Safety Advisory Notice SAN20010244

Safety Advisory Notice issued to: Extended-hours operations

Recommendation details
Output No: SAN20010244
Date issued: 15 April 2002
Safety action status: Closed
Background: Why this Safety Advisory Notice was developed



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 inertia.

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 available.

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 (Bonnet, 1983).
- 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, & Bertini, 2000).
- 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.

Influencing variables

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, including:
- 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, 1971).
- 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 include:
- 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 empirically validated.

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 performance.

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. These include:
- 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 performance.
- Two or more persons: Involving all crewmembers in flight planning and decision making may minimise the likelihood of errors going unnoticed.
- 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 interruptions.

Any sleep can cause sleep inertia: Sleep inertia may occur following sleep during the day as well as sleep during the night.


Balkin, T. & Badia, P. (1988). Relationship between sleep inertia and sleepiness: Cumulative effects of four nights of sleep distribution/restriction on performance upon abrupt nocturnal awakenings. Biological Psychology, 27, 245-258.
Blakey, M. (2002, April 3). NTSB Chairman Highlights Fatigue as Major Cause of Transportation Accidents. NTSB Press Release: SB-02-09. Cited at .
Bonnet, M.H. (1993). Cognitive effects of sleep and sleep fragmentation. Sleep, 16, 565-567.
Bruck, D. & Pisani, D.L. (1999). The effects of sleep inertia in decision-making performance. Journal of Sleep Research, 8, 95-103.
Della Rocco, P.S., Comperatore, C., Caldwell, L., & Cruz, C. (2000). The Effects of Napping on Night Shift Performance. (Office of Aviation Medicine Report DOT/FAA/AM-00/10). Washington D.C.: U.S. Department of Transportation, Federal Aviation Administration.
Dement, W.C. (1999). The Promise of Sleep: The Scientific Connection Between Health, Happiness, and a Good Night's Sleep. New York: Delacorte Press.
Dinges, D.F. (1989). Napping patterns and effects in human adults. In D.F. Dinges & R.J. Broughton (Eds.)., Sleep and Alertness Chronobiological, Behavioural, and Medical Aspects of Napping (pp. 171-204). New York: Raven Press.
Dinges, D.F. (1990). Are you awake? Cognitive performance and reverie during the hypnopompic state. In R. Bootzin, J. Kihistron, & D. Schacter (Eds.)., Sleep and Cognition (159-175). Washington D.C.: American Psychological Society.
Dinges, D.F., Orne, E.C., Evans, F.J., & Orne, M.T. (1981). Performance after naps in sleep conductive environments. In L.C. Johnson, D.I. Tepas, W.P. Coloquhoun, & M.J.Colligan (Ed.)., Biological Rhythms, Sleep an Shift Work (pp. 539-552). New York: Spectrum.
Dinges, D.F., Orne, M.T., Whitehouse, W.G., & Orne, E.C., (1987). Temporal placement of a nap for alertness: Contributions of circadian phase and prior wakefulness. Sleep, 10(4), 313-329.
Ferrara, M. & De Gennaro, L. (2000). The sleep inertia phenomenon during the sleep-wake transition: Theoretical an operational issues. Aviation, Space, and Environmental Medicine, 71(8), 843-848.
Ferrara, M., De Gennaro, L., Casagrande, M., & Bertini, M. (2000). Selective slow-wave sleep deprivation and time-of-night effects on cognitive performance upon awakening. Psychophysiology, 37, 440-446.
Jewett, M.E., & Kronauer, R.E. (1999). Interactive mathematical models of subjective alertness and cognitive throughput in humans. Journal of Biological Rhythms, 14(6), 588-597.
Hawkins, F.H. (1993). Human Factors in Flight (2nd Ed.). England: Ashgate.
House of Representatives Standing Committee on Communications, Transport and the Arts (2000, October). Beyond the Midnight Oil: An Inquiry into Managing Fatigue in Transport. Canberra, Australia: The Parliament of the Commonwealth of Australia.
Naitoh, P., Kelly, T., & Babkoff, H. (1993). Sleep inertia: Best time not to wake up? Chronobiology International, 10(2), 109-118.
Stones, M.J. (1977). Memory performance after arousal from different sleep stages. British Journal of Psychology, 68, 177-181.
Wilkinson, R.T., & Stretton, M. (1971). Performance after awakening at different times of the night. Psychonomic Science, 23, 283-285.

Output text

The Australian Transport Safety Bureau alerts all operators in the transport industry, particularly those involved in extended-hours operations, to the possibility of crew members suffering sleep inertia and suggests that operators take steps to mitigate the effects of sleep inertia. The steps should not include subjecting employees to sleep deprivation.

Initial response
Date issued: 10 January 2003
Response from: Transport Industry
Action status: Response Not Required
Response text:
ATSB response:

File closed CMS 10/1/03

Last update 01 April 2011