James Wyatt of the Rush University Medical Center and his colleagues studied 16 men for 29 days while they stayed in private windowless suites free of time cues from the outside world. The researchers scheduled 42.85-hour days for the subjects, simulating the amount of time many doctors, truckers and emergency services personnel must stay awake. By disrupting the men's circadian rhythms, which promote sleepiness in a cyclical fashion, the scientists instead focused on the homeostatic push for sleep, which becomes stronger the longer someone is awake. Previous analyses of caffeine's ability to energize have shown that when people consume a large amount of caffeine in one sitting, the levels of the compound in their bloodstream will peak and then fall as the day goes on. "Unfortunately, the physiological process they need to counteract is not a major player until the latter half of the day," Wyatt explains.
In the new study, the scientists instead tested the effects of administering an hourly, low dose of caffeine equivalent to about two ounces of coffee to one group, while the second group received a placebo. The caffeinated men performed better on cognitive tests than the control individuals did, and dozed off less often. And though they received the same cumulative dose as subjects in previous, single-dose studies, taking many small doses minimized some of the negative side effects that caffeine can have, such as tremors.
The findings strengthen the hypothesis that caffeine blocks a receptor for adenosine, a messenger involved in the homeostatic sleep cycle, but they also indicate that caffeine cant replace the restorative effects of shuteye: although the caffeine-takers stayed awake more than the control group, they reported feeling sleepier. "While there is no perfect substitute for sleep, our results point the way toward a much better method for using caffeine in order to maintain optimal vigilance and attention," Wyatt says, "particularly when someone has to remain awake longer than the traditional 16-hour wake episode."