A double espresso three hours before bedtime can induce a 40-minute time delay in your body's internal clock, making it harder to go to sleep on time and more challenging to wake up in the morning, scientists have shown for the first time.
The study led by the University of Colorado Boulder and the Medical Research Council's Laboratory of Molecular Biology in Cambridge, 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.
"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.
"It also provides new and exciting insights into the effects of caffeine on human physiology," he said.
For the study the team recruited five human subjects, three females and two males, who went though a double-blind, placebo-controlled 49-day protocol.
The subjects were tested under four conditions: low light and a placebo pill; low light and the equivalent of a 200-milligramme 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.
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