Sleep Debts
Accrue When Nightly Sleep Totals Six Hours or Less
September / October 2003
Those who believe they can function 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. The study, published in 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 six or fewer hours of sleep each
night.
“Routine nightly sleep for six hours or less results
in cognitive performance deficits, even if we feel we have
adapted to it,” says 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. Drs. Dinges,
Van Dongen and their colleagues looked at the effects of
four hours of nightly sleep and six hours of 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 subjects receiving four or six hours of sleep each night
experienced increasing lapses in psychomotor vigilance, resulting
in a decline of performance over 14 days that matched that
of the subjects who went without any sleep for two to three
nights. 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.
In today's 24-hour society many people choose to increase
their wake hours and reduce their sleep so they can accomplish
more. “People who find themselves dozing off in the
middle of the day or falling asleep on the couch at night
are probably sleep deprived. If your patients are not able
to get enough sleep every night they should plan to sleep
in on the weekends or take vacation time on a regular basis
in order to reduce their sleep debt,” explains Dr.
Van Dongen. Although eight hours of sleep each night is still
recommended, it is important to realize that some individuals
need as few as six hours or as many as 10 hours of sleep
each night.
According to Dr. Van Dongen, many experts in the field of
sleep research believed that after several days of not getting
enough sleep, people would adapt to the situation. “We
tested this hypothesis in our study and concluded that after
the first few days of sleep deprivation, the deficits became
significant and the impairment became greater with every
day of continued sleep restriction.
“Most of us are able to relate to the deficit you
build up after two or three days with no sleep whatsoever,
but if you measure it objectively the people with chronic
sleep loss performed just as poorly—they just did not
realize it,” adds Dr. Van Dongen.
Another critical aspect of sleepiness that these Penn scientists
found in an earlier study is “wake-state instability.” Sleep-deprived
people are not necessarily performing poorly at all times
of the day, but their performance may instantly become unreliable,
without warning. When this happens, the brain will work more
slowly in order to maintain accuracy if it can. When fast
reaction times are critical or when there is time pressure,
however, accuracy can no longer be maintained. That is when
errors are made and accidents happen as a consequence of
sleep loss.
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.
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