Why does coffee sometimes make me tired?

Caffeine, the main active ingredient in coffee, has a well-justified reputation for being an energy booster. But caffeine is also a drug, which means that it can impact each of us differently, depending on our consumption habits and our genes.

“The paradox of caffeine is that in the short term, it helps with attention and alertness. It helps with some cognitive tasks, and it helps with energy levels,” said Mark Stein, a professor in the Department of Psychiatry and Behavioral Sciences at the University of Washington, who has studied the impact of caffeine on people with attention-deficit/hyperactivity disorder. “But the cumulative effect — or the long-term impact — has the opposite effect.”

Part of the paradoxical effects of caffeine results from its effects on what researchers refer to as “sleep pressure,” which fuels how sleepy we become as the day wears on. From the moment we wake up, our bodies have a biological clock that drives us to go back to sleep later in the day.

Seth Blackshaw, a neuroscientist at Johns Hopkins University who studies sleep, said that researchers are still learning about how sleep pressure builds up in the body, but that over the course of the day, our cells and tissues use and burn energy in the form of a molecule called adenosine triphosphate, or ATP. As that ATP gets expended — as we think, exercise, run errands or sit on conference calls — our cells generate a chemical called adenosine as a byproduct. That adenosine goes on to bind to receptors in the brain, making us more sleepy.