Par 11 Page 13 efficiently by taking more than the nominal 8 hours of sleep per day. Hence, effective recovery sleep is a shared responsibility of the scheduler and the employee.
b. Napping. Sleep is the only way to reverse sleepiness. Therefore, at times when some amount of sleep is possible but limited, napping is the most effective physiological strategy for restoring alertness levels. Naps have shown to be beneficial for restoring both performance and alertness levels, especially during long periods of wakefulness 4, 5, 35, 60, 71, 72, 89, 91. Even short naps of 25-30 minutes can have beneficial effects. Controlled studies have shown that a nap can yield significant improvements in subsequent pilot alertness and vigilance performance compared to similar pilots who did not receive the nap so methods that augment crews to permit napping outside the cockpit is a viable fatigue countermeasure under current FAA rules. The FAA authorizes in-flight naps for flightcrew if there is an augmented complement so that two pilots are on the flight deck while the augmented crewmembers are resting. Although a number of foreign air carriers authorized in-seat cockpit naps during flight, the FAA does not authorize such in-seat cockpit naps 42.
c. In-Flight Rostering and Bunk Sleep (Flight-Ops Specific). In-flight rostering, although not commonly discussed as a fatigue countermeasure, can help minimize fatigue. It refers to the scheduling of augmented flightcrew to assigned positions on the flight deck, freeing other flightcrew to obtain in-flight rest or bunk sleep. In-flight rostering is directly related to the crew complement, or number of crewmembers assigned to the flight and is determined—in advance— during the scheduling process. Performance and alertness begin to deteriorate after 18-20 hours of continuous wakefulness 7. However, an increased likelihood of incidents and accidents has occurred in shift workers after only 8-9 hour duty periods 39, 81. In a review of aviation accidents, the National Transportation Safety Board (NTSB) found that when “time since awakening” exceeded the median for crew position, there was an increase in overall errors 67. In light of these facts, it is essential to provide a sufficient number of crewmembers so that there are multiple opportunities for rest.
(1) The effects of continuous hours of wakefulness on alertness and performance can be minimized with the use of efficient in-flight rostering, which helps to ensure that at least one flightcrew member is always rested 16. Use the following two principles:
(a) Scheduling bunk sleep periods that minimize the number of hours of extended wakefulness for the landing crew.
(b) Scheduling flightcrew to perform who just had a recent bunk sleep opportunity and have an increased chance of being well-rested during critical phases of flight.
NOTE: It is essential to consider rostering in the planning stages of the flight, educate the crew about the rostering approach, and that the crew adheres to the rostering and napping schedule during the flight.
(2) There is a lack of research concerning the specific number of crewmembers necessary to guarantee adequate sleep opportunities for sufficient performance and safety in the context of extended aviation operations. Although it is clear that more crewmembers are necessary to
Page 14 Par 11 improve the alertness of other crewmembers, an empirical and evidence-based approach is needed to improve the current situation 16.
(a) The ULR Crew Alertness workshops of the FAA’s 2008 Fatigue Symposium showed that ensuring adequate bunk sleep is one of the most important in-flight countermeasures to use to address sleep loss and circadian disruption during extended aviation operations 37. The only way to reverse cumulative sleep debt is to obtain sleep because it addresses the underlying physiology of sleep loss. For example, if flight demands permit, physiological sleepiness can be reduced by utilizing periods of increased sleepiness during circadian low points, which, in turn, can contribute to increased quantity and quality of bunk sleep 17, 24, 26. Thus, in-flight sleep periods are an operationally feasible approach to manage sleep loss associated with complex aviation schedules (e.g., extended wakefulness, crossing multiple time zones, and nighttime duty hours).
(b) Although proper bunk-sleep scheduling is both a feasible and operational approach for managing alertness, scheduled sleep periods often occur at less than optimal time periods due to conflicting crewmember job responsibilities. Therefore, efforts should be made to optimize the in-flight sleep periods for the primary crew, who have responsibility for critical flight maneuvers such as landings and takeoffs. Timing of in-flight sleep for the primary crew must be part of the rest planning process.
(3) Crewmembers have indicated that environmental factors in the bunk facilities influence the quality of in-flight rest periods. A NASA survey revealed that the most common factors that conflicted with quality bunk sleep were ambient temperature, noise from the galley and elsewhere, and background lighting 75. Pilots who completed the survey also indicated that making the bunk facilities more private and having comfortable bedding and blankets would help promote better quality and quantity of bunk sleep. Many of these issues can be addressed during the design stage of bunk facilities.
(4) Finally, the duration of in-flight rest breaks (sleep opportunities) should be limited to no longer than about 6 hours. Studies have shown that even when given longer than 6 hours to sleep in the bunk, relatively few crewmembers can take advantage of that additional time.
Merely providing pilots with more sleep opportunities does not guarantee that they will obtain more sleep. For example, flightcrews given a 5-hour bunk sleep opportunity obtained only around 3 hours of sleep, on average 46. Signal and colleagues found that flightcrews who had a 7-hour sleep opportunity obtained, on average, only 3 hours 25 minutes of bunk sleep 79. This is due to a range of factors that limit the length of time a pilot can remain asleep in the bunk and the need to limit the continuous in-seat time of the relief crew. Given the time to prepare for sleep and then return to duty after sleep, in-flight measurements indicate that actual bunk sleep times seldom last longer than 5 hours. In light of the limitations on rest break utilization, two rest breaks of 3 to 5 hours each are probably better than a single break of 8 hours for long duration flights. Plans for utilization of in-flight bunk sleep opportunities must consider multiple interacting factors.
(a) Length of in-flight sleep needed to remain alert at critical phases of flight;
(b) Circadian timing of the sleep opportunity;
Par 11 Page 15
(c) Need for occasional use of the toilet facility outside the rest facility;
(d) Length of in-seat time of the relief crew;
(e) Comfort, vibration, turbulence, and noise that can fragment sleep;
(f) Time needed to undress and redress for duty; and
(g) Time after sleep to wake up and become fully alert—dissipate sleep inertia.
d. Activity Breaks. Short breaks can serve to increase alertness by reducing the monotony of a highly automated cockpit environment through conscious disengagement with the flying task and, possibly, by allowing mild physical activity, depending on the type of break and the behaviors allowed during the break. Although not as effective as some other countermeasures, anecdotal reports from pilots indicate that many take brief, out-of-the-seat breaks as a fatigue countermeasure.
(1) Several studies indicate an improvement in alertness and performance associated with a cognitive break from continuous tasks. Even short 5-minute breaks can relieve monotony, increase overall productivity, and reduce reports of physical fatigue 40. Others studies have shown that breaks also can have positive effects when individuals are experiencing partial or total sleep loss. Two total sleep deprivation studies ranging from 54 to 64 hours of continuous wakefulness revealed that participants who received 5 20-minute rest breaks exhibited improvements in performance, alertness, fatigue, and overall mood compared to participants who did not receive any breaks 45. These physiological and performance improvements also have been observed in aviation environments, in which rest breaks have helped military pilots working sustained operations to overcome fatigue 2.
NOTE: Rest breaks are not a substitute for adequate sleep and not all studies have shown beneficial effects 83.
(2) Activity breaks that allow an individual to change posture by getting up out of their seat and walking or otherwise physically moving while engaging in increased social interaction can increase alertness levels; however, studies indicate that the beneficial effects can be short-lived, especially when the crew have been awake for more than 18 hours 68, 61.
(3) The beneficial effects of breaks are due in part to postural changes that occur when a pilot temporarily hands off flight-related tasks. There are consistent results from laboratory-based sleep deprivation research examining the effects of posture. Simply standing up can increase physiological arousal, decrease reaction times, and increase measures of attention in drowsy subjects 25.
e. Light. Research has shown that the use of properly timed bright light can shift human circadian rhythms. In addition, some research suggests that light may have an immediate and acute alerting effect on mood and performance, independent of its circadian phase-shifting capacity. The alerting effects of light may be a result of its suppression of melatonin, a neurotransmitter released in the mid- to late-evening. Therefore, light may be a powerful
Page 16 Par 11 mitigator of the usual alertness and performance decline common to nighttime duty, especially during aviation operations.
(1) Bright light is not necessary for measurable improvements in alertness. Cajochen and colleagues showed measurable increases in subjective alertness and reductions in slow eye movements with normal indoor lighting (100-200 lux) 14. Short wavelength light in the blue portion of the spectrum appears to have the greatest alerting effect 56 with the spectrum of typical room light containing enough energy in the shorter wavelengths to be effective.
(2) The alerting effects of light may be independent of time of day, making light a possible daytime countermeasure for those who have experienced prior sleep deprivation, and thus improve alertness and performance.
(3) Finally, while light can have beneficial effects while at work, exposure to sunlight after a night shift could inhibit the ability to get to sleep in the morning 78. This is primarily due to the alerting effects of light and suppression of melatonin. Therefore, avoid sunlight after a night shift and prior to a morning sleep period, when possible.
f. Caffeine. Caffeine can be an effective countermeasure in improving alertness and performance levels 8, 88, 93. Caffeine takes 15-30 minutes to enter the bloodstream after consumption, and thus alertness effects do not occur immediately. However, its effects can persist up to 5 hours after ingestion. Another characteristic of caffeine that makes it a common countermeasure used by pilots to maintain alertness is that it is readily available in beverages such as coffee, tea, and soft drinks. Minimal use of caffeine is not generally associated with the undesirable effects often associated with chronic use (i.e., tolerance, gastrointestinal problems, increased blood pressure, etc.). Individual differences exist in people’s response to caffeine including its effect on performance levels and sleep structure. Before using caffeine as a fatigue countermeasure, individuals should ground test it (as with any countermeasure) to determine its specific effects with their physiology.
12. IDENTIFYING FATIGUE IN AVIATION OPERATIONS.
a. Studies of Fatigue in Simulators and Operational Environments. Both simulator and in-flight studies conducted during flight operations and within shift work environments have documented that fatigue impairs central nervous system functioning.
(1) Long-haul pilots are particularly susceptible to vigilance lapses during low-workload periods, and such lapses could simultaneously appear in both crewmembers at the same time 13.
(2) In-flight recordings of brain activity have found that pilot micro-sleeps occurred most frequently during the cruise portion of long haul operations (in the middle-to-late segments of the flight) and that micro-sleeps were more than 9 times as likely during nighttime flights compared to daytime flights 94. Spontaneous micro-sleeps increase with increasing flight duration 77.
(3) Despite strong motivation to be alert during the final stages of a flight, studies of brain activity and eye-closures indicate that physiological micro-events can occur during the period from top-of-descent to landing 74.
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(4) Of the 1,424 flightcrew members responding to a NASA survey of fatigue factors in regional airline operations, 80 percent acknowledged having “nodded off” during a flight at some time 20.
(5) Results of a survey involving corporate/executive aviation operations, 71 percent of 1,488 flightcrew members reported having nodded off during duty 73.
b. Evidence of Fatigue from Accident Investigations. Fatigue has been, and continues to be, a contributing factor in aviation accidents. Currently, the NTSB has seven aviation fatigue-specific recommendations. Since 1993, the NTSB has determined that fatigue contributed to 7 air carrier accidents within the United States, resulting in 250 fatalities and 52 serious injuries.
Recent events continually highlight the operational relevance of fatigue among flightcrew; it is not uncommon that crew fall asleep while flying. NTSB investigations have found that flightcrew on long duty days (a shift of more than 13 hours) exhibit a disproportionate amount of accidents when compared to those on short duty days (a shift of less than 13 hours). The longer the crews are awake, the more errors they tend to commit, especially cognitive errors such as decisionmaking.
(1) NTSB investigators divide the causes of fatigue into operational and personal factors.
Operational factors contributing to fatigue induced by the workplace include short rest periods between shifts, which can be as short as 8 hours under current regulations, rapid rotation of shift start times, which can disrupt circadian rhythms, working early morning and graveyard shifts, and duration of commute, among others. Equally important are personal drivers of fatigue, which are largely habits and behaviors controlled by the individual, such as ensuring proper duration of rest. However, personal drivers of fatigue also depend on many factors such as the presence of sleep disorders, circadian variability, additional employment, and use of alcohol and stimulants.
(2) The table, below, of recent aviation accidents attributed in part to crew fatigue was presented during the NTSB keynote address at the FAA Fatigue Symposium.
Airline Date Probable Cause Outcome American International 808 1993 Impaired judgment, decisionmaking and flying abilities due to fatigue.
3 serious injuries.
Korean Air 801 1997 Crew failure to prepare for/execute non-precision approach.
228 fatalities;
26 serious injuries.
American Airlines 1420 1999 Flightcrew failure to discontinue approach and ensure that spoilers had extended after touchdown. Contributing factor was the flightcrew’s impaired performance due to fatigue.
11 fatalities;
45 serious injuries.
Federal Express 1478 2002 Crew failure to establish and maintain proper glidepath at night. Fatigue was contributing factor.
3 serious injuries.
Page 18 Par 12 Corporate Airline 2004 Combination of fatigue related factors producing pilot error.
13 fatalities;
2 serious injuries.
Shuttle America 2007 Fatigue contributing factor affecting ability to plan and monitor approach leading to runway overrun.
No fatalities.
13. MEASUREMENT AND MITIGATION OF FATIGUE RISK.
a. Factors in Fatigue Risk. Fatigue due to extended work hours, time of day, and shift work induces reductions in vigilance and reaction time and increases in risk of poor decisions, human error, incidents, and accidents. As described earlier, cognitive fatigue results from the interaction of sleep limitations and circadian drives for sleepiness with time-on-task and cumulative duty time effects. Human error due to fatigue is the result of sporadic losses of brain function and attention that increase in frequency with reductions in sleep and circadian drive for sleep. Lapses in attention are random in time, making it difficult to demonstrate the role of fatigue in specific accident cases. Self-reported, subjective sleepiness cannot be relied upon in this context, because it has a low correlation with actual performance impairment. One way to understand the role of fatigue in accident risk is to consider that fatigue causes random periods of inattention that occasionally coincide with operational conditions that demand attention to avert a serious event or incident. Hence, accident risk increases when either:
(1) Lapses of attention increase in frequency due to sleep loss or circadian factors; or
(2) Work demands increase the need for attention.
b. Mitigation of Fatigue Risk. There are two main strategies to reduce fatigue-related accident risk: a) decrease fatigue factors that drive lapses in attention, or b) alter the job so that the task is less sensitive to lapses in attention.
(1) Factors leading to fatigue reviewed above—in brief, they are:
• Time since awakening,
• Cumulative sleep debt, and
• Circadian rhythm of attention.
(a) Determining how multiple fatigue factors combine to lead to heightened risk is difficult without the aid of a computer simulation of fatigue factors and how they combine to increase lapses in attention and heighten fatigue risk.
(b) Biomathematical fatigue models or simulations can derive from the work schedule and likely sleep under those schedules, and the fatigue factors that reduce performance and attention and increase risk of errors and accidents. Fatigue models can help improve performance and safety in operational settings by pointing to job-related factors, such as work
Par 13 Page 19 scheduling, sleep opportunities, and individual sleep habits, which could be changed to reduce the chances of fatigue-related attention lapses that could lead to errors and accidents.
(c) Arranging for less schedule-induced fatigue is important but is only half of the equation. The staff and crews must use sleep opportunities effectively to be rested and prepared for duty. Training of staff and crews should provide basic information on fatigue as contained in this AC, including the measures that individuals should take to be fully rested for duty. Staff and crews need to know that alert performance requires sufficient sleep prior to duty and recovery sleep following duty to prevent cumulative sleep debt.
(2) Operators can modify the work environment to minimize the consequences of fatigue.
For example, training of crews to work together better as a team—commonly known as Crew Resource Management (CRM)—can reduce the chances of fatigue-related mistakes. CRM training includes a range of knowledge, skills, and attitudes that improve the quality of communications, situational awareness, problem solving, decisionmaking, and teamwork. Taken together, these teamwork skills and procedures can catch fatigue-related errors before they adversely affect operations. In addition, the flight control systems themselves can be modified to provide alerts and warnings to pilots to supplement attention to the instruments, reducing the chances that a lapse of attention will lead to a mishap.
14. FATIGUE RISK MANAGEMENT.
a. Fatigue Risk Management System (FRMS).
(1) Prescriptive flight and duty time limitations and rest requirements reduce, but do not eliminate, the conditions that lead to fatigue. FRMSs potentially offer non-prescriptive procedures to reduce fatigue further by addressing the complexity of aviation operations and fatigue challenges associated with aviation operations. FRMSs are inherently evidence-based and include a combination of processes and procedures that are employed for the measurement, mitigation, management, and monitoring of fatigue risk within a specific operational setting 80.
Fatigue risk management programs provide an interactive and collaborative approach to address performance and safety levels of operations on a case-by-case basis, and, therefore, are more adaptive to the specific conditions that create fatigue in a particular operational environment.
(2) An FRMS employs a multi-layered defense to proactively manage operational fatigue risk. The defenses against fatigue risk can include the following levels of intervention:
(a) Flight and duty time scheduling,
(b) Employee training and individual sleep practices and hygiene,
(c) Teamwork and crew resource management, and
(d) Procedural and flight system barriers to error.
b. FRMS Applicability. An FRMS can be used within the envelope of prescriptive flight and duty time limitations or as an alternative to such prescriptive rules if it provides at least an equivalent level of safety. An FRMS enhances the capability of prescriptive flight and duty time
Page 20 Par 14 limitations to provide an equivalent or enhanced level of safety based upon the identification and management of fatigue risk relevant to the specific circumstances. Use of an FRMS can allow greater operational flexibility and efficiency while maintaining safety by relying on in-flight measurements of sleep and alertness, including subjective reports by crewmembers, to monitor how scheduling affects flight and cabin crew alertness during flight duty.
(1) Commercially available computer models can be used to predict average performance capability from sleep/wake history and normal circadian rhythms. Models embedded within the FRMS process can help operators understand the likely effects on performance of sleep obtained before and during trip patterns. Such models, though not required, encapsulate the latest scientific research on human circadian systems, sleep, and performance capability and can be useful for rapidly estimating the likely fatigue levels associated with proposed new routes or schedule changes. However, certain assumptions and limitations need to be taken into account.
They represent one useful component of an FRMS, but are not a substitute for an FRMS.
(2) An effective FRMS is data-driven and routinely collects and analyzes information and reports related to crew alertness as well as operational flight performance data. An FRMS’s comprehensive range of safeguards helps to control the risk associated with both transient and cumulative fatigue. An FRMS based upon scientific principles and knowledge combined with sound methods of data collection and analysis can help maintain an equivalent level of safety while allowing greater operational flexibility.
c. FRMS as Part of a Safety Management System (SMS). Ideally, an FRMS should be an integral part of an operator’s established SMS and its capability should be commensurate with the risk oversight needs. Whether within an existing SMS or as a stand-alone system, an FRMS applies SMS principles to proactively and continuously manage fatigue risk through a process requiring shared responsibility among management and flight and cabin crewmembers. Since feedback and non-punitive reporting from employees are essential elements of an SMS, a “just culture” is integral to any FRMS program. When properly implemented, an FRMS is a continuous performance improvement process, using feedback on the success or limitations of prior scheduling and fatigue mitigations to suggest improvements for future scheduling and fatigue mitigations.
d. Benefits of FRMS. Aviation carriers, regulators, and groups worldwide are addressing the benefits of incorporating FRMS programs into current aviation operations. Recent examples include easyJet, Air New Zealand, United Airlines, Continental Airlines, the Australia Civil Aviation Safety Authority, the Flight Safety Foundation, and the International Civil Aviation Organization (ICAO). Preliminary data from easyJet’s FRMS program have demonstrated its effectiveness in reducing fatigue 82. Air New Zealand has been a leader in demonstrating the operational benefits of an FRMS as presented at the FAA Symposium, “Aviation Fatigue Management Symposium: Partnerships for Solutions” on June 17—19, 2008 in Vienna, Virginia.
Based on the available research and demonstrated success of such programs, the FAA is exploring ways to implement effective FRMS-based programs in operations where fatigue has been identified as an inherent risk. An FRMS offers a way to conduct safer flights beyond existing regulatory limits and is a promising addition to prescriptive flight and duty time and rest period regulations.
Par 4 Page 21 ORIGINAL SIGNED by Raymond Towles for John M. Allen Director, Flight Standards Service
Appendix 1 APPENDIX 1. REFERENCES
(1) Achermann, P. The two-process model of sleep regulation revisited. Aviation, Space, and Environmental Medicine, 2004, 75 (3 Suppl): A37-43.
(2) Angus, R. G.; Pigeau, R. A.; Heslegrave, R. J. Sustained-operations studies: from the field to the laboratory. In: Stampi, C.; ed. Why we nap. Birkhauser; Boston; 1992: 217-41.
(3) Belenky, G.; Wesensten, N. J.; Thorne, D. R.; Thomas, M. L.;, Sing, H. C.;
Redmond, D. P.; et al. Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose-response study. Journal of Sleep Research, 2003; 12: 1-12.
(4) Bonnet, M. H. Dealing with shift work: physical fitness, temperature, and napping.
Work & Stress, 1990, 4: 261-74.
(5) Bonnet, M. H. The effect of varying prophylactic naps on performance, alertness, and mood throughout a 52-hour continuous operation. Sleep, 1991, 14: 307-15.
(6) Bonnet, M. H. Sleep Deprivation. In M. H. Kryger, T. Roth, & W. C. Dement (Eds.), Principles and practice of sleep medicine (3rd ed). Philadelphia: Saunders; 2000: 53-71.
(7) Bonnet, M. H. Acute sleep deprivation. In: Kryger, M. A.; Roth, T.; Dement, W. C.;
eds. Principles and practice of sleep medicine. Philadelphia: Elsevier Saunders; 2005: 51-66.
(8) Bonnet, M. H.; Balkin, T. J.; Dinges, D. F.; Roehrs, T.; Rogers, N. L.; Wesensten, N. J. The use of stimulants to modify performance during sleep loss: A review by the Sleep Deprivation and Stimulant Task Force of the American Academy of Sleep Medicine. Sleep 2005;
28(9): 1163-87.
(9) Borbély, A. A. A two-process model of sleep regulation. Hum Neurobiol, 1982;
1(3): 195-204.
(10) Borbely, A. A.; Achermann, P. Concepts and models of sleep regulation: an overview. Journal of Sleep Research, 1992, 1 (2): 63-79.
(11) Bourgeois-Bougrine, S.; Carbon, P.; Gounelle, C.; Mollard, R.; Coblentz, A.
Perceived fatigue for short- and long-haul flights: a survey of 739 airline pilots. Aviat Space Environ Med 2003, 74: 1072-7.
(12) Boulos, Z.; Campbell, S, S.; Lewy, A. J.; Terman, M.; Dijk, D. J.; and Eastman, C.
I. Light treatment for sleep disorders: consensus report. VII. Jet lag. J Biol Rhythms 1995, 10(2):
167-76.
(13) Cabon, P.; Coblentz, A.; Mollard, R.; Fouillot, J. P. Human vigilance in railway and long-haul flight operation. Ergonomics 1993, 36: 1019-33.
Appendix 1
(14) Cajochen, C.; Zeitzer, J. M.; Czeisler, C. A.; & Dijk, D. J. Dose response relationship for light intensity and ocular and electroencephalographic correlates of human alertness. Behav Brain Res 2000; 115: 75-83.
(15) Caldwell, J. A.; Mallis, M. M.; Colletti, L. M.; Oyung, R. L.; Brandt, S. L.;
Arsintescu, L.; DeRoshia, C. W.; Reduta-Rojas, D. D.; Chapman, P. M. The effects of ultra-long-range flights on the alertness and performance of aviators (NASA Technical Memorandum 2006-213484), Moffett Field, CA: NASA Ames Research Center.
(16) Caldwell, J. A.; Mallis, M. M.; Caldwell, J. L.; Michel, A. P.; Miller, J. C.; Neri, D.
F. Fatigue Countermeasures in Aviation. Aviat Space Environ Med 2009; 80(1): 29-59.
(17) Carskadon, M. A. Ontogeny of human sleepiness as measured by sleep latency. In:
Dinges DF, Broughton RJ, eds. Sleep and alertness: chronobiological, behavioral, and medical aspects of napping. New York: Raven Press; 1989: 53-69.
(18) Carskadon, M. A.; Dement, W. C. (1987). Daytime sleepiness: quantification of a behavioral state. Neuroscience Biobehavioral Review 1987, 11: 307-17.
(19) Carskadon, M. A.; Roth, T. Sleep Restriction. In T. H. Monk (Ed.), Sleep, Sleepiness and Performance. New York: John Wiley & Sons. 1991: 155-67.
(20) Co, E. L.; Gregory, K. B.; Johnson, J. M.; & Rosekind, M. R. Crew factors in flight operations XI: A survey of fatigue factors in regional airline operations. Moffett Field, CA:
NASA Ames Research Center; 1999. Report No: NASA/TM—1999 - 208799.
(21) Czeisler, C. A.; Duffy, J. F.; Shanahan, T. L.; Brown, E. N.; Mitchell, J. F.;
Rimmer, D. W.; et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science 1999, 284: 2177-81.
(22) DeRoshia, C. W.; & Greenleaf, J. E. Performance and mood state parameters during 30-day 6° head-down bed rest with exercise training. Aviat, Space Environ Med 1993, 64: 522-7.
(23) Dijk, D. J. & Czeisler, C.A. Contribution of the circadian pacemaker and the sleep homeostat to sleep propensity, sleep structure, electroencephalographic slow waves, and sleep spindle activity in humans. Journal of Neuroscience 1995, 15: 3526-38.
(24) Dijk, D. J.; Franken, P. Interaction of sleep homeostasis and circadian rhythmicity:
dependent or independent systems? In: Kryger, M. A.; Roth, T.; Dement, W. C.; eds. Principles and practice of sleep medicine. Philadelphia: Elsevier Saunders; 2005: 418-34.
(25) Dijkman, M.; Sachs, N.; Levine, E.; Mallis, M.; Carlin, M. M.; Gillen, K. A.; et al.
Effects of reduced stimulation on neurobehavioral alertness depend on circadian phase during human sleep deprivation. Sleep Res 1997; 26: 265.
(26) Dinges D. F. Differential effects of prior wakefulness and circadian phase on nap sleep. Electroen Clin Neuro 1986, 64: 224-7.