Why is it that in the afternoon the British pour a spot of tea, the Spanish succumb to the siesta, and the American reaches for an icy Diet Coke?
Around the globe, we all have ways of coping with the natural patterns of human biology. The afternoon slump, when eyelids droop and shoulders sag, is the result of a complicated dance of the body's chemical messengers.
Caffeine, the drug found in tea, coffee, colas and chocolate, is how billions of people perk up when modern life doesn't allow for a catnap. Yet scientists have long puzzled over how caffeine delivers its zing. Now researchers at the University of Texas Southwestern Medical Center say caffeine doesn't so much perk us up as prevent us from powering down.
After several hours of hard work, a busy brain has its own mechanism for recharging; it seeks a rest. It triggers a release of adenosine, a neurotransmitter that, like a key opening a lock, attaches to special receptors on the surface of nerve cells throughout brain and body. Once the chemical has opened the lock and delivered its payload to the brain cell, the connection causes drowsiness, promoting sleep.
"Neurons in the brain do things such as talk to each other, process information and coordinate body activities," says Robert W. Greene, a professor of psychiatry at the University of Texas. As the brain senses that the workday should be coming to a close, it causes the body to release a steady stream of adenosine. Eventually, the accumulation of the chemical will be overwhelming. "It's like it's telling [your brain cells]: 'You guys have worked to hard; take it easy and refresh yourselves,'" says Greene.