A new University of Colorado at Boulder study shows that people who awaken after eight hours of sound sleep have more impaired thinking and memory skills than they do after being deprived of sleep for more than 24 hours.
The study showed test subjects had diminished short-term memory, counting skills and cognitive abilities during the groggy period upon awakening known as sleep inertia, said CU-Boulder Assistant Professor Kenneth Wright, lead study author. The new study has implications for medical, safety and transportation workers who are often called upon to perform critical tasks immediately after waking, since cognitive deficiencies following 24 hours of sleep deprivation have previously been shown to be comparable to the effects of alcohol intoxication, he said.
The study appears in the Jan. 11 issue of the Journal of the American Medical Association. Study authors included Wright and Adam Wertz of CU-Boulder's integrative physiology department and Joseph Ronda and Charles Czeisler of Brigham and Women's Hospital in Boston, which is affiliated with Harvard Medical School.
"This is the first time anyone has quantified the effects of sleep inertia," Wright said. "We found the cognitive skills of test subjects were worse upon awakening than after extended sleep deprivation. For a short period, at least, the effects of sleep inertia may be as bad as or worse than being legally drunk."
Following six nights of monitored sleep lasting eight hours per night, the study participants were given a performance test that involved adding randomly generated, two-digit numbers, said Wright. Based on the results, the researchers concluded the subjects exhibited the most severe impairments from sleep inertia within the first three minutes after awakening, he said.
The most severe effects of sleep inertia generally dissipated within the first 10 minutes, although its effects are often detectable for up to two hours, according to the study authors.
Studies conducted by Dr. Thomas Balkin and colleagues at the Walter Reed Army Institute of Research in Washington, D.C., have shown cortical areas of the brain like the prefrontal cortex take longer to come "on-line" following sleep than other areas of the brain, Wright said. The prefrontal cortex is thought to be responsible for problem solving, emotion and complex thought.
The CU-Boulder study has implications for medical professionals who are often called on to tend patients in crisis on a moment's notice, often at odd hours, Wright said. Medical residents, for example, who may work 80 hours or more per week and who "catnap" at times, could be prone to make simple math mistakes when calculating dosages of medicine during bouts of sleep inertia, he said.
The results also have implications for emergency medical technicians and firefighters who may be hastily awakened and called upon to drive a vehicle to an emergency scene, putting themselves and others at risk, said Wright. The study also has implications for commercial truck drivers, who frequently pause for quick naps in their vehicles' sleeping berths during cross-country excursions, he said.
Wright and his colleagues said further studies are needed to measure the effects of nap interruption and "recovery sleep" in on-call, sleep-deprived individuals.
The study also illuminates the challenges faced by everyday people who are forced to make crucial decisions following abrupt awakening. "If a person is awakened suddenly by a fire alarm, for example, motivation alone may be insufficient to overcome the effects of sleep inertia," he said.
The paid study volunteers, nine of whom were included in the sleep inertia study, slept eight hours per night during the month leading up to the study, had no medical, psychiatric or sleep disorders and were free of medication including alcohol, nicotine, recreational drugs and caffeine. The subjects also spent several hours each day during their six-day, in-patient stay practicing the math test used to quantify sleep inertia.
"These were very healthy people who had performed the test hundreds of times, making the results even more profound," said Wright.
Located at Boston's Brigham and Women's Hospital, the sleep lab used in the study contained infrared video cameras, audio equipment and physiological recorders that monitored sleep patterns of participants through the night, he said. Wright previously was a neuroscientist on the hospital staff and faculty instructor at Harvard Medical School before coming to CU-Boulder. The data was analyzed at CU-Boulder.
The study was funded primarily by the National Space Biomedical Research Institute, a consortium of 12 institutions headquartered in Houston and established in 1997 by NASA.
CU-Boulder's Biological Sciences Initiative and Undergraduate Research Opportunities Program provided additional funding for the study.
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