Boulder, Colorado - It’s no secret that slugging down caffeinated drinks in the evening can disrupt sleep.

But a new study led by the University of Colorado Boulder and the Medical Research Council’s Laboratory of Molecular Biology in Cambridge, England shows for the first time that evening caffeine delays the internal circadian clock that tells us when to get ready for sleep and when to prepare to wake up. The research team showed the amount of caffeine in a double espresso or its equivalent three hours before bedtime induced a 40-minute phase delay in the roughly 24-hour human biological clock.  

The study also showed for the first time how caffeine affects “cellular timekeeping” in the human body, said CU-Boulder Professor Kenneth Wright, who co-led the study with John O’Neill of the Medical Research Council’s Laboratory of Molecular Biology (LMB) in Cambridge. While it has been known that caffeine influences circadian clocks of even primitive creatures like algae and fruit flies, the new study shows that the internal clocks in human cells can be impacted by caffeine intake.

“This is the first study to show that caffeine, the mostly widely used psychoactive drug in the world, has an influence on the human circadian clock,” said Wright, a professor in CU-Boulder’s Department of Integrative Physiology. “It also provides new and exciting insights into the effects of caffeine on human physiology.”

A paper on the subject led by Wright and O’Neill is being published online in the Sept 16 issue of Science Translational Medicine.