They found that people awakening after eight hours of sleep have more impaired thinking and memory skills than someone kept awake for 24 hours -- equivalent to being drunk.

"This is the first time that anyone has quantified the effects of sleep inertia," said Kenneth Wright, a researcher at the University of Colorado-Boulder and lead author of the study, published Wednesday in a letter to the Journal of the American Medical Association.

The study, funded mainly by NASA, was carried out in the sleep lab at Boston's Brigham and Women's Hospital, where Wright was a staff neuroscientist and instructor at Harvard Medical School before joining the Colorado program.

Worse Than Extended Sleep Deprivation

The findings have implications for medical, safety and transportation workers who are often called upon to perform critical tasks immediately after waking up.

"We found that the cognitive skills of test subjects were worse upon awakening than after extended sleep deprivation," Wright said. "For a short period, at least, the effects of sleep inertia may be as bad or worse than being legally drunk."

The study participants were nine paid volunteers, all but one men, ranging in age from 20 to 41, with no medical, psychiatric or sleep disorders.

They stayed in the clinic for six days. Following six nights of monitored sleep lasting eight hours per night, participants were given a performance test that involved adding two randomly generated, two-digit numbers. They'd also been asked to spend several hours doing the task each day they were in the lab. They continued to take the test every two hours while they were kept awake over the next 24 hours.

Each subject did substantially worse on the test than he or she did during any of the later tests.

Some Parts of Brain Take Longer to Come Online

The results showed the most severe impairment from sleep inertia in the first three minutes after awakening, and that level of grogginess dissipated within another seven minutes, although effects were detectable in some subjects for up to two hours.

"These were very healthy people who had performed the test hundreds of times, making the results even more profound," Wright said.

Other studies conducted by Dr. Thomas Balkin and his team at Walter Reed Army Institute of Research in Washington have shown that some parts of the brain critical to analysis and complex thinking take longer to come "online" following sleep than other areas of the brain.

The new findings may be particularly important for medical professionals, such as resident physicians in hospitals who may work 80 hours a week or more and often take "catnaps" at slow times during their shifts, then have to respond to a crisis at a moment's notice.

Wright said those waking suddenly could be more prone to making simple math mistakes on tasks such as deciding medicine doses.

Sleep inertia also takes a toll on emergency medical technicians and firefighters who may be suddenly awakened and have to drive a vehicle to an emergency scene, and for commercial drivers who pause for quick naps during long hauls.

The study also illustrates the challenges that everyone faces when forced to make judgments when abruptly awakened, whether by a crying child or an alarm.

Wright said that more studies need to be done to measure the effects of nap interruption and "recovery sleep" in people who are on-call and sleep-deprived.