DEAR DR. BLONZ: How long will I be affected by the caffeine I consume? If I have it at lunch, is there any reason it would cause sleep problems at night? -- J.C., San Jose, California
DEAR J.C.: Caffeine is a general stimulant that increases heart rate, revs up the release of stomach acid, speeds the transport of food through the digestive system, and relaxes the smooth muscles, such as those found in the lungs. Once in the body, caffeine goes just about everywhere. Because it's a foreign substance, the body starts to eliminate caffeine as soon as it appears.
The answers to your questions depend on the amount consumed, how regular your caffeine intake is, and your particular degree of sensitivity.
The "half-life" is a common way to consider how long the effect of a substance lingers. This is the length of time it takes the body to metabolize and eliminate half the amount present. The amount left after a half-life can still be buzz-worthy: A 16-ounce cup of coffee can contain well over 300 milligrams of caffeine, and after one half-life, there would still be 150 milligrams -- the amount in a double espresso or a regular-sized cup of coffee.
Caffeine's "half-life" for an average healthy individual will be about five to seven hours, but the range is extensive when considering a more general population. Those who break down caffeine fastest are smokers and, strangely enough, children. It takes them about three hours to eliminate half their body's caffeine. For women taking birth control pills, the rate increases to 13 hours. The half-life in pregnant women is 18 to 20 hours, but the rate of breakdown returns to normal within a month after delivery. A newborn does not gain any real ability to metabolize caffeine until he or she is several days old. Nursing mothers need to be aware that any caffeine received through breast milk during this period has a half-life of about three to four days.
Another consideration is the element of tolerance. After about three to five days of regular use, the normal, healthy human body adapts to the presence of caffeine. That means that the buzz you get on Day 1 is not the same buzz you will get after several days of habitual intake. The process of adaptation involves adenosine, a neurotransmitter that conveys directives to slow down and relax. Caffeine binds with adenosine receptors, preventing them from responding to normal directives, with the net effect that the body stays more alert. But after a number of days of regular caffeine consumption, the body compensates by adjusting the number and sensitivity of its adenosine receptors. Some people are better at this than others, which explains how some people can have coffee with dinner and suffer no effect on sleep.
If a habitual caffeine user abruptly stops, it leaves their body hyper-responsive to adenosine, because caffeine is no longer present to moderate adenosine's effects. This circumstance is believed to be responsible for the headache, malaise and flu-like symptoms often associated with an abrupt cessation of caffeine intake. Those who fast for blood tests, medical procedures or religious holidays are familiar with the sensation. Having caffeine reverses these effects. (It has been argued that the continued consumption of caffeine may depend as much on the avoidance of the withdrawal effects as on any enjoyment of the caffeine beverage itself.)
If one were to stop caffeine intake entirely, the adenosine receptors in the body would shift back to their pre-caffeine state over a few days.
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