze), and is even used by athletes as a performance aid. Some are more sensitive than others (women, lower body weight, people with mitral valve prolapse). Use in moderation or do decaf.