Healthcare and Medicine Reference
In-Depth Information
basal need for sleep appears to be between 7.5 and 8.5
hours per day in healthy adult humans. This number was
based on a study in which prior sleep debt was completely
eliminated through repeated nights of long-duration sleep
that stabilized at a mean of 8.17 hours. 15 A similar value
was obtained from a large-scale dose-response experiment
on chronic sleep restriction that statistically estimated
daily sleep need to average 8.16 hours per night to avoid
detrimental effects on waking functions. 4
“optional” sleep. There is, instead, recovery sleep, which
may or may not be optional, although there have very few
studies of the sleep needed to recover from varying degrees
of chronic sleep restriction.
Adaptation to Sleep Restriction
One popular belief is that subjects may be acutely affected
after initial restriction of sleep length and may then be able
to adapt to the reduced sleep amount, with waking neuro-
cognitive functions unaffected further or returned to base-
line levels. Although several studies have suggested that
this is the case when sleep duration is restricted to approxi-
mately 4 to 6 hours per night for up to 8 months, 6 , 9 there
is also evidence indicating that the adaptation is largely
confined to subjective reports of sleepiness but not objec-
tive cognitive performance parameters. 4 This suggests that
the presumed adaptation effect is actually a misperception
on the part of chronically sleep-restricted people regarding
how sleep restriction has affected
Core Sleep versus Optional Sleep
In the 1980s, it was proposed that a normal nocturnal sleep
period was composed of two types of sleep relative to
functional adaptation: core and optional sleep. 16 , 17 The
initial duration of sleep in the sleep period was referred
to as core, or “obligatory,” sleep, which was posited to
“repair the effects of waking wear and tear on the cere-
brum.” 16, p. 57 Initially, the duration of required core sleep
was defined as 4 to 5 hours of sleep per night, depending
on the duration of the sleep restriction. 16 The duration of
core sleep has subsequently been redefined as 6 hours of
(good-quality, uninterrupted) sleep for most adults. 14
Additional sleep obtained beyond the period of core sleep
was considered to be optional, or luxury, sleep, which “ills
the tedious hours of darkness until sunrise.” 16, p. 57 This core
versus optional theory of sleep need is often presented as
analogous to the concept of appetite: Hunger drives one
to eat until satiated, but additional food can still be con-
sumed beyond what the body requires. It is unknown
whether the so-called optional sleep serves any function.
According to the core sleep theory, only the core portion
of sleep—which is dominated by slow-wave sleep (SWS)
and slow-wave activity (SWA) on an electroencephalogram
(EEG)—is required to maintain adequate levels of daytime
alertness and cognitive functioning. 16 The optional sleep
does not contribute to this recovery or maintenance of
neurobehavioral capability. This theory was strengthened
by results from a mathematical model of sleep and waking
functions (the three-process model) that predicted that
waking neurobehavioral functions were primarily restored
during SWS, 18 which makes up only a portion of total sleep
time. However, if only the core portion of sleep is required,
it would be reasonable to predict that there would be no
waking neurobehavioral consequences of chronically
restricting sleep to 6 hours per night, and that cognitive
deficits would be evident only when sleep durations were
reduced below this amount. Experimental data have not
supported this prediction. 10 For example, findings from the
largest sleep dose-response study to date, which examined
the effects of sleep chronically restricted to 4, 6, or 8 hours
of time in bed per night, 4 found that cognitive performance
measures were stable across 14 days of sleep restriction to
8 hours time in bed, but when sleep was reduced to either
6 or 4 hours per night, significant cumulative (dose-depen-
dent) decreases in cognitive performance functions and
increases in sleepiness were observed. 4
It appears, therefore, that the “core” sleep needed to
maintain stable waking neurobehavioral functions in
healthy adults aged 22 to 45 years is in the range of 7 to 8
hours on average. 18 Moreover, because extended sleep is
thought to dissipate sleep debt caused by chronic sleep
restriction, 3 it is not clear that there is such a thing as
their cognitive
capability.
One factor thought to be important in adaptation to
chronic sleep restriction is the abruptness of the sleep
curtailment. One study examined the relationship between
rate of accumulation of sleep loss, to a total of 8 hours, and
neurobehavioral performance levels. 19 After 1 night of total
sleep deprivation (i.e., a rapid accumulation of 8 hours of
sleep loss), neurobehavioral capabilities were significantly
reduced. When the accumulation of sleep loss was slower,
achieved by chronically restricting sleep to 4 hours per
night for 2 nights or 6 hours per night for 4 nights, neu-
robehavioral performance deficits were evident, but they
were of a smaller magnitude than those following the night
of total sleep loss. A greater degree of neurobehavioral
impairment was evident in those subjects restricted to 4
hours for 2 nights than in those subjects allowed 6 hours
per night, leading to the conclusion that during the slowest
accumulation of sleep debt (i.e., 6 hours per night for 4
nights), there was evidence of a compensatory adaptive
mechanism. 19
It is possible but not scientifically resolved that different
objective neurobehavioral measures may show different
degrees of sensitivity and adaptation to chronic sleep
restriction. For example, in the largest controlled study to
date with statistical modeling of adaptation curves, cogni-
tive performance measures showed little adaptation across
14 days of sleep restriction to 4 or 6 hours per night, com-
pared with 8 hours per night, 4 whereas waking EEG mea-
sures of alpha and theta frequencies showed no systematic
sleep dose-dependent changes over days. 14 Consequently,
different neurobehavioral outcomes showed markedly dif-
ferent responses to chronic sleep restriction, with neuro-
cognitive functions showing the least adaptation, subjective
sleepiness measures showing more adaptation, and waking
EEG measures as well as non-rapid eye movement (non-
REM), SWS measures showing little or no response. 4 , 14
The reliability of the latter findings may depend on the
dose of restricted sleep and other factors.
Two-Process Model Predictions of
Sleep Restriction
Biomathematical models of sleep-wake regulation have
been used to make predictions about recovery in response
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