Why this Safety Advisory Notice was developed
SLEEP INERTIA AND WAKE-UP EFFECTS ON PERFORMANCE
The Australian Transport Safety Bureau (ATSB) recently
investigated sleep inertia and the potential safety issues
associated with it. Research on the factors that influence sleep
inertia and the effects of sleep inertia on performance was
reviewed by the ATSB and it became apparent that sleep inertia was
a safety issue that should be considered by all transport
operators, especially those providing an extended-hours service. A
number of aviation operators were contacted and the majority was
unaware of sleep inertia or the impact it could have on their
operations. As a result of that evidence, the ATSB identified a
need to advise operators of the effects of sleep inertia and
provide information on potential means to mitigate the impairments
that sleep inertia may cause. This Safety Advisory Notice is the
result of that finding.
Fatigue in transportation
Fatigue in transportation is now recognised as a central safety
issue. In 2000, The House of Representatives Standing Committee on
Communications, Transport and the Arts (HRSCCTA) released a report
summarising its inquiry into managing fatigue in transport.
According to this report, "human fatigue is now recognised around
the world as being the main cause of accidents in the transport
industry" (HRSCCTA, 2000, p. 1). A similar concern regarding
fatigue in transportation was raised by the United States Congress
in 1980. The NASA Ames Research Centre carried out a major research
programme during the decade that followed, including several
research projects examining fatigue, jet lag and fatigue
countermeasures. Today, fatigue is still recognised as a primary
cause of serious transportation accidents throughout the United
States (Blakey, 2002).
A definition of fatigue readily applicable to the transportation
industry is that "fatigue is the result of inadequate rest over a
period of time and that fatigue leads to physical and mental
impairment" (HRSCCTA, 2000, p. 2). Sleep inertia is one component
of fatigue relevant to the transportation industry. Sleep inertia
countermeasures should be incorporated into fatigue management
systems or organisational risk/safety audits.
Sleep inertia defined
Sleep inertia refers to the period of poorer task performance that
results immediately after awakening (Naitoh, Kelly, & Babkoff,
1993). It is commonly reported as a feeling of mental dullness or
sluggishness immediately after awakening (Wyatt & Bootzin,
1994) or the poor performance related to the process of arousal
from sleep (Bonnet, 1993).
During a period of sleep inertia, people demonstrate all the
outward physical signs of being awake, but are not cognitively
awake (Naitoh, Kelly, & Babkoff, 1993). Individuals affected by
sleep inertia typically report feeling sleepy, disorientated,
confused and sluggish.
In operations where employees are allowed to sleep (including nap)
while on duty, the effects of sleep inertia may not be mitigated.
Sleep inertia may impair performance and thereby increase the
likelihood of performance errors, reducing transport safety.
When awoken from sleep, a person's ability to perform a range of
tasks is reduced for a time period ranging from a few seconds to 75
minutes (Ferrara & De Gennaro, 2000). The performance
impairment is due to sleep inertia and occurs as a result of
reduced alertness while waking.
Sleep inertia (or sleep drunkenness) is the feeling of
disorientation, mental dullness or sluggishness when a person wakes
(Wyatt & Bootzin, 1994). When awakening from sleep normally,
the effects of sleep inertia are believed to last for less than 5
minutes. When abruptly awoken, the effects of sleep inertia have
been identified as typically lasting up to 30 minutes, but possibly
in excess of 1-2 hours.
Sleep inertia affects reaction time, performance accuracy and
decision making. For example, upon being awoken, an Emergency
Medical Services (EMS) pilot could be required to perform tasks
that may be affected by any and all of these deficits. Estimating
fuel requirements and flight planning involve problem recognition,
problem solving and decision making. Errors in these calculations
may occur if the pilot is under the influence of sleep
If a pilot is asleep when s/he receives a call to carry out a
task, it is probable that in the 6 minutes (the typical response
time reported by most EMS operators) prior to becoming airborne,
the pilot is experiencing some sleep inertia. If present, sleep
inertia effects may influence the pilot's ability to make
decisions. Sleep inertia could contribute to a pilot departing on
an incorrect flight plan or with insufficient fuel.
It should be noted that it is not recommended that operators deny
employees sleep in order to avoid sleep inertia. Rather, it is
recommended that operators acknowledge the potential impact that
sleep inertia may have on performance and take actions to mitigate
these effects. There are several options available to
extended-hours operators to mitigate the effects of sleep inertia,
including increasing response times to provide adequate wake-up
time, the use of automated flight planning, and the use of
non-abrupt means to wake pilots. Other options may be
The effects of sleep inertia
Sleep inertia has been shown to affect memory, performance
accuracy and reaction time, as well as decision making.
- Memory: Sleep inertia has been shown to reduce memory ability
- Performance accuracy and reaction time: Sleep inertia has been
shown to impair performance and reaction time on tasks ranging from
arithmetic tasks, to simple motor tasks such as grip strength and
finger tapping (Balkin & Badia, 1988; Ferrara & De Gennaro,
2000). Performance accuracy is more impaired by sleep inertia than
performance reaction time (Ferrara, De Gennaro, Casagrande, &
- Decision making: Decision making is a cognitively complex
process that involves recognition of the need to make a decision,
generation of decision alternatives and selection of a decision
alternative. Within the first 3 minutes of waking, decision making
performance can be as low as 51% of the person's best decision
making ability before sleep (Bruck & Pisani, 1999) and decision
making performance may still be 20% below optimum performance 30
minutes after waking.
When awoken, a person usually experiences some degree of sleep
inertia. The degree of impairment that sleep inertia has on
performance is influenced by a number of variables,
- Abruptness of awakening: Being abruptly awoken from sleep
increases both the effects and the duration of sleep inertia (Bruck
& Pisani, 1999).
- Stage of sleep interrupted: If awoken from deep or slow wave
sleep the effects of sleep inertia are more pronounced (Dinges,
1989, 1990). Slow wave sleep is more likely to occur during the
early stages of sleep.
- Sleep deprivation: Sleep deprivation increases the effects of
sleep inertia (Ferrara & De Gennaro, 2000).
- Type of task performance: The effects of sleep inertia vary
among different types of tasks. For example, performance accuracy
is more impaired by sleep inertia than reaction time (Ferrara, De
Gennaro, Casagrande, & Bertini, 2000).
- Time between awakening and time of performance: Sleep inertia
will cause less impairment as the time between awakening and task
performance increases (Bruck & Pisani, 1999).
Most researchers agree that sleep inertia lasts a minimum of five
minutes, however, some authors suggest that sleep inertia lasts 15
minutes (Dement, 1999), while others suggest it may require two or
more hours to dissipate completely (Jewett et al., 1999). The most
measurable decrement in performance occurs in the first few minutes
after awakening (Hawkins, 1993). The duration of sleep inertia
depends upon the influencing variables listed above and the task
being performed, such that:
- Sleep inertia can last up to 15 minutes for reaction time and
arithmetic tasks (Dinges, et al., 1981, Wilkinson & Stretton,
- Sleep inertia can last in excess of 30 minutes on more complex
tasks such as decision-making (Bruck & Pisani, 1999).
- The most commonly reported duration for sleep inertia is from a
few seconds up to 20-30 minutes, dependent on the performance being
assessed (Ferrara & De Gennaro, 2000).
Some variables have been shown not to have an impact upon the
effect of sleep inertia on task performance. These variables
- Time of day: The effects of sleep inertia are most apparent when
the individual is abruptly awoken from sleep, regardless of whether
the sleep occurs as a daytime nap or occurs during the night (Bruck
& Pisani, 1999). The exception to this is naps that end during
the low point in the alertness cycle. Sleep inertia will last
longer following naps ending between 0300-0700h (Naitoh, Kelly,
& Babkoff, 1993).
- Circadian rhythm: The stage of the circadian rhythm does not
affect sleep inertia. (Naitoh, Kelly, & Babkoff, 1993). Note,
though, that this is inconsistent with the effect of time of day
described above, and further research is warranted to determine the
effect of circadian rhythm on sleep inertia.
- Sleepiness: No evidence of any relationship between sleepiness
and sleep inertia has been found (Balkin & Badia, 1988).
Preventing sleep inertia
Sleep deprivation has not been recommended to avoid the effects of
sleep inertia. Napping can enhance alertness during sustained
wakefulness, but the importance of the temporal placement of the
nap remains controversial. The exact time a nap should be limited
to depends upon the stage of the circadian rhythm in which the nap
occurs. For this reason, no preferred nap duration can be
recommended. (For a review of the napping literature see Della
Rocco, Comperatore, Caldwell & Cruz, 2000.) Napping to avoid
sleep deprivation can significantly improve alertness,
communication and performance. Rather, it is important that the
potential effects of sleep inertia following a nap be acknowledged
and that actions are engaged to mitigate the effects.
There have been few attempts to identify the effects of counter
measures to sleep inertia. Ferrara and De Gennaro (2000) suggest
that the use of alerting factors upon awakening, such as washing
one's face in cold water, bright lights, loud noise and physical
exercise, may assist in minimising the effects of sleep inertia.
The effectiveness of these alerting factors, however, has not been
The use of automated facilities, for example automating flight
planning and fuel calculations, would reduce the opportunity for
sleep inertia errors in performance. In addition, involving all
crewmembers in any flight planning or decision making that occurs
may reduce the likelihood of errors going unnoticed.
199701060: A late night return duty (flight pairing) was reported
as going against the normal sleep cycle. Pilots were already sleep
deprived due to a 0430 wake up and the early sign-on the previous
morning. The duty involved around 25 hours from the original early
morning sign-on, with four to six hours sleep in the afternoon. The
sleep/work cycle poses the opportunity for sleep inertia to effect
pilot performance following the afternoon nap.
199901850: A pilot conducting a night freight operation was
probably suffering fatigue due to lack of sleep during the day.
After the pilot had levelled the aircraft at the intended cruising
altitude, he fell asleep. As the flight progressed, the pilot
occasionally woke and made slight corrections to the heading, but
he did not identify a tracking error and continued the flight on
the incorrect heading. The aircraft sustained minor damage during
the subsequent landing, however, the pilot was not injured. It is
possible that the pilot was under the influence of sleep inertia
when he awoke in the cockpit during the flight. Sleep inertia
provides a possible explanation for why the pilot failed to notice
the aircraft's incorrect heading.
200003130: The helicopter was operating a medical evacuation
flight. The helicopter lost engine power and impacted the ground in
a paddock. The helicopter was destroyed and all occupants received
fatal injuries. Based on the available evidence it was likely that
the pilot had retired to bed early which would have been a
significant period of time before the departure call. If the pilot
was asleep at the time of the call, he would need to have awoken,
dressed, proceeded to the aerodrome, prepared to depart and
departed, all within a 14 minute period. If the pilot had been
abruptly awoken from deep sleep, his decision making performance
would probably have been affected during the flight planning stage,
as well as the early part of flight. The extent to which the pilot
actually experienced sleep inertia could not be determined.
Literature examining the effects of sleep inertia on human
performance was examined. The results of this literature review
indicated that sleep inertia may compromise safety. Knowledge of
sleep inertia and the means to mitigate its effects should be
distributed within the transport industry to minimise the risk of
sleep inertia affecting the performance of safety-critical workers.
The results are not limited to the aviation industry or aircrew
It is not recommended that sleep deprivation occur to avoid the
effects of sleep inertia. Avoiding sleep deprivation by allowing
napping can significantly improve alertness, communication and
performance. However, the potential effects of sleep inertia
following a nap should be mitigated against.
For operators susceptible to sleep inertia, a range of options is
available to assist in mitigating the effects of sleep inertia.
- Awareness of sleep inertia: If a person is likely to suffer
sleep inertia, ensure that s/he is aware that his/her performance
may be affected by sleep inertia.
- Automating complex decisions: The use of automated facilities,
for example automating flight planning and fuel calculations, may
minimise the opportunity for sleep inertia errors in
- Two or more persons: Involving all crewmembers in flight
planning and decision making may minimise the likelihood of errors
- Wake-up time: Factoring additional time into the response times
to accommodate the effects of sleep inertia. Many EMS operators
quoted a 6 minute response time which would not allow pilots who
were deeply asleep to recover from sleep inertia prior to being
airborne. Pilot decision making (such as flight planning and fuel
calculations) prior to being airborne in 6 minutes is most
susceptible to sleep inertia.
- Avoid abrupt awakenings: Telephone calls are an example of an
abrupt awakening which may induce sleep inertia effects. Operators
should decide if it is necessary for the pilot to be awoken by
emergency calls. Does the pilot need to respond in the first
instance? If it is not necessary, then the pilot should be awoken
in a non-abrupt manner. Operators should also check whether the
ringing telephone is waking the pilot, causing unnecessary sleep
Any sleep can cause sleep inertia: Sleep inertia may occur
following sleep during the day as well as sleep during the
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