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.

1.0 OPERATIONAL OVERVIEW

This report is the seventh in a series on the physiological and psychological effects of flight operations on flight crews, and on the operational significance of these effects. This section presents a comprehensive review of the major findings and their significance. The rest of the volume contains the complete scientific description of the work.

To document the psychophysiological effects of flying overnight cargo operations, 41 B-727 crew members were monitored before, during, and after one of two typical 8-day trip patterns. On the Destination-Layover pattern, crews stayed in layover hotels between consecutive nights of flying. After three nights on duty, they deadheaded home and had about 45 hr off duty before deadheading out to begin another three nights of flying. On the Out-and-Back pattern, crews returned home after each night of flying. After five nights on duty, they had about 45 hr off duty before flying for two additional nights. The average duty "day" on the Destination-Layover pattern was 3.5 hr longer than on the Out-and-Back pattern, with double the number of flight segments and 52 min more flight time; the average layover was 6.1 hr shorter. All flights took place in the eastern and central United States, with a maximum time zone change of 1 hr per day.

Thirty-four volunteers provided sufficient data to be included in the analyses. Their average age was 37.6 yr and they had flown for an average of 4.7 yr with the participating company. Throughout their participation in the study, they wore a portable biomedical monitor that recorded average heart rate, wrist activity, and core body temperature every 2 min. In a logbook, they rated their fatigue and mood every 2 hr while awake and kept a detailed record of their daily activities including: duty times; sleep timing, quantity, and quality; food and fluid consumption; and any occurrences of 20 different medical symptoms. They also completed a Background Questionnaire that included basic demographic information, sleep and lifestyle habits, and four personality inventories. Subjects were accompanied on all study flights by a NASA cockpit observer who kept a detailed log of operational events.

Flying at night required crews to sleep during the day. Daytime sleep episodes were about 3 hr (41%) shorter than nighttime sleep episodes and were rated as lighter, less restorative, and poorer overall. The incidence of sleeping more than once in 24 hr tripled on days with duty, compared to days without duty. Overall, crew members averaged 1.2 hr less sleep per 24 hr on duty days than on pretrip days.

The circadian temperature rhythm did not adapt completely to the inverted wake-rest schedule on duty days, being delayed by about 3 hr. As a result, the average temperature minimum occurred about an hour after coming off duty, at around 0820 local time. The time of the temperature minimum corresponds to the daily low point in alertness and in performance capabilities in the laboratory, in flight simulators, and in other 24-hour industries. Crew members were also accumulating a sleep debt across the 8 days of the trip patterns.

The way that crews organized their sleep between successive nights of flying reflected the interaction of duty timing with circadian physiology. Regardless of the time that they went to sleep after coming off duty in the morning, they tended to wake up around 1410 local time, even after as little as 4-5 hr of sleep. This clustering of wake-up times coincides with the timing of the circadian "wake-up signal" identified in laboratory studies. Anecdotal reports from crew members indicate that they often awaken spontaneously around this time but do not feel well rested. Because it is difficult to sleep past the circadian wake-up signal, getting off duty earlier enables crews to sleep longer in the morning. If late off-duty times are unavoidable due to operational constraints, then longer layovers (the present data suggest at least 19 hrs) would accommodate a second sleep episode in the evening. Layovers in which crew members slept twice ended 4-7 hr later (around 0330 local time) than layovers in which they slept only once. Because of the evening wake maintenance zone, crew members need to be aware that they risk having difficulty falling asleep if they do not go to sleep again before about 2300 local time. This is a part of the circadian cycle when it can be difficult to fall asleep, even after sleep loss.

The night off in the middle of a sequence of duty nights provided an important opportunity for recuperation. Crew members averaged 41 min more sleep per 24 hr than pretrip and 115 min more than during daytime layovers. It was effectively positioned in the sequence of night duties to offset the cumulative sleep loss imposed by the schedules. On the Destination-Layover pattern, one third of all crew members had lost more than 8 hr sleep after three nights of flying. It was clearly prudent not to add a fourth consecutive night of duty in this case. In contrast, on the Out-and-Back pattern, only one quarter of the crew members had lost more than 8 hr of sleep after five nights of flying. The amount of sleep lost varied greatly, even among crew members on the same trip pattern. It was not correlated with any of the individual attributes previously reported to predict adaptability to shift work and time zone changes (i.e., amplitude of circadian rhythms, morningness/eveningness, extraversion, and neuroticism).

When they were awake on duty at night, subjects rated their fatigue and negative affect as higher, and their activation and positive affect as lower, than when they were awake during the day pretrip. Subjective fatigue and activation appear to be influenced by both the circadian cycle and the duration of wakefulness, with minimum fatigue (peak activation) occurring 8-10 hr after awakening. Flying at night disrupted the normal relationship between these two components. The data did not permit a precise description of these changes. However, highest fatigue and lowest activation occurred around the time of the temperature minimum, as has been reported for night workers in other industries.

Crew members reported eating more snacks on duty days than on pretrip days. However, unlike the daytime short-haul air-transport crews in other NASA field studies, they did not increase their consumption of caffeine on duty days. Used appropriately, caffeine can be a convenient operational countermeasure for fatigue. Ensuring that caffeine and information about its use are readily available could help crew members maintain their alertness during night flights. However, caffeine also disturbs sleep so its use close to bedtime is not recommended. On duty days, by comparison with pretrip days, reports of headaches quadrupled, reports of congested nose doubled, and reports of burning eyes increased ninefold.

The responses of overnight cargo crew members to duty demands were compared with those of daytime short-haul air-transport flight crews for whom the same measures were available. In both cases, crews crossed no more than one time zone per 24 hr. The overnight cargo crews had shorter duty "days" (by 3 hr), with 2 hr less flight time, fewer, shorter flight segments, and longer layovers (by 2.4 hr). They were also 5.4 yr younger on average. Nevertheless, while on duty, they lost a comparable amount of sleep per 24 hr, had shorter individual sleep episodes, and had more broken sleep than their daytime short-haul counterparts. This is consistent with the finding that 62% of shift workers in other industries report sleep complaints, compared with 20% of day workers. The daytime sleep of night-shift workers is also reduced by about a third relative to a normal night of sleep at home.

Reports of headaches were more than twice as common among overnight cargo crews than among short-haul fixed-wing crews and were approaching the incidence reported by helicopter crews who flew daytime air-transport operations in cockpits in which overheating, poor ventilation, and high levels of vibration were common. Overnight cargo crews also reported that trips had a negative effect on appetite, whereas daytime short-haul fixed-wing crews reported no change.

Over the past 45 years, there has been a significant increase in scientific knowledge regarding sleep loss, circadian disruption, and their effects on performance and alertness. Laboratory studies have demonstrated that reducing sleep by 2 hr on one night is sufficient to significantly decrease subsequent alertness and performance. These studies have shown that sleep loss accumulates over time into a cumulative sleep debt. As this "debt" increases, people become increasingly sleepy. Acute sleep loss and a cumulative sleep debt, combined with poor sleep quality, all have the potential to decrease waketime alertness progressively with the number of days of reduced sleep. In laboratory studies, the combination of working through the circadian temperature minimum with a sleep debt produces the poorest performance.

Data for this study were collected between November 1987 and November 1988. Since that time, there have been a number of changes in the operations of the participating company. In domestic operations, so-called "morning" Out-and-Back patterns of the type studied here have been almost eliminated. Destination-Layover patterns are still common, but longer layovers have been introduced. "Evening" Out-and-Back patterns are also common (Clive Seal, personal communication, 1994). The maximum number of consecutive nights of flying has been extended to six, and there has also been an expansion into international cargo operations. This has resulted in some schedules which include both transmeridian and back-of-the-clock flying (David Wells, personal communication, 1994). The impact of these changes on circadian disruption and duty-related sleep loss deserves investigation. The company has also banned smoking in the cockpit. Of the 34 crew members included in the analyses in this study, only one reported being a smoker. Thus, it is unlikely that smoking in the cockpit was related to the physical symptoms reported during these trips.

Clearly, no one study can address in detail all the issues in overnight cargo operations, which are rapidly evolving and expanding in response to market demands and other forces. Schedules are varied and changeable, and logistical and cost factors limit the number of crew members who can be studied. However, night work has some generic physiological consequences which stem from trying to override the day-active orientation dictated by the human circadian clock. The present study illustrates that overnight cargo operations, like other types of night work, can require people to work through the circadian lowpoint in alertness and performance and displace sleep to a part of the circadian cycle where its quality and quantity are reduced. Currently, there are no countermeasures, which have been shown to be safe and effective in operational settings, to overcome the incomplete adaptation of the circadian clock to night work. However, the present study indicates several approaches for minimizing sleep loss. In trip construction, particular attention can be given to the timing and duration of rest periods and to the number of consecutive nights of flying. Education and training on sleep and circadian physiology, and its operational significance, can enable crew members to develop better personal strategies for coping with the demands of overnight cargo flying. The participating company addresses fatigue issues in an ongoing way through listening to crew members and its Flight Safety Department, monitoring innovations in the industry, and as part of its recurrent training and Crew Resource Management training curriculum.

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