Caffeine consumption in the evening delayed the internal circadian clock that tells us when to get ready for sleep and when to prepare to wake up, a new study published in Science Translational Medicine showed.

Researchers from the University of Colorado Boulder and the Medical Research Council’s Laboratory of Molecular Biology in Cambridge, England showed that the amount of caffeine in a double espresso or its equivalent, taken 3 hours before bedtime, induced a 40-minute phase delay in the roughly 24-hour human biological clock. Kenneth Wright, a professor at CU-Boulder, stated that the study also showed for the first time how caffeine affects “cellular timekeeping” in the human body.

For the study, the team recruited five human subjects to be tested under four conditions: low light and a placebo pill; low light and the equivalent of a 200mg 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 throughout the study for levels of melatonin.

RELATED: FDA: Significant Safety Risk With Pure Powdered Caffeine

Results showed that subjects who took the caffeine pill under low light conditions had a roughly 40-minute delay in their nightly circadian rhythm compared to those who took the placebo pill under low light conditions. The study also showed that bright light alone and bright light with caffeine induced circadian phase delays of about 85 minutes and 105 minutes, respectively. There were no significant differences between the dim light with caffeine combination and the bright light with placebo combination or between the bright light with placebo and bright light with caffeine combinations.

Wright stated that these 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.”

For more information visit