A collaborative study from University of Colorado, Boulder (USA) and MRC Laboratory of Molecular Biology, Cambridge (United Kingdom) states that caffeine intake at night disrupted the circadian rhythm of humans. The scientists have elucidated the mechanism by which caffeine resets the internal clock.
Our bodies operate on a circadian rhythm (also known as the internal biological clock or inner clock), in which physical and biological changes follow a 24-hour cycle. The circadian rhythm, together with the pressure to sleep, controls the sleep-wake cycle. Previous research has shown that a disruption in the circadian rhythm had profound effects on the metabolism and behavior of mice. Diseases like cancer have been linked to a disruption of the circadian rhythm as well.
Given the importance of the biological clock, it is only natural that scientists are curious about drugs that could upset its rhythm. The most used psychoactive drug in the world is caffeine. It is consumed in the form of coffee, tea, in sodas, energy drinks, in supplements, etc. Caffeine stimulates the central nervous system and is reported to alter circadian rhythms.
Because caffeine could influence cellular timekeeping, the scientists in the current study investigated the effect of a nighttime cup of coffee on the circadian rhythm of five volunteers. Three hours before bedtime, the study subjects were given capsules containing either caffeine (equivalent to a double espresso and adjusted to the participants’ body weight) or a placebo and were studied for 49 days. The participants were also subjected to bright or dim light exposure (bright light is known to delay circadian rhythm). The saliva of the study subjects was tested for melatonin, a known sleep hormone.
The results showed that:
- Caffeine delayed the body clocks of study participants by 40 minutes.
- Bright light brought about a much longer delay than caffeine (about twice).
- Caffeine directly influenced the adenosine receptor in cells (as gleaned through cell culture experiments).
- The impact of caffeine on adenosine receptors resulted in the increase of a cellular messenger molecule, called cyclic adenosine monophosphate or cAMP (cAMP is a key component involved in the regulation of cellular timekeeping).
Commenting on the results, Dr. Wright, the senior author of the study, says (as told to NPR), “We found that caffeine did indeed, in the evening, shift your clock later.” This kind of shift is enough to make people struggle to wake up at the right time the next day, according to co-author Dr. O’Neill.
The question then arises if drinking coffee at any time of the day shifts the Circadian clock. Not necessarily, according to Dr. O’Neill. He tells NPR that drinking coffee several hours before bedtime should not have the effects as drinking coffee right before bedtime. His advice?
“Leading a lifestyle where you're consistently not getting enough sleep, or where your body clock is continually out of sync with the natural world, is bad for you.”
Written by Mangala Sarkar, Ph.D.
Burke, T., Markwald, R., McHill, A., Chinoy, E., Snider, J., Bessman, S., . . . Wright, Jr, K. (2015). Effects of caffeine on the human circadian clock in vivo and in vitro. Science Translational Medicine, 7(305), 305ra146-305ra146.
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