A double espresso three hours before bedtime can induce a 40-minute time delay in our body's internal clock, leading to disrupted sleep, the result of a first-ever human trial has revealed.
The study, led by University of Colorado (CU) Boulder and the Medical Research Council's Laboratory of Molecular Biology in Cambridge, found 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 study also showed for the first time how caffeine affects 'cellular timekeeping' in the human body," said professor Kenneth Wright from CU-Boulder.
While it has been known that caffeine influences circadian clocks of even primitive creatures like algae and fruit flies, the new findings show that the internal clocks in human cells can be impacted by caffeine intake.
For the study, the team recruited three females and two males who went though a double-blind, placebo-controlled 49-day protocol.
The participants 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.
Melatonin hormone 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," said Wright.
The magnitude of delay from the caffeine dose was about half that of the delay induced in participants 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 participants 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," Wright emphasised.
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 new results could benefit frequent travellers.
Properly timed caffeine use could help shift the circadian clocks of those flying west over multiple time zones.
"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," the authors concluded.
Wright co-led the study with John O'Neill of the Medical Research Council's Laboratory of Molecular Biology (LMB) in Cambridge.
A paper on the subject led by Wright and O'Neill is forthcoming in the journal Science Translational Medicine.
