How we react to caffeine is largely dependent on our genes. Depending on what variant of the genes we have, can determine how fast caffeine is broken down in our bodies and how sensitive and tolerant we are to the effects of caffeine. Some caffeine sensitive people may experience unpleasant symptoms by drinking as little as one cup of coffee or even tea per day. Those who are not caffeine sensitive and built up caffeine tolerance may have more than 5 cups of coffee per day and barely notice the effects.
Caffeine sensitivity is not the same as caffeine intolerance (caffeine allergy).
Read more about the role of genes in caffeine sensitivity and the caffeine sensitivity symptoms here.