https://www.science.org/doi/10.1126/scitranslmed.aac5125
Your daily drug resets your clock
Your morning cup of coffee may be shifting your circadian clock. Burke et al. show that caffeine—widely available, legal, and psychoactive—inserts a delay into the ~24-hour metabolic rhythm that keeps your body running in time with the world.
In a sensitive, within-subject experimental design, five people were kept under highly controlled conditions for 49 days. Before bedtime, they were given various treatments: either a double-espresso caffeine dose, exposure to bright or dim light, or a placebo. The caffeine delayed their internal clock by 40 min, a shift about half as long as bright light, a stimulus known to robustly lengthen the circadian phase.
The authors used cultured cells to determine that the drug acted directly on the adenosine receptor, which increases the intracellular messenger molecule cyclic AMP. The fact that cyclic AMP forms a key cog in the inner workings of the clock links caffeine’s biochemical effects to its delay of the circadian rhythm.
Not only do these results reinforce the common advice to avoid caffeine in the evening, but they also raise the intriguing possibility that caffeine may be useful for resetting the circadian clock to treat jet lag induced by international time zone travel.