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THE SLEEP INERTIA PHENOMENON DURING THE SLEEP-WAKE
TRANSITION: THEORETICAL AND OPERATIONAL ISSUES
MICHELE FERRARA, PH.D.
, LUIGI DE GENNARO, PH.D.
Department of Psychology, University of Rome "La Sapienza", Italy
Address reprint requests to:
Michele Ferrara, Ph.D.
Dipartimento di Psicologia
Università degli Studi di Roma “La Sapienza”
Via dei Marsi, 78; 00185 Roma, Italia
Sleep researcher at the Sleep Psychophysiology Laboratory, Department of
Psychology, University of Rome “La Sapienza”, and consultant of Italian Air Force
Aerospace Medicine Department (C.S.V.), Pratica di Mare AFB (Rome), Italy.
This material has been partly published by the Research and Technology Agency,
North Atlantic Treaty Organization (RTO/NATO) in MP-31, Individual Differences
in the Adaptability to Irregular Rest-Work Rhythms. Status of the Use of Drugs in
Sleep-Wakefulness Management, 1999.
Running Head: Sleep Inertia: theoretical and operational issues
Sleep Inertia (SI) defines a period of transitory hypovigilance, confusion,
disorientation of behavior and impaired cognitive and sensory-motor performance
that immediately follows awakening. SI, the cognitive and behavioral correlate of the
transition from sleep to wakefulness, has been incorporated in several models of sleep
and vigilance regulation. Monitoring of several physiological parameters during the
awakening period cleary indicate that this transition process is very slow. On the
cognitive and behavioral side, SI has relevant operational implications. SI is one of the
most serious contraindications to the use of napping during quasi-continuous
operations if the individual may be required to perform complex tasks immediatly
after sudden awakening at unpredictable times. The studies on SI modulating factors
showed that SI is strongly affected by slow wave sleep amount and sleep depth, while
the outcomes concerning the modulation of SI by circadian factors are not consistent.
Cognitive tasks involving high attentional load seem to be much more affected by SI
than simple motor ones, performance accuracy being more impaired than speed.
Finally, some possible countermeasures against the detrimental effects of SI to be
applied in operational settings have been provided.
Index Terms: Sleep inertia, Sleep management, Performance upon awakening, Sleep-
THE SLEEP INERTIA PHENOMENON DURING THE SLEEP-WAKE
TRANSITION: THEORETICAL AND OPERATIONAL ISSUES
“Transitional states are among the most difficult to characterize and understand.
They occur during times of shifting priorities and constitute hybrid conditions which
borrow features from the more distinctive parent states. It follows that the more
complex and the greater the differences between the parent states, the more likely it
will be that the transitional processes will also be complex. For these reasons, there
has been a tendency to consider these states as somewhat confusional conditions to be
recognized and controlled for, but not often the subject of detailed study”(43). This
statement perfectly applies in particular to the transition from sleep to wakefulness.
In fact, while sleep onset has received increasing attention in the last two decades, the
emergence from sleep still remains a poorly understood phenomenon.
In this review we will focus our attention on Sleep Inertia (SI), a period of
transitory hypovigilance, confusion, disorientation of behavior and impaired
cognitive and behavioral performance that immediately follows awakening (30). SI
has been considered a "paradoxical" phenomenon (30) since performance upon
awakening is worse than before sleep. However, if physiological sleep phenomena
are best described by sinusoidal rather than by square-wave functions (5),
consequently the underlying behavioral states cannot readily be switched on and off
when shifting to another state. For this reason, if we consider the transition from sleep
to wakefulness as a complex process that takes some time to be completed, more than
an exact shifting point from one state of consciousness to another, SI simply becomes
the cognitive-behavioral face of this transition process.
SI has interesting theoretical and operational implications. It has been included as
an important component in several models of alertness and performance (1, 3, 20). It
integrates the influence of two other major components: a 24-h circadian component
(Process C), with a sinusoidal shape; and a homeostatic component (Process S), that
increases exponentially during wakefulness and is reversed during sleep. The
wakeup component, or Process W, was originally incorporated to take into account
the fact that people take some time to wake up properly (20). It takes the form of a
deviation from Process S that decreases in an asymptotic manner as a function of the
logarithm of hours awake, and ceases about 2-3 hours after awakening (20). Although
in the above-mentioned performance and alertness models SI acts independently of
Processes C and S, it is not possible to exclude that Process W may interact with
Process S in a non-linear manner (28). Consequently, the magnitude and/or the time
constant of the dissipation of SI may increase as a consequence of sleep deprivation
(so that S is very high).
SI is a robust phenomenon that must be taken into account in many operational
settings. The effects of sleep deprivation and chronobiological variations in
performance are undoubtedly among the most pervasive limitors of human ability in
all situations that require sustained periods of continuous performance and in
around-the-clock work settings (e.g. 12). These work scenarios are becoming
increasingly common, often involving highly skilled and dedicated personnel as in
sustained military operations, space flight preparation and launching, crisis and
catastrophe management (38). In all these situations, the negative effects of sleep loss
during sustained operations must be compared to the adverse effects of SI upon
abrupt awakening from sleep due to a possible emergency (11, 12). SI is one of the
most serious contraindications to the use of napping during quasi-continuous
operations if the individual may be required to perform complex tasks immediatly
after sudden awakening at unpredictable times (10).
From a physiological point of view, a clear dissociation between different
parameters is evident during the awakening period. Based on the standard EEG
scoring system (44), the awake EEG is identified by a predominant alpha rhythm.
However, the EEG represent only a fraction of all the state-determining factors. In
other words, "the presence of all polygraphic features of one state does not mean that
no (unmonitored) variables of another state are present" (36). As an example,
Broughton (8) showed that visual evoked potentials (VEP) recorded upon awakening
are more similar to those obtained during sleep than to baseline waking values.
Following one-fourth of the arousals from slow-wave sleep (SWS), VEP contained an
apparent carry-over of typical SWS components. Even when there was no such carry-
over, the VEP regularly showed decreased amplitude and increased latency of 100-
300 msec components. No similar changes in visual evoked potentials were observed
after awakenings from REM sleep. The author ascribes these results to an impairment
of cerebral responsiveness ("functional deafferentation") after SWS awakenings, also
responsible for the behavioral changes (namely confusion) anecdotally reported only
after awakenings from slow-wave sleep.
Other indications of a slow shift from the sleep EEG substrate to that of
wakefulness come from the study of EEG power spectra during spontaneous sleep-
wake transitions (42). The spectral analysis (Fast Fourier Transform, FFT) of EEG
sampled during behaviorally identified (button pressing to stop a tone) spontaneous
arousals from sleep showed a non-predicted gradual and continued drop of theta
and delta power well into the first few minutes of wakefulness. Although delta
power decreased by almost 50% at the first behavioral response, there was a
statistically significant difference between sleeping and waking delta only after the
subject had responded to three consecutive tones (i.e., about 70 sec after the first
response). Theta power trend was very similar to that seen for delta frequencies.
Similarly, studies on cerebral blood flow - CBF - (e.g. 37) and cerebral blood flow
velocities - CBFV - during sleep (e.g. 24, 31) as indirect but reliable indexes of the
underlying neuronal metabolism and activity (e.g. 49), also suggest that the period
immediatly following nocturnal and morning awakenings have blood flow
characteristics that are not comparable to daytime levels. Moreover, Hajak and co-
workers (24) showed that upon morning awakening, subjects required up to half an
hour to reach CBFV values corresponding to the waking state of the previous
evening. The delayed increases in CBFV after awakening suggest an uncoupling of
cerebral electrical activity and cerebral perfusion and provide another example of
dissociation between different physiological parameters of sleep-wake transition,
further stressing the slowness of this transition.
SI and Sleep Management
SI has relevant operational implications. As already mentioned in the Introduction,
from a sleep-logistic perspective the main problem is to weigh the effects of sleep loss
on sleepiness and performance against the adverse effects of SI upon abrupt
awakening from sleep due to a possible emergency. From this point of view, one of
the most critical factors on SI concerns its duration and time course.
However, although SI has been incorporated in several models of sleep and
vigilance regulation (1, 3, 20), only a few attempts have been made to experimentally
quantify its time course. Most authors have typically made only one performance
assessment after awakening (e.g. 41), not allowing the determination of the time
course and duration of SI. Due to this methodological limitation, SI has been
generally reported to be short-lasting, being comprised between 1 and 20 minutes
(11, 26, 27, 34, 48).
Achermann and co-workers (2) addressed this issue by assessing subjective
alertness and reaction times in a memory task every 20 minutes (4 times) during the
first hour after awakening from nighttime sleep or from an evening nap, and finally
after three hours from each awakening. For both alertness and performance
measures, they found SI to persist for slightly less than one hour, and to subside
according to exponential functions with time constants of 0.45 and 0.30 hour,
respectively. More recently Jewett and coll. (28) reported that subjective alertness and
performance in an addition task show a sharp rise in the first hour after awakening
and begin to level off about 2 hours after awakening, reaching the baseline waking
values between 2 and 4 hours after awakening. Also in this case an asymptotic
dissipation of SI has been suggested, since a saturating exponential function
provided a good fit to the data for each measure. The time constants for the
dissipation of sleep inertia were 0.67 h in subjective alertness and 1.17 h in cognitive
performance (number of additions performed). Finally, in another study (17) it has
been found that performance accuracy in a subtraction task reaches the baseline level
after 30-45 minutes from the morning awakening, showing an increasing linear trend
during the first 75 minutes after awakening. On the other hand, although sensory-
motor (auditory reaction times) and simple motor (finger tapping) performances
were less affected by SI, they were still below baseline levels in the same period of
time, never reaching the baseline level during the testing period.
The differences in SI duration and time-course reported in the above-mentioned
studies (2, 28, 17) may be due to some relevant methodological differences between
them: as an example, Achermann et al. (2) did not assess SI between 1 h and 3 h after
awakening, while Ferrara et al. (17) gave no tests after the first 75 min from the
awakening. In addition, in the latter study SI was assessed upon awakenings placed
between 8 a.m. and 9 a.m. after nocturnal sleep episodes of 7.5 h characterized by
different sleep homeostasis conditions (comprising SWS deprivation and recovery
nights), while in the former SI was assessed at 7 a.m. after nocturnal sleep, as well as
at about 9 p.m. after an early evening nap. Furthermore, the Jewett et al. data (28)
were collected in an environment free of time cues, and during the third day of the
experiment subjects were exposed to very dim light during a constant routine
Differences in reported results can also be due to possible differential sensitivity of
the performance tasks used to assess SI. As an example, the time constant for the
dissipation of SI in cognitive performance (number of additions performed) found by
Jewett et al. (28) is much larger than that reported for reaction times in a short term
memory task by Achermann et al. (2), suggesting that some neurobehavioral
functions may be more sensitive to SI than others.
In conclusion, the discrepancies in the reported time-course and duration of SI are
accounted for by large differences in the experimental paradigms used, leading to
uncontrolled interactions between homeostatic and circadian (time-of-day) processes
regulating sleep and wakefulness, as well as to the use of several different tasks and
variables to assess SI.
SI: Modulating Factors
SI duration and magnitude can be modulated by several factors. There are
differential effects of REM/NREM sleep stages on performance upon awakening.
More specifically, SWS awakenings have often been reported to have greater
negative effects on subsequent performance than REM sleep awakenings. These
effects have been demonstrated with a wide array of tasks: simple motor tasks (54,
55); sensory-motor tasks (16, 47); and cognitive tasks (47, 51). However, it is not clear
whether the above-mentioned differential effects are due to neurophysiological,
psychophysiological and functional differences between REM and NREM states, or
whether to uncontrolled temporal and circadian influences (time-of-night effects), or
to an interaction between the two. All these variables should require further
exploration with a more controlled research methodology.
Furthermore, it has been claimed that sleep structure is also very important in
determining SI (11). The increased sleep depth (in terms of both amount of SWS and
sleep stage at awakening) caused by sleep deprivation dramatically exacerbate SI and
cognitive impairment upon awakening from recovery sleep (10). It has also been
found that cognitive decrements after abrupt awakenings from 1 and 2 hour naps
show a linear relationship with SWS amount during the nap (10, 15).
Moreover, the negative influence of sleep deprivation on SI seems to interact with
time-of-night or circadian factors in producing even more dramatic effects. As an
example, Naitoh (40) reported that, after a 2-hour nap taken early in the morning
(0400-0600 a.m.) following 45 hours of continuous work without sleep, both task
performance and self-rating of mood, sleepiness and fatigue remain deteriorated up
to 6 hours. This long-lasting sleep inertia effect was not observed when a nap of the
same duration was taken at 1200-1400, after 53 hours of wakefulness; in fact,
following this midday nap, sleep inertia disappeared within 1 hour and was then
replaced by improvements.
More generally, the outcomes concerning the modulation of SI by circadian factors
- mainly linked to body temperature rhythm - are not consistent. Conflicting
evidence comes from studies of napping with and without previous sleep
deprivation (e.g. 7, 41, 52), as well as from repeated awakenings during nocturnal
sleep (e.g. 4, 45, 46). As an example, Bonnet & Arand (7) reported a worsening of SI
effects following awakening at 5:00 a.m., but some other studies showed greatest
performance impairment upon awakenings placed in the first part of the night (22,
23, 52). It is evident that these approaches necessarily confound the effects of the
circadian phase with those of homeostatic variables (i.e., the amount of prior sleep or
of prior sleep loss, if sleep deprivation is involved). However, this problem is difficult
to solve, if possible at all, since both of these factors are temporal dimensions that
covary with each other (10). For these reasons, the available evidence for circadian
modulation of SI can not be considered definitive, and a more accurate description of
circadian influences on SI needs the support of further empirical data collected with
Moreover, SI seems to dramatically depend on the type of task used, highly
demanding cognitive and attentional tasks being much more affected than simple
motor ones (39). As regards the impairment of simple sensory-motor performance
(auditory reaction times and finger tapping task) upon awakening, a recent study
(18) reported that it is accounted for by: a general decrement in overall response
speed (median of RT); a decrease in response speed in the "optimum response
domain"; and an increase of lapsing. Consequently, behavioral performance slowing
upon awakening is not simply due to lapses or failure to respond, but should be
ascribed to a general decline in the ability to allocate attention to the task and to give
the required motor response as fast as possible. As regards cognitive performance, at
variance with physiological sleepiness, which in self-paced tasks affects speed of
performance more than accuracy (6, 13, 14), it has been claimed that SI exerts a
negative influence on both, but particularly on the latter (4, 41). Some recent evidence
confirmed that the lowered level of brain arousal upon awakening adversely affects
cognitive performance accuracy more than performance speed. (19).
In conclusion, although it is often difficult to compare results of studies on SI, since
several different experimental designs and tasks have been used, a few clear
indications seem to emerge. The intensity of SI is strongly influenced by some
homeostatic sleep variables linked to SWS amount and, more generally, to depth of
sleep (indexed by awakening thresholds or by sleep stage at awakening). Moreover,
circadian factors and previous sleep loss exacerbate SI by adding their simple effects.
Finally, cognitive tasks involving high attentional load seem to be much more
affected by SI than simple motor ones, performance accuracy being more impaired
From a review of the literature on the physiological basis and modulating factors
of SI, we will try to suggest some countermeasures against the detrimental effects of
SI on performance upon awakening, to be applied when it is possible in operational
The first countermeasure could be to reduce the probability of awakening out of
SWS, since it is well known that SWS awakenings yield the greatest performance
decrements. One possibility is to allow sleep when the occurence of SWS is very low
(e.g., in the morning). Another strategy can be to allow naps of about 80-100 minutes
(i.e., the mean range of duration of normal NREM-REM sleep cycles), or,
alternatively, of about 20 minutes, minimizing the probability of a SWS awakening.
Some experimental data confirm the usefulness of this strategy, by showing that SI
magnitude after a 20-min and a 80-min nap are very similar, while the worst
performance upon awakening is recorded after a 50-min nap (50). Obviously, a 80-
min nap should be preferred to a 20-min nap because of its greater restorative power.
Another very important strategy to minimize SI is to avoid a long period of
wakefulness before allowing a nap, since the increase of sleep depth caused by sleep
deprivation dramatically exacerbates SI (10, 40). As already suggested by others (10),
sleep opportunities should be provided before sleep loss accumulates beyond 36
hours, since longer and more severe performance decrements have been reported on
awakening from naps taken after this time as compared to naps taken within 30
hours of wakefulness (10, 40).
In addition, awakening near the circadian nadir of body temperature should also
be avoided, especially if the sleep period follows sleep deprivation (40).
It has been reported that washing one's face with cold water immediately after
awakening is a simple but effective tool to fight SI (32, 33). More generally, every
"alerting" factor (i.e., noise, light, physical exercise) should be useful in counteracting
SI, even though - at present - only few attempts have been made to assess their
effectiveness. As an example, pink noise (75 dBA) administered during the first hour
after awakening improves response speed at 0500 but not at 0800, when it has
detrimental effects on performance (53). Although the authors claimed that all
subjects experienced the same amount of prior sleep debt, since each of them slept
for three hours during the experimental night, it has to be noted that the group
woken up at 0800 possibly experienced a greater homeostatic pressure for sleep,
because their sleep time was postponed by two hours as compared to the group
woken up at 0500. Consequently, the different homeostatic pressure acted as a
possible confounding variable, casting some doubts on the interpretation of results.
More recently, it has been reported that following the "normal morning routine"
(i.e., getting out of bed, taking a shower, having breakfast) does not abolish SI as
compared to a constant routine in bed (28). In the same experiment (28), it was found
that exposure to normal room light (about 150 lux) upon awakening did not improve
performance as compared to very dim light (about 20-25 lux). However, it has to be
noted that in the above-mentioned study (28) the exposure to very dim light was
introduced on the awakening of the third experimental day together with a constant
routine condition. Consequently, it is difficult to dissociate the effects of the dim light
condition and those of the constant routine. Moreover, any question about a possible
alerting effect of bright light remains unanswered; it may be that, to detect an
alerting effect, a very bright light (i.e. 2000 lux) upon awakening should have been
used (see below).
SI: Open Questions
SI is still a poorly understood phenomenon from both the point of view of its
physiological substratum, which could be approached in the near future with
neuroimaging techniques, and of the sleep-related modulating factors and
psychological and personality variables that may influence it. However, a few
research areas that should be explored to give important answers on SI, also to be
applied in operational fields, will be pointed out.
The first unexplored topic is the role of individual differences in reactions to the
effects of SI. It is anecdotally well-known that individuals show a wide range of
variation with respect to their perceived ability to function immediately after
awakening. However, the literature on SI has definitely ignored this problem,
relegating individual differences to a role of "confusing variable" to be controlled.
The study of individual difference modulation of SI will add very important
knowledge to the definition of the psychophysiological profile of tolerance of
irregular work hours. As an example, it could be intersting to explore the relation
between diurnal type and SI, since the morningness-eveningness dimension has been
associated with the adjustment to shift work (21, 25, 29).
The same applies to the role of psychological factors, like motivation, in the
modulation of SI. As an example, one should believe that motivation can be a strong
and efficient countermeasure to SI for a fighter pilot sleeping on-call, when he is
requested to be in the cockpit at 5000-10000 metres a.s.l. just 5 minutes after abrupt
awakening. However, this topic should be specifically evaluated.
For operational purposes, the duration and time course of SI after naps taken at
different times of the day should also be further assessed, since available data are
inconclusive. Varying nap duration may also be necessary for a complete
understanding of these aspects of SI.
It would be very important to have some pharmacological countermeasures to SI,
such as very fast acting stimulants, to be used in operational settings when the need
for high levels of alertness and performance immediately after awakening should
arise. To our best knowledge, the use of stimulants to counteract SI effects has never
been tried, not even in laboratory settings.
Non-pharmacological countermeasures to SI could also be very useful,
particularly because pharmacological measures are currently lacking. Generally
speaking, any alerting factor could be assessed to counteract SI: physical and/or
mental exercise, external noise, bright light. As regards noise, although in at least one
study pink noise has been administered for one hour after awakening with non-
univocal results (53), the effectiveness of different types of noise with different
intensities and durations should be assessed. Bright light might also be effective
against SI, since its alerting effects are well established (e.g. 9).
Several observations on the physiological correlates of the sleep-wake transition (8,
24, 31, 37, 42) are in line with evidence coming from human studies on cognitive (e.g.
51) and behavioral (e.g. 54) features of awakening from sleep, pointing out the need
to re-define the sleep-to-wake transition period as a neurophysiologically distinct
state. In other words, emerging from sleep can not be identified with an exact shifting
point from one state of consciousness (sleep) to another (wakefulness), but is better
described as a complex and slow process that takes some time to be completed.
During this transition period, that shares some features with both the wake and sleep
states, a clear dissociation between different parameters (physiological, cognitive and
behavioral) is evident, since they show different rates of change from the sleep
pattern to the wake pattern.
Consequently, in such a situation the subject may still be able to conduct some
simple social interaction (e.g. 11) but the "functional deafferentation", proposed by
Broughton (8) to explain the low levels of brain reactivity upon awakening, will make
it difficult to obtain more demanding and complex performance. For these reasons, in
all these situations requiring highly skilled performance immediatly after an abrupt
awakening (e.g., sustained military operations, medical emergency management), the
unavoidable adverse effects of SI have to be considered in advance, providing
personnel with simple tools to wash out these effects. At the present, the use of any
feasible alerting factor (physical exercise, external noise, bright light, cold water) can
only be suggested. Further research is needed to experimentally clarify which are the
most effective tools to be applied in operational settings.
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