Human Factors In Aviation: Circadian Rhythmicity

2245 words - 9 pages

Circadian RhythmLiving organisms on this planet have adapted to the daily rotation of the earth on its axis. By means of "endogenous circadian clocks" that can be synchronized to the daily and seasonal changes in external time cues, most notably light and temperature. Most people anticipate environmental transitions, perform activities at biologically advantageous times during the day, and undergo characteristic seasonal responses. The effects of transmeridian flight and shift work are stark reminders that although modern technologies can create "cities that never sleep" we cannot escape the recalcitrance of endogenous clocks that regulate much of our physiology and behavior. Recent progress in understanding the molecular mechanisms underlying circadian rhythms has been remarkable. In its most basic form, circadian clocks are comprised of a set of proteins that generate a self-sustaining feedback loop with a free-running period of about 24 hours. One or more of the clock components is acutely sensitive to light, thus it can be synchronized to local time. (Edery, 2000)Levels of light and the Circadian RhythmThe sleep-wake cycle, or circadian rhythm, in people is largely governed by exposure to light. Bright light enters the eye and follows the optic nerve to where the nerves for both eyes cross, and then stimulates a small bit of tissue that begins a cascade of chemical and nervous events stimulating wakefulness. In particular, a substance called melatonin is prevented from being produced by the stimulation of light. When people are exposed to low light levels or darkness, as in a Com Center or radar ARTCC room, the brain interprets this as sleep time and begins to produce melatonin and other substances that tell the body to begin to shut down. This change in the body clock has both physiological and psychological effects. The digestive system slows secretion of digestive juices, coordination and reaction time slow, body temperature drops, and general alertness and level of arousal decrease. Fighting this physiological switch that turns on the body's rest and sleep apparatus can be extremely difficult and uncomfortable when one is forced to stay awake by the requirements of the job (Monk, 1990).1+2Social Factors as one of the types of zeitgebersThe specific environmental time cues which synchronize it to a 24-hour day are known by the German term "zeitgebers," meaning "time-givers." Currently, two types of zeitgebers have been identified: exposure to bright light and social factors. There is some evidence that the human circadian clock may be synchronized by certain social factors, including the work/rest schedule. However, the specific aspects of the social environment that constitute time cues have not yet been identified, and the mechanisms by which they affect the clock remain unknown. (NASA 1999)Maximal Sleepiness PeriodsWe are physiologically programmed for two periods of maximal sleepiness in a usual 24-hour period. The period 3-5 A.M. is a circadian low point for temperature, performance, and alertness. During this time, the brain triggers sleep and sleepiness.The other period of increased sleepiness is roughly 3-5 P.M. Most individuals have experienced an afternoon wave of sleepiness. These windows can be used to schedule sleep periods or naps because the brain provides a period of maximal sleepiness and an increased opportunity for sleep. Performance and alertness can be decreased during the nocturnal window, which is from 2 A.M. until 6 A.M. For some, the afternoon window of sleepiness may occur between 2 P.M. and 4 P.M. Mental activity invlolved in flight at these times should help maintain alertness.(NASA 1999)Falling asleep earlier is harder than sleeping laterThe problem with having to get up earlier than usual is that it is very difficult, if not impossible, to fall asleep sufficiently early the night before to compensate (even when the duty schedule permits). It is not simply a question of discipline or motivation. The circadian clock effectively opposes falling asleep earlier than the habitual bedtime. Just as there are preferred times in the circadian cycle for falling asleep, there are also times when sleep onset is very unlikely. These times have been labeled "wake maintenance zones," and one of them occurs just before the habitual bedtime. In addition, because the "biological day" dictated by the circadian clock tends to be longer than 24 hours, it is easier to go to sleep later than to go to sleep earlier. Going to sleep later also means staying awake longer, which allows more time for the homeostatic "sleep pressure" to build up.(NASA 1999)Long Haul Operations and the biological clockField studies specific to different aviation environments and using a range of measures (e.g., performance, physiology, and behavior) have revealed a number of factors related to fatigue. For example, in long-haul operations, the non-24-hr duty/rest cycles, the circadian desynchronization associated with transmeridien flights, and the sleep loss accompanying nighttime flying are all associated with fatigue In overnight cargo crews, even regular nighttime flying often results in incomplete circadian adaptation. Additionally, duty periods ending in the morning hours lead to sleep loss due to an increasing signal for wakefulness from the biological clock during this time. (Gander 1998) (6)DesynchrnizationThe effects of transmeridian flight and shift work on the human circadian timing system likely occur at two levels. "Desynchronization not only occurs between the external environment and the SCN rhythm generator but also affects phase alignments between the different peripheral clocks. Different rates of resynchronization cause jet lag and other abrupt changes in light-dark cycles."(Yamazaki 2000) (201). Melatonin, a naturally produced hormone that is under circadian regulation, has been used to alleviate jet lag (Brzezinski 1997). Another successful approach for treating jet lag has been the use of phototherapy. It is estimated that more than 20% of the U.S. work force is subjected to shifting work schedules (Yamazaki 2000) (201). This includes a wide variety of occupations where fatique could have disastrous consequences for many people, such as medical personnel, pilots, air traffic controllers and other systems administrators, security and military personnel, and commercial truck drivers.In flight problems of forced internal desynchronizationThis forced internal desynchronization has a number of important consequences. First, the bi-circadian (twice per cycle) peaks in sleep tendency can occur in-flight, thus increasing the risk of inadvertent napping in the cockpit. Second, the part of the circadian cycle during which sleep normally occurs may or may not be contained within a layover, and may or may not coincide with local night. The interplay of these factors will have a major effect on how much sleep flight crewmembers are able to obtain en route and on layover. Third, the circadian rhythms in digestive function may or may not coincide with the patterns of meal availability in-flight and during layovers. Gastrointestinal problems can result from repeatedly eating at inappropriate times in the circadian cycle. (Gander 1998)Sleep, circadian rhythms, and air transport operationsSleep and circadian rhythms interact physiologically. Air transport operations bring a unique set of circumstances and challenges to these two physiological processes. Sleep and circadian rhythms interact in several ways. The two factors can work against one another, thereby weakening or negating each other's effect, or they can work in the same direction, thereby intensifying the effect they each have on sleepiness or alertness. When the factors which affect sleepiness (e.g., time of continual wakefulness, prior sleep quantity and quality) favor sleepiness during a low in the circadian cycle (a time of circadian sleepiness), a person trying to maintain wakefulness has both sets of physiological factors to fight. Less dramatic, but important to an industry which requires peak performance during a crisis, are the following possibilities: even a well-rested person who has slept recently can be affected by a circadian low-point; conversely, a person at a peak in the circadian rhythm (which favors wakefulness) can show degraded performance if sleep deprived. Finally, even after a long-duty day, a crewmember may not be able to fall asleep during the rest period if it coincides with a circadian high. Both factors must be considered in determining the likelihood of someone being vulnerable to sleepiness, or oppositely, being at peak performance at a given time.(Rosekind 1994)(7)Interaction with the flight operations environmentIn addition to interacting with one another, sleep and circadian rhythms interact with the flight operations environment. Flight operations bring distinct factors to bear on the problem: for example, duty at unusual times of day or night, changing schedules, extended duty periods with rest periods at unusual times, and time-zone changes. Like all 24-hour operations, flight opera-tions require people to work at times when their bodies are programmed for sleep. This can lead to increased sleepiness during duty and the associated performance decrements. In addition to requiring unusual sleep/wake times, the duty hours change according to flight schedules and bid lines. This requires a person's body to make changes more frequently. Further, air transport operations often require long duty periods, whether comprised of one extended flight or several short "hops." This increases a crewmember's time of continual wakefulness and, therefore, the chance that underlying physiological sleepiness will surface. Complicating the issue, rest periods often fail to coincide with a crewmember's normal circadian sleep time. This can make the crewmember unable to obtain adequate sleep on layover, and perpetuate the waking sleepiness problem. Finally, many air transport operations entail crossing multiple time zones, which can lead to the circadian disruption previously discussed. All of these factors require the body to adapt to changes in the sleep/wake cycle.(Rosekind 1994)(7)Fatique and the circadian rhythm causes safety issues"There was a study done from 1993 to 1997 on long haul, it was called the Long Haul Field Study, and the major contributors to fatigue in those cases were the fact that you didn't tend to have a 24-hour duty cycle, so you had staggered start and stop times, which made it difficult to get your body acclimated. You also had a circadian rhythm desynchronization because you were crossing time zones. Quite often during night flying, particularly between the hours of 2 o'clock and 6 o'clock, is when your body cycle is at its lowest level, so it is very difficult to stay awake during those periods of time. So the long haul had fatigue caused by those factors. We did a study in 1994 to 1998 looking at regional airlines, which are much shorter, and the fatigue contributors there tended to be long duty days, very short night layovers, and progressively earlier report dates. ...[T]here was significant sampling from air crews, ... 88 percent of the crews reported that fatigue was a common occurrence, and 92 percent reported that when fatigue did occur, it was a significant safety issue on their part."(Mann 1999)1) Swenson, David X.Ph.D. Keep the Lights On For Me: Coping with Com Center Shiftwork by Police & Security News, September/October 19972) Monk, T. H. (1990). The relationship of chronology to sleep schedules and performance demands. Work and Stress, 4(3), 227-236.3)NASA. Ames Research Center. Crew Factors in Flight Operations X: Alertness Management Moffett Field, CA 94035-1000 April 19995) Statement of Michael B. Mann Deputy Associate Administrator Office of Aero-Space Technology National Aeronautics and Space Administration Hearing onPilot Fatigue Before the Aviation Subcommittee of the Committee ontransportation and Infrastructure United States House of Representatives August 3, 19994) Edery, Isaac. Circadian rhythms in a nutshell.Department of Molecular Biology and Biochemistry, Rutgers University, Center for Advanced Biotechnology and Medicine, Piscataway, New Jersey 08854 Physiol Genomics 3: 59-74, 2000.6) Gander, P. H., Gregory, K. B., Connell, L. J., Graeber, R. C., Miller, D. L., & Rosekind, M. R. (1998). Flight Crew Fatigue IV: Overnight Cargo Operations. Aviation, Space, and Environmental Medicine, 69(9), B26-B36.13) Brzezinski A. Melatonin in humans. N Engl J Med 336: 186-95, 1997.(201) Yamazaki S, Numano R, Abe M, Hida A, Takahashi R, Ueda M, Block GD, Sakaki Y, Menaker M, and Tei H.7) Fatigue Countermeasures: Alertness Management in Flight OperationsCo, E. L., Rosekind, M. R., Johnson, J. M., Weldon, K. J., Smith, R. M., Gregory, K. G., Miller, D. L., Gander, P. H., Lebacqz, J. V. (1994). Fatigue Countermeasures: Alertness Management in Flight Operations. Southern California Safety Institute Proceedings, Long Beach, 1994, 190-1978) Mann, Michael B., Deputy Associate Administrator, Aero-Space Technology, National Aeronautics and Space Administration. TESTIMONY: PILOT FATIGUE (106-33) HEARINGS BEFORE THE SUBCOMMITTEE ON AVIATION OF THE COMMITTEE ON TRANSPORTATION AND INFRASTRUCTURE HOUSE OF REPRESENTATIVES ONE HUNDRED SIXTH CONGRESS FIRST SESSION AUGUST 3 and SEPTEMBER 15, 1999


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