An after-dinner espresso may taste great, but it can keep you up. Now, scientists have shown that evening caffeine not only disrupts sleep, it also resets the body’s circadian clock, the set of molecular signals that keeps a person on a 24-hour schedule. Drinking the equivalent of a double espresso 3 hours before bedtime shifts the clock back by an average of 40 minutes, researchers report online today in Science Translational Medicine. Although caffeine’s effects on alertness and sleep are well known, researchers weren't sure how it affects the circadian clock in humans. Experiments on five people who were given a caffeine pill or a placebo pill a few hours before their normal bedtime and then exposed to dim or bright light showed that caffeine’s effects were about half as strong as exposure to bright lights—one of the best-studied ways to shift the body’s clock. (Each subject was exposed to each set of conditions, so they could serve as their own controls.) The researchers also found that human cells in the lab exposed to caffeine lengthened their circadian clocks, an effect that the researchers linked to molecules known to play a role in cells’ 24-hour rhythms. The work also suggests that drinking coffee at the right time can help people adjust more quickly to jet lag, which is due to the body’s clock being out of sync with the local time zone. The solution for those who enjoy an espresso after dinner, but don’t like insomnia? Indulge only after a long flight west.
*Correction, 18 September, 1:03 p.m.: This item has been corrected to reflect that caffeine's effects on the human circadian clock were unknown. Other research has shown that caffeine affects the circadian clock in animals such as fruit flies.