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The aviation industry requires 24-hour activities to meet operational demands. Growth in global long-haul, regional, overnight cargo, and short-haul domestic operations will continue to increase these round-the-clock requirements. Flight crews must be available to support 24-hour-a-day operations to meet these industry demands. Both domestic and international aviation can also require crossing multiple time zones. Therefore, shift work, night work, irregular work schedules, unpredictable work schedules, and dm zone changes will continue to be commonplace components of the aviation industry. These factors pose known challenges to human physiology, and because they result in performance-impairing fatigue, they pose a risk to safety. It is critical to acknowledge and, whenever possible, incorporate scientific information on fatigue, human sleep, and circadian physiology into 24-hour aviation operations. Utilization of such scientific information can help promote crew performance and alertness during flight operations and thereby maintain and improve the safety margin.

To document the psychophysiological effects of flying overnight cargo operations, 41 B-727 crew members (average age 38 yr) were monitored before, during, and after one of two typical 8-day trip patterns. During daytime layovers, the average sleep episode was 3 hr (41%) shorter than nighttime sleeps and was rated as lighter, less restorative, and poorer overall. Sleep was frequently split into several episodes and totaled 1.2 hr less per 24 hr than on pretrip days. Each trip pattern included a night off, which was an effective countermeasure against the accumulating sleep debt. The organization of sleep during daytime layovers reflected the interaction of duty timing with circadian physiology. The circadian temperature rhythm did not adapt completely to the inverted wake-rest schedule on duty days, being delayed by about 3 hr. Highest subjective fatigue and lowest activation occurred around the time of the temperature minimum. On duty days, reports of headaches increased by 400%, of congested nose by 200%, and of burning eyes by 900%. Crew members also reported eating more snacks. Compared with daytime short-haul air-transport operations, the overnight cargo trips included fewer duty and flight hours, and had longer layovers. Overnight cargo crews also averaged 5.4 yr younger than their daytime short-haul counterparts. On trips, both groups lost a comparable amount of sleep per 24 hr, but the overnight cargo crews had shorter individual sleep episodes and more broken sleep. These data clearly demonstrate that overnight cargo operations, like other night work, involve physiological disruption not found in comparable daytime operations.

Seventy-four pilots were monitored before, during, and after 3- or 4-day commercial short-haul trip patterns. The trips studied averaged 10.6 hr of duty per day with 4.5 hr of flight time and 5.5 flight segments. The mean rest period lasted 12.5 hr and occurred progressively earlier across successive days. On trip nights, subjects took longer to fall asleep, slept less, woke earlier, and reported lighter, poorer sleep with more awakenings than on pretrip nights. During layovers, subjective fatigue and negative affect were higher, and positive affect and activation lower, than during pretrip, in-flight, or posttrip. Pilots consumed more caffeine, alcohol, and snacks on trip days than either pretrip or posttrip. Increases in heart rate over mid-cruise were observed during descent and landing, and were greater for the pilot flying. Heart-rate increases were greater during takeoff and descent under instrument meteorological conditions (IMC) than under visual meteorological conditions (VMC). The following would be expected to reduce fatigue in short-haul operations: regulating duty hours, as well as flight hours; scheduling rest periods to begin at the same time of day, or progressively later, across the days of a trip; and educating pilots about alternatives to alcohol as a means of relaxing before sleep.

This study examined the effectiveness of a planned cockpit rest period to improve alertness and performance in long-haul flight operations. The Rest Group (12 crew members) was allowed a planned 40 minute rest period during the low workload, cruise portion of the flight, while the No-Rest Group (9 crew members) had a 40 minute planned control period when they maintained usual flight activities. Measures used in the study included continuous ambulatory recordings of brain wave and eye movement activity, a reaction time/vigilance task, a wrist activity monitor, in-flight fatigue and alertness ratings, a daily log for noting sleep periods, meals, exercise, flight and duty periods, and the NASA Background Questionnaire. The Rest Group pilots slept on 93 percent of the opportunities, falling asleep in 5.6 minutes and sleeping for 25.8 minutes. This nap was associated with improved physiological alertness and performance compared to the No-Rest Group. The benefits of the nap were observed through the critical descent and landing phases of flight. The nap did not affect layover sleep or the cumulative sleep debt. The nap procedures were implemented with minimal disruption to usual flight operations and there were no reported or identified concerns regarding safety.

Thirty-two helicopter pilots were studied before, during, and after 4-5 day trips providing support services from Aberdeen, Scotland, to rigs in the North Sea oil fields. Early on-duty times obliged subjects to wake up 1.5 hours earlier on trip days than on pretrip days. Consequently, they slept nearly an hour less per night on trips. They reported more fatigue on post-trip days than on pretrip days, suggesting a cumulative effect of duty-related activities and sleep loss. Fatigue and negative affect were higher, and activation lower, by the end of trip days than by the end of pretrip days. The earlier a subject went on duty, the lower his activation by the end of the day. Caffeine consumption increased 42 percent on trip days. The incidence of headache doubled, of back pain increased twelve fold, and of burning eyes quadrupled. In the aircraft studied, thermal discomfort and high vibration levels were common. The longer pilots remained on duty, the more negative their mood became.

This study investigated changes in sleep after multiple transmeridian flights. The subjects were 12 B747 airline pilots operating on the following polar flight: Tokyo (TYO)-Anchorage (ANC)-London (LON)-Anchorage-Tokyo. Sleep polysmonograms were recorded on two baseline nights (B1, B2), during layovers, and, after returning to Tokyo, two recovery nights were recorded (R1, R2). In ANC (outbound), total sleep time was reduced and, sleep efficiency was low (72.0 percent). In London, time in bed increased slightly, but sleep efficiency was still reduced. On return to ANC (inbound), there was considerable slow wave sleep rebound and multiple awakenings reduced sleep efficiency to 76.8 percent. Sleep efficiency on R2 was significantly lower than on B1 but not different from R1. To sum up, sleep of aircrews flying multiple transmeridian flights is disrupted during layovers and this effect persists during the two recovery nights. As a result, there is a marked cumulative sleep loss during multilegs polar route trip in comparison to single leg flights. These findings suggest that following such extensive transmeridian trips, crews should have at least three nights of recovery sleep in their home time zone before returning to duty.

How flight crews organize their sleep during layovers on long-haul trips is documented. Additionally, environmental and physiological constraints on sleep are examined. In the trips studied, duty periods averaging 10.3 hr alternated with layovers averaging 24.8 hr, which typically included two subject-defined sleep episodes. The circadian system had a greater influence on the timing and duration of first-sleeps than second-sleeps. There was also a preference for sleeping during the local night. The time of falling asleep for second-sleeps was related primarily to the amount of sleep already obtained in the layover, and their duration depended on the amount of time remaining in the layover. For both first- and second-sleeps, sleep durations were longer when subjects fell asleep earlier with respect to the minimum of the circadian temperature cycle. Naps reported during layovers and on the flight deck may be a useful strategy for reducing cumulative sleep loss. The circadian system was not able to synchronize with the rapid series of time-zone shifts. The sleep/wake cycle was forced to adopt a period different from that of the circadian system. Flight and duty time regulations are a means of ensuring that reasonable minimum rest periods are provided. This study clearly documents that there are physiologically and environmentally determined preferred sleep times within a layover. The actual time available for sleep is thus less than the scheduled rest period.

Long-duration space missions require adaptation to work-rest schedules which are substantially shifted with respect to earth. Astronauts are expected to work in two-shift operations and the environmental synchronizers (zeitgebers) in a spacecraft differ significantly from those on earth. A study on circadian rhythms, sleep, and performance was conducted by exposing four subjects to 6 deg head-down tilt bedrest (to simulate the effects of the weightless condition) and imposing a 12-h shift (6 h delay per day for two days). Bright light was tested in a cross-over design as a countermeasure for achieving faster resynchronization and regaining stable conditions for sleep and circadian rhythmicity. Data collection included objective sleep recording, temperature, heart rate, and excretion of hormones and electrolytes as well as performance and responses to questionnaires. Even without a shift in the sleep-wake cycle, the sleep quantity, circadian amplitudes and 24 h means decreased in many functions under bedrest conditions. During the shift days, sleepiness and fatigue increased, and alertness decreased. However, sleep quantity was regained, and resynchronization was completed within seven days after the shift for almost all functions, irrespective of whether light was administered during day-time or night-time hours. The time of day of light exposure surprisingly appeared not to have a discriminatory effect on the resynchronization speed under shift and bedrest conditions. The results indicate that simulated weightlessness alters circadian rhythms and sleep, and that schedule changes induce additional physiological disruption with decreased subjective alertness and increased fatigue. Because of their operational implications, these phenomena deserve additional investigation.

The results of a NASA-sponsored study examining the effectiveness of a brief, preplanned cockpit rest period to improve pilot alertness and performance in nonaugmented long-haul flight operations are discussed. Four regularly scheduled trans-Pacific flight legs were studied. The shortest flight legs were about 7 h and the longest about 9.5 h, with duty periods averaging about 11 h and layovers about 25 h. Three-person B747 crews were divided randomly into two volunteer pilot groups. These crews were nonaugmented, and therefore no relief pilots were available. The rest group, consisting of four crews, was allowed a 40 min opportunity to rest during the overwater cruise portion of the flight. On a preplanned, rotating basis, individual crew members were allowed to nap. It is concluded that a preplanned cockpit nap is associated with significantly better behavioral performance and higher levels of physiological alertness and that this can be accomplished without disrupting normal flight operations or compromising safety.

The adjustment of sleep-wake patterns and the circadian temperature rhythm was monitored in nine Royal Norwegian Airforce volunteers operating P-3 aircraft during a westward training deployment across nine time zones. Subjects recorded all sleep and nap times, rated nightly sleep quality, and completed personality inventories. Rectal temperature, heart rate, and wrist activity were continuously monitored. Adjustment was slower after the return eastward flight than after the outbound westward flight. The eastward flight produced slower readjustment of sleep timing to local time and greater interindividual variability in the patterns of adjustment of sleep and temperature. One subject apparently exhibited resynchronization by partition, with the temperature rhythm undergoing the reciprocal 15-h delay. In contrast, average heart rates during sleep were significantly elevated only after westward flight. Interindividual differences in adjustment of the temperature rhythm were correlated with some of the personality measures. Larger phase delays in the overall temperature waveform (as measured on the 5th day after westward flight) were exhibited by extraverts, and less consistently by evening types.

Eight middle-aged males were studied in a temporal isolation experimental lasting 15 d. After 5 d and nights of entrainment to his own habitual routine, each subject experienced an acute unheralded 6-h phase advance in routine, accomplished by truncating his sixth sleep episode. For the remaining 10 d of the study, subjects were held to a routine 6-h phase advanced to the original. Significant symptoms of jet lag appeared in mood, performance efficiency, sleep, and circadian temperature rhythms. When plotted as a function to days postshift, some variables showed a fairly monotonic recovery to baseline levels, but other variables showed a zig-zag recovery pattern, suggesting the interaction of two competing processes, and reinforcing the need for greater sophistication in the development of jet-lag coping strategies.

Recent statistical and experimental studies on the role of circadian rhythms in aircrew fatigue and aviation accidents are reviewed from a human-factors perspective, and typical data are presented in extensive graphs. Consideration is given to the biological clock and the limits of endurance, circadian desynchronization, sleep and sleepiness, short-haul and long-haul operational studies, and the potential advantages of cockpit automation.

The findings of this cooperative study of layover sleep have direct implications for flight operations. In the consensus view of the principal investigators, these can be divided into their relevance for eastward or westward flight. Eastward flight produced more sleep disruption than westward. Different sleep and scheduling strategies are recommended for each flight direction, and the importance of individual crewmember factors is discussed in relation to age and circadian type. Despite the limitations of this study with regard to trip simplicity and the baseline data, the results for each airline are highly consistent and should be applicable to a wide range of long-haul crewmembers and carriers.

An international research team has carried out an electroencephalographic study of sleep and wakefulness in flight crews operating long-haul routes across seven or eight time zones. Following baseline recordings, volunteer crews (n = 56) from four airlines spent their first outbound layover at a sleep laboratory. This paper provides an overview of the project's history, its research design, and the standardization of procedures. The overall results are remarkably consistent among the four participating laboratories and strongly support the feasibility of cooperative international sleep research in the operational arena.

Physiological recordings of sleep and wakefulness in operating international (B-747) flight crews were obtained. Crews spent their first layover (48 h) of a trip in a sleep laboratory where standardized EEG, electro-oculograph (EOC), and electromyograph (EMG) sleep recordings were carried out whenever volunteers chose to sleep. During periods of wakefulness they underwent multiple sleep latency tests every 2 h in order to assess daytime drowsiness. The same standardized recordings were carried out at a home-based laboratory before departure. Approximately four crews each participated in flights over 7 to 9 time zones on five routes. All participants were encouraged to use whatever sleep-wake strategies they thought would provide them with the most satisfactory crew rest. Overall, layover sleep quality was not seriously disturbed, but eastward flights produced greater sleep disruption. The contributors of individual factors and the usefulness of various sleep strategies are discussed in the individual laboratory reports and in an operational summary.

Physiological and psychological disruptions caused by transmeridian flights may affect the ability of flight crews to meet operational demands. To study these effects, 9 Royal Norwegian Airforces P3-Orion crewmembers flew from Norway to California (-9 hr), and back (+9 hr). Rectal temperature, heart rate and wrist activity were recorded every 2 min, fatigue and mood were rated every 2 hr during the waking day, and logs were kept of sleep times and ratings. Subjects also completed 4 personality inventories. The time-zone shifts produced negative changes in mood which persisted longer after westward flights. Sleep quality (subjective and objective) and duration were slightly disrupted (more after eastward flights). The circadian rhythms of sleep/wake and temperature both completed the 9-hr delay by day 5 in California, although temperature adjusted more slowly. The size of the delay shift was significantly correlated with scores on extraversion and achievement need personality scales. Response to the 9-hr advance were more variable. One subject exhibited a 15-hr delay in his temperature rhythm, and an atypical sleep/nap pattern. On average, the sleep/wake cycle (but not the temperature rhythm), completed the 9-hr advance by the end of the study. Both rhythms adapted more slowly after the eastward flight.

Experiments were conducted to estimate the magnitude of the masking effect produced in humans by alternate periods of physical activity and rest or sleep on the circadian rhythms of heart rate and core temperature. The heart rate, rectal temperature, and nondominant wrist activity were monitored in 12 male subjects during 6 days of normal routine at home and during 6 days of controlled bed-rest regimen. The comparisons of averaged waveforms for the activity, heart rate, and temperature indicated that about 45 percent of the range of the circadian heart rate rhythm during normal routine and about 14 percent of the range of the circadian temperature rhythm were attributable to the effects of activity. The smaller effect of activity on the temperature rhythm may be partially attributable to the fact that core temperature is being more rigorously conserved than heart rate, at least during moderate exercise.

Two Van der Pol oscillators with reciprocal linear velocity coupling are utilized to model the response of the human circadian timing system to abrupt displacements of the environmental time cues (zeitgebers). The core temperature rhythm and sleep-wake cycle simulated by the model are examined. The relationship between the masking of circadian rhythms by environmental variables and behavioral and physiological events and the rates of resynchronization is studied. The effects of zeitgeber phase shifts and zeitgeber strength on the resynchronization rates are analyzed. The influence of intrinsic pacemakers periods and coupling strength on resynchronization are investigated. The simulated data reveal that: resynchronization after a time zone shift depends on the magnitude of the shift; the time of day of the shift has little influence on resynchronization; the strength of zeitgebers affects the rate and direction of the resynchronization; the intrinsic pacemaker periods have a significant effect on resynchronization; and increasing the coupling between the oscillators results in an increase in the rate of resynchronization. The model data are compared to transmeridian flight studies data and similar resynchronization patterns are observed.