OK, it's Monday, where's the coffee?
Why do so many people crave coffee first thing in the morning?
Yes, we need it to feel awake. But why coffee? Why not an egg roll? Or some yogurt?
Because coffee, and not eggs or yogurt, contains caffeine, a substance that stimulates the human nervous system and makes it think the body is still full of energy. Yeah, I don't need sleep!
Caffeine, the kicker in coffee (and tea, and Coca Cola, etc) is a psychoactive drug, one that alters your brain function, temporarily changing your perception, mood, consciousness, and behavior. Many of these changes are helpful (e.g. that increased alertness that coffee brings on), and this positive reinforcement makes us depend on it.