It’s not just the caffeine that keeps you from falling asleep after a cup of coffee, according to a new study published this week in the journal Science Translation Medicine – it’s the effect that the beverage has on your circadian rhythms that prevent you from napping after a latte.
In fact, drinking a double espresso can shift a person’s internal clock, which tells the body when to go to sleep and what time to wake up, by an average of 40 minutes, researchers from Harvard Medical School, the University of Colorado and elsewhere reported in their new study.
Furthermore, the effect is worse when combined with bright light, according to NBC News. A three-hour period of exposure to light prior to bedtime can shift a person’s sleep cycle by nearly 1.5 hours, and adding coffee into the mix throws the body clock off by one hour, 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,” Kenneth Wright from the University of Colorado Department of Integrative Physiology, explained. “It also provides new and exciting insights into the effects of caffeine on human physiology.”
So what time should be “last call” for coffee drinkers?
Wright and his colleagues recruited five individuals, and over a period of seven weeks, they exposed these volunteers to bright light and dim light, and gave them caffeine or placebos to learn what impact these would have on sleep rhythms. Participants agreed not to drink alcohol, consume caffeine to take any drugs on their own during the 49-day study.
Analysis of saliva, blood and cells taken from the volunteers found that caffeine blocks the cell receptors that grant entry to adenosine, a neutrotransmitter carrying chemical signals which are used by a person’s system to promote sleep. The findings, they said, could explain why coffee drinkers tend to stay up late, and could led to new ways to use the beverage to fight jet lag.
So when should a person stop drinking coffee? One of the study authors, Dr. John O’Neill from the Medical Research Council’s Laboratory of Molecular Biology in Cambridge, told BBC News that it would be “complete speculation” to establish a cut-off time – adding that he personally did not ever drink the beverage himself after 17:00 (or 5:00pm local time).
“Individuals differ in their sensitivity to caffeine, and if coffee drinkers experience problems with falling asleep, they may try to avoid drinking coffee in the afternoon and evening,” added Professor Derk-Jan Dijk, from the University of Surrey, who said that the findings indicate that caffeine intake is “part of the reason why we sleep so late.”
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