Subjective ratings and EEG parameters describe different dimensions of the multidimensional alertness phenomenon ( Broughton, 1992). Alertness ratings show a smooth time course representing overall assessment of capacities, whereas alpha power in the EEG measures spontaneous dozing off and the susceptibility to sleep.
Sleepiness that may be caused by monotony or that may follow high workload is not part of the alertness model described by Folkard and Akerstedt ( 1992). It cannot be ruled out that monotony in the second part of the flights may have added to the observed sleepiness.
The findings in the subjective sleepiness assessments are supported by results obtained from night nurses ( Folkard et al., 1978). Like night nurses that work on different shifts as opposed to permanent night workers, pilots experience two consecutive night flights as an unusual schedule and are less prone to cope with the situation than permanent night workers.
As a consequence of increased sleepiness, countermeasures have to be considered. Naps are reported to alleviate spontaneous sleepiness ( Dinges, Orne, Whitehouse, & Orne, 1987; Minors & Waterhouse, 1989; Mullaney, Kripke, Fleck, & Johnson, 1983; Naitoh, Englund, & Ryman, 1982). Timing and duration of naps, however, seem to be of crucial importance to assess the benefit of a nap for job performance ( Dinges, Whitehouse, Orne, & Orne, 1988; Rosa, 1993). This awaits further research. Recently, the use of preplanned naps in three crew operations was discussed in detail as one of the possible measures to cope with the situation of extended night flights ( Rosekind, Graeber, Dinges, Connell, Rountree, Spinweber, & Gillen, 1992). The possibility of recommending napping for two crew operations provided napping is compatible with crew management and does not compromise flight safety should also be considered.
The cooperation of the pilots and the airline management is gratefully acknowledged. The work was in part funded by the CEU, Directorate-General for Transport, contract no. C2, B93, B2-7020, SIN 004101.