Double espresso before bedtime induces
40-minute time delay in internal clock
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The
results of this study 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. Credit: © patcharaporn1984/Fotolia
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It's
no secret that slugging down caffeinated drinks in the evening can disrupt
sleep.
But
a new study led by the 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 CU-Boulder Professor Kenneth Wright,
who co-led the study with John O'Neill of the Medical Research Council's
Laboratory of Molecular Biology (LMB) in Cambridge. 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
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."
A
paper on the subject led by Wright and O'Neill is being published online in the
Sept 16 issue of Science Translational
Medicine.
For
the study the team recruited five human subjects, three females and two males,
who went through a double-blind, placebo-controlled 49-day protocol through
CU-Boulder's Sleep and Chronobiology Laboratory, which is directed by Wright.
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, which is produced naturally by the pineal
gland when directed to do so by the brain's "master clock." The
master clock is re-set by exposure to light and coordinates cellular clocks
throughout the human body. 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, said Wright.
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 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. 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, Wright said.
In
addition, researchers at O'Neill's lab at the LMB in Cambridge used
"reporter" genes that made cells glow when the clock genes were
expressed to measure changes caused by caffeine. O'Neill's group showed that
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, said Wright.
The
new results could benefit travelers. Properly timed caffeine use could help
shift the circadian clocks of those flying west over multiple time zones, said
Wright.
In a 2013 study, Wright and
his research team showed one week of camping in the Rocky Mountains with no
artificial light, not even flashlights, synchronized the circadian clocks of
the eight study subjects with the timing of sunrise and sunset.
Originally published in ScienceDaily
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