Everyone knows that people who say they can drink a cup of coffee and fall right asleep are just fooling themselves. A new study shows why.
Caffeine, it found, disrupts the circadian rhythm — the internal clock that tells the body when to sleep and when to wake up.
The amount of caffeine in a double espresso shifts this clock by 40 minutes, on average, the team at the University of Colorado, Harvard Medical School and elsewhere found.
It’s even worse when combined with bright light. A three-hour dose of bright light before bedtime shifts the sleep cycle by 85 minutes — nearly an hour and a half — and combining caffeine with bright light shifts it by an hour and 45 minutes.
"This is the first study to show that caffeine, the most widely used psychoactive drug in the world, has an influence on the human circadian clock," said Kenneth Wright of Colorado’s Department of Integrative Physiology. "It also provides new and exciting insights into the effects of caffeine on human physiology."
Studies have shown caffeine can disrupt the circadian rhythms of small organisms from bread mold to fruit flies, and it can affect the release of the sleep hormone melatonin in people.
The team recruited five volunteers who put up with 49 days of intensive study, coming to the lab to be dosed with bright light, dim light, caffeine and placebos to see what they would do to the sleep-wake cycle.
The volunteers agreed not to drink alcohol or caffeine or take any drugs on their own during the study.
The researchers ran saliva and blood tests and studied cells taken from the volunteers. They found caffeine can block cell receptors — molecular doorways — that let in adenosine, a message carrying chemical or neurotransmitter that promotes sleep.
The same team did a study that showed when they stuck eight people out in the mountains without any artificial light, their body clocks switched over to send them to sleep at sunset and awaken at sunrise.
The new findings may help explain why heavier coffee drinkers tend to be night owls, they said. It might lead to strategic ways to use coffee to fight jet lag — rather than simply throwing back a few triple espressos and hoping for the best.
“Trials are needed to test the latter, and it will be important to monitor for caffeine-induced sleep disruption under such conditions, which could worsen jet lag,” they wrote in the journal Science Translational Medicine.