According to a new report, caffeine effectiveness is determined by the hour at which the coffee is consumed because of the body’s cortisol cycle.

The body’s circadian rhythm is responsible for dictating when a person feels sleepy, hungry, happy, or sad. This clock controls sleep-wake cycles, hormone release, body temperature, and other functions. Cortisol — a stress hormone that increases sugar in the bloodstream — is typically released shortly after waking up in the morning, according to Medscape. Healthy people experience a 50 percent increase in levels during the first 20 to 30 minutes after waking up in the morning. Since cortisol levels are at their peak in the bloodstream, they promote alertness and help regulate the circadian rhythm — a natural boost without the caffeine.

Steven Miller, a neuroscientist at the Uniformed Services University of the Health Sciences in Bethesda, Md., wrote in Washington-based science news blog NeuroscienceDC that people should hold off on consuming caffeinated beverages early in the morning when cortisol concentration is at its peak. “Drug tolerance is an important subject, especially in the case of caffeine since most of us overuse this drug,” he wrote. “Therefore, if we are drinking caffeine at a time when your cortisol concentration in the blood is at its peak, you probably should not be drinking it.”

Those who drink coffee when cortisol levels are high — commonly between 8 a.m. and 9 a.m. — are more likely to develop a tolerance to caffeine. This increased tolerance means the drug will become less effective in the body. Miller explains this by referring to one of the key principles of pharmacology: use a drug when it is needed.

“In other words, the same cup of morning coffee will become less effective and this is probably why I need a shot of espresso in mine now,” he wrote, the Telegraph reports.

To maximize the benefits of your caffeinated beverage, coffee should be consumed between 9:30 a.m. and 11:30 a.m. when cortisol levels are dropping before the next surge. Cortisol levels peak in the bloodstream between 8 a.m. and 9 a.m., between noon and 1 p.m., and between 5:30 p.m. and 6:30 p.m. Miller acknowledged that cortisol levels at various times throughout the day vary from person to person, which means the best time to drink coffee varies from person to person. Early birds will commonly experience a drop of cortisol levels earlier than those who sleep in.

Coffee can not only increase alertness, but also benefit your health. In a study published in the journal Hepatology, a morning cup of coffee or tea was found to protect drinkers from liver failure. Researchers used cell culture and mice to examine the effects of caffeine on non-alcoholic fatty livers (NAFLD). The consumption of caffeine stimulated the metabolization of lipids stored in the liver cells, which decreased the fatty liver of the mice on a high-fat diet. These findings may be applied to humans if they consume four cups of coffee or tea per day to protect against the progression of NAFLD, especially those who tend to have a high-fat diet.

According to the National Coffee Association's 2013 online survey, 83 percent of adults in the U.S. drink coffee, averaging three cups of day per person. The U.S. continues to remain the world’s biggest consumer of caffeinated beverages.