DRINK TO YOUR HEALTH:

Frequent, small amounts of caffeine help keep you awake and mentally alert better than more caffeine consumed less often, according to researchers from the Harvard Medical School , Brigham and Women's Hospital and Rush University Medical Center at the Rush's Sleep Disorders Center.

In the study, Rush University sleep researcher James Wyatt, PhD and his team discovered that caffeine works by blocking the homeostatic system, one of two physiological processes that govern the sleep-wake cycle. Caffeine appears to block the brain receptor for adenosine, a critical chemical messenger involved in that process.

The homeostatic system induces sleep over time, increasing adenosine levels the longer one is awake. The other process, the circadian system, induces sleep cyclically, like an internal clock that periodically releases melatonin and other hormones. If both systems worked in tandem, the drive for sleep would be overwhelming, and the two processes usually oppose one another.

Aligning Processes

According to the study findings, caffeine would be most effective if it were consumed in sync with growing pressure from the homeostatic system. That means taking smaller amounts of caffeine over time to offset the hormone's uptake as it increases during the day. As a result, the researchers proposed an innovative regimen of frequent small doses of caffeine to help shift workers, medical residents, truck drivers and others who need to stay awake for long periods of time to get enhanced benefit from the coffee they drink.

According to Dr. Wyatt, consuming larger amounts of caffeine in the morning is not the way to promote wakefulness and alertness. "Unfortunately, the physiological process they need to counteract is not a major player until the latter half of the day." Instead, caffeine levels are falling through the day at the same time adenosine is increasing.

Testing the Hypothesis

To test their suspicions, the scientists studied 16 men housed in private rooms with no time cues for 29 days. The subjects were kept on a schedule of 42.85 hours, rather than 24, to disrupt their circadian system and maximize the effects of the homeostatic need for sleep, as well as to simulate the extended wakefulness required of those who work longer days. Some subjects received a small, hourly dose of caffeine, proportional to body weight, while others got a placebo.

Those who consumed caffeine exhibited fewer accidental sleep onsets and performed better on cognitive tests. Placebo subjects were unintentionally asleep 1.57 percent of the time during wake periods, compared with .32 percent for those receiving caffeine. However, despite the enhanced wakefulness, the caffeine-taking subjects reported feeling sleepier than those who took the placebo, suggesting that the wakefulness induced by caffeine does not replace the restorative effects gained during sleep.

These behavioral differences confirmed that caffeine imparts its sleep-reducing benefits by working on the homeostatic rather than the circadian system. Dr. Wyatt added, "Where 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, particularly when someone has to remain awake longer than the tradition 16-hour wake episode."