It's no wonder coffee is such a popular morning drink. A new study shows that the grogginess we experience when we first awake from sleep may affect our ability to think clearly, known as "sleep inertia."
The study, published in Tuesday's issue of the Journal of the American Medical Association, is especially important for people who are expected to perform immediately upon awakening -- such as physicians, pilots, truck drivers and military personnel, said lead author Dr. Kenneth Wright, of the University of Colorado at Boulder.
The study involved nine healthy individuals who stayed at a sleep laboratory for several nights. After a full night's rest, they were awakened and given a simple math test, variations of which were repeated for the next 24 hours.
The researchers noted that the people in the study performed the worst within the first three minutes of awakening. Previous studies have shown that sleep inertia may affect cognitive performance for up to two hours, the study authors note.
Dr. William Dement, director of Stanford University's Sleep Disorder Clinic, called the study "very important" -- especially for doctors in training, who are often required to make quick, critical decisions upon awakening.
"There are mistakes that are made in the medical field because of fatigue," he said.
Psychologist Rosalind Cartwright agreed.
"Sleep is good for us but there is a needed transition time from sleep to waking when good judgment is called for," said Cartwright, a professor and chairman of the Department of Behavioral Sciences at Rush University Medical Center in Chicago.
The impact of sleep inertia varies for each person, scientists say.
"There appears to be some diversity in this trait -- some people jump out of bed and hit full speed immediately, whereas others struggle through a shower, two to three cups of coffee, before they wake up," said Dr. Robert Ballard, director of the Sleep Center at National Jewish Medical and Research Center in Denver.
Wright did acknowledge several limitations to the study, including the low number of study subjects. He also said the study was conducted under ideal conditions, and that further studies will need to be conducted under "real world" circumstances, where specific job-related tasks could be measured.
Dr. Mark Wernick is with Beth Israel Deaconess Medical Center in Boston.