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.

The study published in Science Translational Medicine showed for the first time how caffeine affects "cellular timekeeping" in the human body.

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 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.

There were no significant differences between the dim light/caffeine combination and the bright light/placebo combination. Nor were there significant differences between the bright light/placebo and bright light/caffeine combinations.

The results may indicate a "ceiling" was reached in the phase delay of the human circadian clock due to the external factors, author said.

Caffeine can block cell receptors of the neurotransmitter adenosine, which normally promotes sleep and suppresses arousal.

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.

http://www.colorado.edu/news/releases/2015/09/16/caffeine-night-delays-human-circadian-clock-0