“What are the symptoms of caffeine addiction?”

I'm not sure there's really such a thing as "caffeine addiction," but certainly we become TOLERANT to caffeine and suffer withdrawal if we stop suddenly. As you note, headaches are one symptom of caffeine withdrawal, and persistent sleepiness is the other. These last a few days to a week after stopping caffeine cold turkey. They're unpleasant, but neither will hurt you. If you want to stop drinking coffee, just taper how much you drink over 2-3 weeks, then stop. It seems obvious that coffee and caffeine should be bad for you. Scientific studies suggest the opposite: they're good for your heart and may reduce cancer risk. That said, too much caffeine can cause heart rhythm irregularities, irritability, and insomnia in susceptible people.