This is a great scientific review article (evaluates other existing studies and summarizes the conclusions) on caffeine withdrawal. It specifically endorses a number of symptoms, including: headache, fatigue, decreased energy/activeness, decreased alertness, drowsiness, decreased contentedness, depressed mood, difficulty concentrating, irritability, and foggy/not clearheaded. Flu-like symptoms, nausea/vomiting, and muscle pain/stiffness were judged likely to represent valid symptom categories.

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Like many articles, it uses the common timeline of symptoms lasting 2-9 days. However, we believe a limitation of the existing science is that most studies only followed participants for days/weeks (not months/years).

This shows some effects of caffeine on receptors in the mouse brain. Of course, not everything from animal studies can be translated to humans, but similar phenomena may be responsible for the withdrawal syndromes we've encountered!