@Liliki @process: Could be the different caffeine and/or tannins levels between dark and light (or medium) roasts. The body can react with headaches and heartburn upon initially switching.

Tannins and caffeine dosage changes cause headaches quiet commonly.

Higher acidity in dark roasts can indeed cause heartburn.

As with any drug/chemical changes, it can take time for the body to adjust. When I switched to dark roast in evenings, I experienced heartburn for about a week. No headaches, tho.