People given a dose of Coffee, tea, and other caffeinated drinks a few hours before their normal bedtimes exhibited a delay in their circadian rhythms of more than half an hour. According to a paper published in Science Translational Medicine .
“We already knew that caffeine is a stimulant and can keep you awake and make it difficult to fall asleep at night if taken too close to bedtime,” said behavioral neurologist Charmane Eastman of Rush University in Chicago, who was not involved in the work. “This study shows that caffeine can also make your internal circadian body clock later, which could make it difficult to fall asleep the next night even if you don’t take caffeine again.”
Caffeine is thought to induce wakefulness by binding adenosine receptors on brain cells, causing the release of excitatory neurotransmitters. But recent evidence suggests this might not be caffeine’s only effect. “There had been research done in other species like algae and fruit flies suggesting caffeine could impact the circadian clock,” said Kenneth Wright in his latest results , “but nothing had been done in humans.”
For the current work, Wright and colleagues gave study participants caffeine pills—equivalent to a double espresso—three hours before the subjects’ habitual bedtimes and took saliva samples each half hour or so to measure the degree of melatonin—a hormone that accumulates in the body in the hours or darkness and signals the onset of sleep. The caffeine pills caused the subjects’ normal nightly melatonin peaks to be delayed by close to forty minutes.
“One interesting property of the circadian clock is that whereas a light pulse late in the evening delays our rhythm, a light pulse in the morning advances it,” said molecular neurobiologist Malcolm von Schantzof the University of Surry, U.K., “This paper only reports the results of a single time point, and one could hypothesize that in a similar fashion, a dose of caffeine in the morning may help advance our rhythms.” If so, he added, “then a strong coffee in the morning will both help our wakefulness and the synchronization of our body clocks.”
Wright and his colleagues conceive to examine the clock-shifting effects of caffeine within the morning and at different times of day. “Now we have to think about properly timed use of caffeine,” he said, because although caffeine consumption in the evening is generally ill-advised, it could help people overcome jet lag when traveling westward.