It’s no secret that downing caffeinated drinks in the evening can disrupt sleep, but according to a new study, it actually goes one step further than that.
The study, led by 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 the authors of the study. 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 Kenneth 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.”
For the study the team recruited five human subjects, three females and two males, who were tested for 49 days in CU-Boulder’s sleep laboratory. The subjects were tested under four conditions: low light and a placebo pill; low light and the equivalent of a 200-milligram caffeine pill dependent on the subject’s weight; bright light and a placebo pill; and bright light and the caffeine pill.
Saliva samples of each participant were tested periodically during the study for levels of the hormone melatonin. Melatonin levels in the blood increase to signal the onset of biological nighttime during each 24-hour period and decrease at the start of biological daytime.
Those who took the caffeine pill under low-light conditions were found to have a roughly 40-minute delay in their nightly circadian rhythm compared to those who took the placebo pill under low light conditions. The magnitude of delay from the caffeine dose was about half that of the delay induced in test subjects by a three-hour exposure to bright, overhead light that began at each person’s normal bedtime.
The study also showed that bright light alone and bright light combined with caffeine induced circadian phase delays in the test subjects of about 85 minutes and 105 minutes respectively.
The results may help to explain why caffeine-drinking “night owls” go to bed later and wake up later and may have implications for the treatment of some circadian sleep-wake disorders. The findings could also benefit travelers, however, as properly timed caffeine use could help shift the circadian clocks of those flying west over multiple time zones.
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