Researchers studying the effects of caffeine on our circadian clock have made a startling, yet unsurprising discovery: Coffee before bed delays our internal timekeeping system. The findings were published in Science Translational Medicine this week. 

We live on a roughly 24-hour cycle. The master clock in our brain that coordinates cellular timekeeping throughout our entire body is reset by exposure to light. Researchers from the University of Colorado Boulder and MRC Laboratory of Molecular Biology recruited five healthy individuals for a 49-day study. On some days, the participants took a placebo, but on others, they were given a caffeine pill – equivalent to two shots of espresso – three hours before going to bed. 

Caffeine, the team found, lengthened the participants’ circadian period strongly. One post-dinner dose delayed their internal clocks by about 40 minutes on average. "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," CU-Boulder's Kenneth Wright said in a statement. 

The team also wanted to see how nighttime caffeine compared with nighttime exposure to bright light – an environmental cue known to lengthen the circadian phase and disrupt sleep. So, subjects taking either a caffeine or placebo pill were also were exposed to either dim or bright overhead lighting for three hours beginning at each person's normal bedtime. Turns out, the shift caused by consuming bedtime caffeine is about half as long as that caused by evening exposure to bright light. But while bright light induced a delay of about 85 minutes, volunteers combining the caffeine pill with bright light experienced a 105-minute delay in their circadian phase. 

When the researchers exposed cultured human cells to chronic levels of caffeine, they found that the substance prevents adenosine, a sleep-promoting neurotransmitter, from binding to its receptor. And this influences cyclic AMP signaling, which plays a key role in the control of the circadian clock.

The findings suggest that late-night coffee drinking may contribute to the rise in sleep problems, but properly timed caffeine consumption may help reset the circadian clock for people suffering from jetlag after flying westward.