'Sleep Debts' Accrue When Nightly Sleep Totals Six Hours Or Fewer; Penn Study Find People Respond Poorly, While Feeling Only 'Slightly' Tired

Those who believe they can function well on six or fewer hours of sleep every night may be accumulating a "sleep debt" that cuts into their normal cognitive abilities, according to research conducted at the University of Pennsylvania School of Medicine. What's more, the research indicates, those people may be too sleep-deprived to know it.

The study, published in the March 15 issue of the journal Sleep, found that chronically sleep-deprived individuals reported feeling "only slightly sleepy" even when their performance was at its worst during standard psychological testing. The results provide scientific insight into the daily challenges that confront military personnel, residents and on-call doctors and surgeons, shift workers, parents of young children, and others who routinely get fewer than six hours of sleep each night.

"Routine nightly sleep for fewer than six hours results in cognitive performance deficits, even if we feel we have adapted to it," said Hans P.A. Van Dongen, PhD, Assistant Professor of Sleep and Chronobiology in Penn's Department of Psychiatry and corresponding author of the study. "This work demonstrates the importance of sleep as a necessity for health and well-being. Even relatively moderate sleep restriction, if it is sustained night after night, can seriously impair our neurobiological functioning."

David F. Dinges, PhD, Professor of Psychology in the Department of Psychiatry and Chief of the Division of Sleep and Chronobiology, served as principal investigator for the study.

Dinges, Van Dongen and their colleagues looked at the effects of four hours nightly sleep and six more hours nightly sleep on healthy volunteer subjects aged 21 to 38, over a two-week period. They compared the results of the subjects' accumulating performance deficits, determined by standard psychomotor vigilance and other cognitive tests, with similar test results obtained from subjects who had gone without sleep for more than three nights.

The first group of subjects experienced increasing lapses in psychomotor vigilance over days, resulting in a decline of performance that matched that of the subjects who went without sleep for 88 hours. At that level, the subjects suffered lapses in their ability to react that would put them at risk driving or flying an airplane. They were also less able to multi-task successfully.

"The physiologic expression of sleep in humans appears to have multiple functions, ranging from the metabolic to the neurocognitive," Van Dongen said.

Other scientists who worked on the study are Greg Maislin, MS, MA, also of Penn, and Janet M. Mullington, PhD, of Beth Israel Deaconess Medical Center and Harvard University.

The study was funded by the National Institute of Nursing Research of the National Institutes of Health with additional financial assistance from the National Center for Research Resources and the National Heart, Lung and Blood Institute.