A warm cup of coffee is a necessary part of the morning routine for millions of people around the world. And as the end of daylight saving time messes yet again with our sleep patterns, plenty of people in the U.S. may reach for an extra cup or two to power through the drowsiness. (Find out if you live in one of the best coffee cities in the U.S.)

But for some sleep-deprived souls, a cup of coffee is never the answer. No matter the delivery method, even a small amount of caffeine may give these people the jitters or keep them up all night. Why is it that caffeine affects people in such dramatically different ways? The answer, in part, lies in your genes.

“What we’re finding is that we have built-in genetic factors that help us with self-regulating our caffeine intake,” says Marilyn Cornelis, a caffeine researcher at Northwestern University in Chicago, Illinois. “It’s interesting how strong of an impact our genetics have on that.”