In the first bleary-eyed moments of wakefulness, cognitive function and short-term memory may be more muddled for 8-hour-a-night sleepers than for the sleep-deprived, a new study suggests. The new finding suggests that that highly touted cure-all--a full night's sleep--may actually have a downside.
In the past, studies have suggested that cognitive function is impaired during so-called sleep inertia--the period immediately after waking. Scientists reasoned that sleep inertia, which can last from 1 to 3 hours, would have only a modest impact on clear thinking compared to cognition problems in the sleep deprived. But when physiologist Kenneth Wright and his colleagues at the University of Colorado at Boulder took a closer look, they found the opposite.
Wright's team recruited nine adults and had them sleep 8 hours a night for 4 weeks. The volunteers slept the first 3 weeks at their homes to adjust to the sleep schedule. During the last week, participants lived at the laboratory so that scientists could analyze their cognitive function after an 8-hour monitored sleeping period. In the lab, the study participants were repeatedly put through a math drill for 6 days: They had 2 minutes to add a string of two-digit numbers in their heads and type the answers on a keyboard. Most performed best within 2 to 4 hours of waking up. On the seventh day, they were given the test within 1 minute of waking and performed only about 65% as well, the group reports in the 11 January Journal of the American Medical Association.
Then, the participants were kept awake for 26 hours and given the test every 2 hours. Despite the sleep loss, they performed better than they did after sleeping 8 hours a night, suggesting that cognitive function is greatly impaired during the first moments of wakefulness--even after a good night's sleep. "People are performing worse first thing in the morning than when we kept them awake all night," Wright said.
"The real question is, how does this relate to performance in the real world under less controlled circumstances," says Timothy Roehrs, a psychophysiologist at Henry Ford Hospital in Detroit, Michigan, who cautions against drawing broad conclusions from such a small study. Wright's lab is now relying on a larger group of volunteers to examine the impact of sleep inertia on physical performance as well as other cognitive functions. That study will end later this year.