As a nervous system stimulant, caffeine in coffee, soft drinks and chocolate can make you feel as if someone pressed a "fast-forward" button on your body. While caffeine has many effects, one that is not common is causing you to feel drowsy. However, when caffeine wears off, you may notice the effects of fatigue are more pronounced. Understanding how caffeine affects your system can help you understand why you may associate drowsiness with caffeine.