What coffee does to your brain

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Why can some people drink three espresso coffees before bed and still get a good night's sleep, while just one would have others anxious and up all night? Astrid Nehlig, from the French Institute of Health and Medical Research, explains what caffeine does to our brains and tells Emily Thomas that our reactions to it are down to genetics. (Picture: A headless man pouring coffee down his neck. Credit: Getty Images)