Subjects were 29 male flight crew members (average age of 52 yrs) flying B747 aircraft on one of four commercial international trip patterns. This report combined the data from the four trip schedules. The Pilot Daily Logbook was completed prior to, during, and following the trip schedule to collect self-reports of duty times, sleep timing, duration, and quality, etc. Core body temperature, heart rate, and activity using the Vitalog portable biomedical monitor were collected in 2 minute samples. The core body temperature, measured with a rectal thermister, was used as a marker of the underlying circadian time-keeping system.

On average, the duty periods lasted about 10.3 hrs followed by 24.8 hrs of layover. During layovers there were generally 2 sleep episodes. The average sleep/wakefulness pattern was 19 hrs awake, 5.7 hrs of sleep, 7.4 hrs awake, and 5.8 hrs of sleep. The first sleep of the layover was generally rated as better quality, with pilots reporting falling asleep more easily and obtaining deeper sleep. As the length of sleep increased, there was a concomitant increase in the sleep quality ratings. The circadian system appeared to exert a greater influence on the timing and duration of the first sleep episode (as compared to the second sleep of a layover), with a preference for sleeping during local night and/or wakeup occurs after the temperature minimum. The exception was after eastward flights that crossed five or more time zones that produced a high accumulated sleep debt. The time of falling asleep for the second sleep episode was related to the amount of sleep already obtained, typically occurred during local night, and the duration was related to the remaining time available before duty. The duration of both sleep episodes was longer when crew members fell asleep earlier with respect to the minimum of their circadian temperature cycle.

Subjective report of naps taken during layovers were obtained from the daily logbook. When the first sleep episode of a layover was identified as a nap it was generally longer (average length was 2 hrs) than other naps. It also followed significantly longer periods of wakefulness. These first naps typically occurred in response to acute sleep loss associated with overnight eastward flights or westward flights crossing 5 time zones or more. Other naps occurred just prior to the next duty period and effectively shortened the length of continuous wakefulness.

Crew members also reported in their logbooks the occurrence of naps on the flight deck. (Inflight rest on the flight deck is not sanctioned under current federal regulations.) The average duration of the naps reported on the flight deck was 46 mins (range 10-130 mins). Research observers accompanying the crews also noted naps not reported in the logbooks. Data combining the research observers' notes and logbook data suggest that, on average, 11% of flight crew members were taking the opportunity to nap when conditions permitted. The data do not indicate whether these were planned naps or occurred spontaneously in response to sleep loss and circadian disruption.

This study provides unique insights into the physiological and subjective effects of flying long-haul commercial operations. The information is both scientifically provocative and can be translated into operationally relevant considerations. The following are some of the scientific considerations that emerge from the results. The flight schedules pushed the sleep/wake cycle into a period (25.7 hrs) different from that of the circadian system, though the two systems did not become completely uncoupled. Though the circadian system continued to influence the timing and duration of sleep episodes, it was unable to resynchronize and quickly adapt to the rapid, multiple time-zone shifts. It is clear that a variety of external/environmental factors (e.g., light, activity, social cues) interact with internal/physiological factors to affect sleep timing, duration, and quality. The operational relevance of the data are also easily translated. For example, current flight and duty time regulations are intended to ensure that reasonable minimum rest periods are available for flight crews. However, this study demonstrated that in commercial long-haul flight schedules there are physiologically and environmentally determined preferred sleep times within a layover and therefore, the time available for sleep may be less than the off-duty time available.