AM 65-16 

PILOT FATIGUE: INTERCONTINENTAL JET FLIGHT 

l. Oklahoma City <-> Tokyo 

George T. Hauty, Ph. D. 

Thomas Adams, Ph. D. 

Approved by Released by 

STANLEY R. MoHLER, M.D. M. s. WHITE, M.D. 

DIRECTOR, CARl FEDERAL Am SuRGEON 

March 1965 

FEDERAL AVIATION AGENCY 

Office of Aviation Medicine 

Civil Aeromedical Research Institute 

Oklahoma City, Oklahoma 
Acknowledgments 

The authors gratefully acknowledge the technical assistance provided by 

Dr. Walter Nelson and Mr. Frank Jones, who made the biomedical assessments 

and who also participated as subjects; the assistance given by Dr. David K. 

Trites, Mr. Rollo S. Beebe, Mrs. Bernice Butler, and Miss Jane Caskey in the 

statistical analysis and graphic representation of the obtained data; the 

efforts of Mr. Earl Blanchard and Mr. George Doil, who not only made 

possible the transportation of the subjects but who also provided invaluable 

assistance during the period in Tokyo; and the cooperation of the subjects 

Mr. Carl Erickson, Mr. Dennis Paulson, Mr. Gary Thomas, and Mr. Russell 

Wendorff. Remaining to be ackno,Yledged is the advice and efforts of Dr. 

Franz Halberg, who collaborated in the design and execution of the study. 

Qualified requestors may obtain Aviation Medical Reports from Defense Documentation 

Center. The general public may purchase from Clearinghouse for Federal Scientific 

CAT. F€1R and Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151 

LOAN STAC!(C~-~ 

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1965'.'/~-3~ 

PILOT FATIGUE: INTERCONTINENTAL JET FLIGHT 

Oklahoma City ~~ Tokyo 

I. Introduction 

Of the several conditions that produce pilot 

fatigue, one has been selected for specific and 

systematic experimental study. This condition, 

for the intercontinental-air-carrier crew, reputedly consists of the, disruption of physiological 

day-night cycling occurring as a consequence of 

rapid translocation through many time zones. 

Due to biological lag time, these physiological 

readjustments can be expected to have the additional effect of attenuating recuperative processes. 

The predicted result, which in fact has been 

claimed by operating air crews, is an undesirable 

level of both acute and chronic fatigue. 

Unfortunately, the scientific literature does not 

provide sufficient empirical, reproducible, or 

critically relevant information pertinent to these 

human physiological changes. Most of the experimental studies of human periodicity (e.g., Aschoff 

and Wever, 1962; Hauty, 1963; Kleitman, 1963; 

Lewis and Lobban, 1957) have been prompted by 

etiological considerations necessitating environmental conditions and controls such as to preclude 

fruitful .extrapolation. Studies that have dealt 

directly with the problem concerned and that do 

permit extrapolation provide limited information. 

Such studies are few in number (Burton, 1956; 

Flink and Doe, 1959; Gerritzen, 196~; Sasaki, 

1964), and with but one notable exception 

(Bugard and Henry, 1961) the respective subject 

population size was small, and, for the most part, 

only a single function rather than a spectrum 

of biological activities was assessed. The reason 

must not be attributed to any inadequacy on the 

part of the research investigators involved but, 

instead, to the costs and to the problems of logistics and experimental control likely to be encountered in such research. 

Accordingly, a project consisting of a series 

of intercontinental flights was designed :for the 

purpose of obtaining information that would reveal, firstly, the severity of the problem as it 

relates to the proficiency, reliability, and health 

1 

of the pilot and, secondly, what might be done 

to mitigate those consequences found to be adverse 

or undesirable. The first phase of such flights 

will consist of an East-vVest, 1Yest-East, and a 

North-South round-trip jet flight that will permit 

a comparative analysis of bidirectional time-zone 

displacements and, by using the North-South 

flight as a "control," an appraisal of those effects 

solely attributable to prolonged flights. 

To date, the East-West and the 1Vest-East 

flights have been completed. To be reported here 

are the findings obtained from a preliminary 

flight that, as a pilot study, served two purposes: 

determination of the feasibility of such a research 

project and collection of information relevant to 

the research problem concerned. In the case of 

the former, determinations had to be made of 

the logistical problems occasioned by the critically 

scheduled shipment of a considerable amount of 

scientific equipment, of the tolerance limits of 

human volunteer subjects, and of the reliability 

of assessment techniques. In the case of the latter, 

information was sought pertinent to those basic 

questions most obviously in need of initial and 

definitive answer: 

A. 1Vhat is the extent of time lag for biological 

phase shifts under these circumstances? 

B. To what extent, if any, is proficiency ad-

''ersely affected during the period of transition? 

C. To what extent do individuals differ in 

lag time and adverse effects upon proficiency? 

D. ·what is the extent and nature of differences manifested during the primary (Oklahoma 

City -~ Tokyo) and the back (Tokyo -~ 

Oklahoma City) shifts and their respective transitional periods? 

Finally, one additional element of the rationale 

underlying this first phase of the project should 

be mentioned. The subjects were selected, not 

from intercontinental-air-carrier cre,vs, but from 

a group whose daily habits of work and sleep 

were more representative of the general, adult, 

440 
male population. The reason consists of the obvious need for a reference against which to compare the findings to be obtained from intercontinental-air-carrier crews during the second 

phase of the project. It is only with such a 

basis for comparison that an evaluation can be 

made of the respective roles of habituation 

that would act to \lttenuate fatigue-induced effects upon proficiency and health and of chronic 

exposure that could be expected to exacerbate 

such deleterious effects. 

II. Method. 

Following 3 consecutive days of biological assessment at Oklahoma City (preflight) conducted 

for the purpose of obtaining a reference of bio·· 

logical time set to Central Standard Time, subjects enplaned at 0810 hours (CST) for a jet 

flight to Tokyo. After 18 total elapsed hours, 

they arrived in Tokyo (1700 hours local time) 

and remained there for 10 days, during which 

biological assessments were repeated on alternate 

days beginning with the first full day in Tokyo. 

On the 11th day, subjects left Tokyo at 0930 

hours (local time) on a jet flight and, after 17% 

total elapsed hours, arrived in Oklahoma City 

at 1100 hours (CST) where they \Yere subjected 

to 3 consecutive days of biological assessment 

(postflight) beginning with the first full day of 

their return. 

The data to be reported include: 

A. Rectal temperature-A calibrated rectal 

thermistor probe was wom by each subject continuously throughout the 2±-hour day of assessment. During the waking portion of the day, 

the subject read and recorded his own temperatures to the nearest 0.1° C at 30-minute intervals 

on a portable meter. During the remainder of 

the day, a technician read and recorded at 30-

minute intervals the temperatures of the sleeping 

subject. 

B. Reaction time-The speed of the subject's 

manual response to (1) a single auditory stimulus, (2) a single visual, and (3) one of several 

possible visual stimuli ("multiple-choice''\ ,\·as 

measured at 4-hour intervals beginning at 0700 

to 2300 hours for each day of assessment. 

C. Critical flicker-fusion-Three ascending and 

three alternating descending thresholds were determined at the above times of assessment for 

each day of assessment. In indicating the highest 

2 

rate at which this type of input ean be adequately 

processed, these thresholds are generally considered representative of the state of efficiency of 

cerebral function, and, when used as an index of 

fatigue, an im·erse relationship between the 

flicker-fusion threshold and hours of work (e.g., 

driving a truck) is generally reported. 

D. Subjective fatigue-The level of fatigue 

perceived by the subject to be representative of 

his state of feelings was also measured at the 

above times of assessment during each assessment 

day by a scaled check list. 

During the preflight and postflight periods in 

Oklahoma City, subjects were quartered in a clinical research ward. 1Vhile in Tokyo, they were 

quartered in a hospital at a r.s. military base, 

which assured access to accustomed food. At 

both locations, subjects were instructed to conform to the extent possible to their own daily 

living habits, particularly the hours of local time 

for retiring to and arising from bed. 

The subjects consisted of six male volunteers 

drawn from a medical-school population. The 

ages of three ranged from 19 to 23 years. Ages 

of the remaining three ranged from 40 to 48 

years. 

III. Results and Discussion. 

A. OKLAHOMA CITY --') TOKYO 

~Iean assessment values made during the 3-day 

preflight period in Oklahoma City and during 

the 10-day period in Tokyo are reported in Figure 

1. In the case of rectal temperatures obtained 

during the 3 consecutive days of preflight assessment, means were computed for each hour for 

all subjects and for all 3 days. During the period 

in Tokyo, means were computed for all subjects 

for each hour of the 24-hour day of assessment. 

In the case of the remaining assessments, daily 

means were computed. 

1. Rectal tempemtm·e. Examination of the preflight curve of rectal temperatures reveals the 

periodicity and amplitude of stored heat typically 

obtained under normal circumstances; i.e., a 

steady increase in internal temperature begins at 

awakening, peaks between 1400 and 1700 hours, 

and is followed by a steady decrease that continues to the beginning of the second biological 

day. Rectal temperatures during the first day in 

Tokyo appear somewhat less regular, apparently 

peaking about 4 or 5 hours out of phase with 
MEAN OF ALL SUBJECTS 

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Fmt:RE 1. Mean values of rectal temperature, reaction time (solid circles), subjective fatigue (solid squares), decision time (solid triangles), and critical flicker-fusion based upon six subjects assessed prior to flight and following arrival in Tokyo. 

Tokyo time. By the third day in Tokyo, however, 

rectal temperatures appear to have shifted into 

phase with Tokyo time. 

2. Reaction time. The values plotted for reaction 

time (solid circles) can be taken to represent 

the average speed with which the subjects reacted 

manually and appropriately to a variety of different stimuli throughout each givm1 day of assessment. "Csing the preflight value (mean of all 

subjects of all ;) preflight days) as the basis for 

comparison, reaction time is seen to increase substantially (i.e., slower) during the first day in 

Tokyo. On the third day, reaction time is decreased (improves in speed) to a level comparable 

to that attained during the preflight period. An 

analysis of the variance attributable to replications (or days) reveals this difference in reaction 

time for these 3 days to be highly significant 

(P < 0.001). Throughout the subsequent days 

3 

in Tokyo, reaction time continues to decrease. 

This gradual improvement in speed of reaction 

time is attributable, of course, to "practice effect," 

which, in turn, was occasioned by the necessity of 

having to include two subjects who did not have 

sufficient practice or training prior to the period 

of preflight assessment. 

3. Decision time. The values plotted for what is 

termed decision time (solid triangles) were obtained by subtracting the mean time of reacting 

in a single way to a single visual stimulus from 

the mean time of reacting correctly in one of 

several possible ways to one of several possible 

stimuli (multiple-choice). The Yalue thus obtained can be taken to represent the average time 

required by the subjects to "decide" which of 

the possible responses was the correct response 

to be made. Again, using the preflight plotted 

value as the basis for comparison, a considerably 
greater amount of time is required for such de· 

cision during the first day in Tokyo. As in the 

case of reaction time, decision time is decreased 

to the preflight level on the third day and, for 

the most part, this level is maintained throughout 

the remainder of the period in Tokyo. In using 

the above-mentioned model for analysis of 

variance, the increase in decision time noted 

for the first day in Tokyo was significant at a 

< 0.10 > 0.05 level of confidence. 

4. Subjectiv.e fatigue. The index of subjective 

fatigue was scaled from extremely fatigued to 

extremely alert; the higher the mean values obtained, the more fatigued the subjects felt themselves to be. From the curves seen thus far, 

subjective fatigue would be expected to be greater 

on the first day in Tokyo compared to the preflight period, and such is borne out. And, again, 

on the third day, the mean value has returned 

to the preflight level. This increment on the 

first day in Tokyo "·as also statistically significant 

(P = < 0.05), whereas the increment in fatigue 

seen to occur on the fifth day in Tokyo and to 

be maintained for the remainder of the period 

is not. 

5. Critical flicker-fusion. On the basis of the 

literature, the curve of mean flicker-fusion 

thresholds was expected to take a form of agreement with the curves presented thus far. Had this 

occurred, a substantial decrease would have been 

noted for the first day in Tokyo, which would 

have been interpreted as a decrease in cerebral 

physiological efficiency. Since this did not occur, 

it must be concluded that physiological efficiency 

was either una.ffected or improperly assessed. This 

conclusion was confirmed by the results of the 

analysis of the variance attributed to the day of 

assessment. To test these possibilities, a more 

precise and sophisticated instrument will be used 

on subsequent flights. 

B. ToKYO _ __,. 0KLAHOlUA CrrY 

Figure 2 presents the mean assessment values 

obtained on the last day in Tokyo (day + 9), 

which can be regarded as the baseline for biological references set to Tol\:yo time against which 

to compare the mean values of data obtained following the return to Oklahoma City (days + 1, 

+ 2, and+ 3). 

1. Rectal temperature. As compared to the baseline curve of rectal temperatures, the curve for 

the first day in Oklahoma City is seen to be in 

4 

phase or very nearly so with Oklahoma City 

time, suggesting that the lag time of the back 

shift is much less than for the primary shift. 

In view of its obvious implications, it should be 

mentioned that this finding is in agreement with 

earlier speculation (Bunning, 1964, p. 58) and 

experimental evidence (Aschoff, 1958; Gerritzen, 

1962). The depressed amplitude of the rectaltemperature curve during the first day back in 

Oklahoma City is surprising, since a corresponding thermal depression did not occur during any 

of the days in Tokyo. On the second day, however, the amplitude of the curve does approach 

more closely that of the baseline reference obtained in Tokyo and during the preflight period 

in Oklahoma City. 

2. Reaction time. As noted in Figure 1, reaction 

time increases on the first day back in Oklahoma 

City and then decreases on the second day. The 

increment in this case, however, is less than that 

'vhich occurred in Tokyo. 

3. Decision time. In contrast with reaction time, 

decision time is seen to inorease on the first day 

and to a greater extent on the second clay in 

Oklahoma City. This latter might be partially 

explained by the following. 

±. Subjective fatigue. A valid assessment of subjective fatigue is particularly difficult because 

of its susceptibility to the effects of other states 

of feelings. One such state, of course, is boredom 

and/or relief from boredom. Since the subjects 

did report boredom to have been generated by the 

long period of assessment in Tokyo, it is possible 

that the value plotted for the first clay back in 

Oklahoma City may reflect relief from such boredom, followed by a return to this state on successive days. Debriefing of the subjects revealed 

boredom to be a matter of such importance that 

the regimen of assessment during subsequent 

flights was changed for the specific purpose of 

mitigating the effects of this state upon subject 

motivation. 

5. Critical flicker-fusion. As in the case of Figure 1, little change in threshold value is to be 

seen. 

IV. Summary. 

A summary comparison of the primary and 

back shifts and their associated consequences is 

given by Figure 3. Under preflight are plotted 

the two bases of reference; i.e., the mean data 
PREFLIGHT 

ITOKYO 

MEAN OF ALL SUBJECTS 

DAY+ I DI.Y+2 DAY+3 

-- --- ---<~ --....... ______ ------- -----· 

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LOCAL TIJoiE (TOKYO) CIMTULTHIII(O/t) CIMTULTIId(OfCl CIMTitAI.TIMI(OIC) 

CII'ITIAL TIMI(O!C) LOCAL TIME (TOitYO) LOCAL Tlllll (TOUOJ 

FIGURE 2. ::\lean values of rectal temperature, reaction time (solid circles), subjective fatigue (solid squares), decision time (solid triangles), and critical flicker-fusion based upon six subjects assessed prior to return flight 

and following return to Oklahoma City. 

values obtained in Oklahoma City during the 

preflight period (dashed lines) and during the 

last day in Tokyo (solid lines). Data are plotted 

in the next column (day 1) for mean values during the first day in Tokyo (dashed lines) and 

during the first day back in Oklahoma City. Mean 

values for the third day in Tokyo (dashed lines) 

and the third day back in Oklahoma City (solid 

lines) are plotted in the last column. 

Since the subject group comprised two distinct age categories differing by about 20 years, 

the relationship of age with phase shifting 

(Kletitman and Kleitman, 19M) should be revealed clearly by the plotting of age-group means. 

An examination of Figure 4 reveals little difference between the rectal-temperature curves of the 

older (subjects 1, 2, 5-dashed line) and the 

younger groups (subjects 3, 4, 6-solid line) during the preflight period in Oklahoma City and 

the first and third days in Tokyo. On the fifth 

and seventh days in Tokyo, differences in phase 

and amplitude are indicated, but on the ninth day, 

the two curves again appear superimposable. For 

subjective fatigue, a considerable difference is revealed; the older group reports a higher level of 

fatigue the first day in Tokyo. Further, this 

higher level is shown up to the last day in Tokyo. 

Finally, it was expected that the older group 

\Yonld be characterized by slower reaction time, 

and this, as can be seen, is borne out. The question of interest was \Yhether increase in reaction 

time during the transitional period would be differentially manifested; it is not evident to any 

appreciable extent. Differences in decision tinw 
are less consistent; the younger group shows the 

greatest increase during the first day in Tokyo 

and the greatest reduction during the subsequent 

days. 

In contrast, Figure 5 reveals a profound difference in amplitude of the rectal-temperature 

curves of these age groups. Such a difference 

cannot be attributed to a consistent experimental 

error because of the technique selected for internal-body-temperature measurement (Mead and 

Bommarito, 1949). Since the level and diurnal 

pattern of human internal temperature has been 

shown to depend to some extent on subject vol31.1 

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untary activity (Vaughan and Adams, 1964; 

Sasaki and Carlson, 1964), it might be suggested 

that the impact of fatigue late in the experimental schedule resulted in a reduced level of 

physical activity, which in turn would be reflected in internal-temperature measurements. If 

this interaction were to occur, it is not unreasonable that its effect might be more manifest in 

the older subjects. It should be noted that the 

daily rectal-temperature patterns for the older 

subjects become progressively depressed, beginning with the preflight measurements in Oklahoma City when the subjects could not have been 

---. 

CI!MTU.L TI.WI tTOitYO) CI!MTRALTIIII!(OIC) CIEMTIAL TIMI! (OIC) 

FIGURE 3. Comparison of mean values of rectal temperature, action time, subjective fatigue, and decision time obtained during the primary phase shift (dashed lines) with mean values of the same parameters during the back 

shift (solid lines). 

6 
fatigued, through the 9 test days in Tokyo (Figure 4). The depression of the rectal-temperature 

curves for the three older subjects more closely 

follows the pattern of the accumulated fatigue 

reported by the group than for the younger team 

members. Nonetheless, until measurements of subject activity levels in terms of whole-body 

metabolism or heart rate are made simultaneously 

with internal temperature in a test schedule 

paralleling the one reported here, this question 

must remain open. 

Lastly, the expected differences in reaction time 

and decision time are again manifested except for 

the last day. For this day, the performance of 

one of the three subjects was responsible for the 

marked increase in the mean values. 

V. Conclusions. 

From what has been presented and from inspection of curves prepared for each individual 

subject (Appendix) certain conclusions can be 

derived. 

MEANS FOR AGE GROUPS 

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FIGURE 4. Comparison of mean values of rectal temperature, subjective fatigue, reaction time (solid circles), and 

decision time (solid triangles) based on the three older subjects (dashed lines) assessed prior to flight and following arrival in Tokyo with the mean values based on the three younger subjects (solid lines) assessed during 

the same periods. 

7 

----------------------------------------------------------------------
1. As indicated by mean rectal temperatures, 

3 to .J days ·were required for the primary phase 

shift and 1 day £or the back shift. 

2. Interindividual differences in lag time were 

profound in that a completed phase shift was 

shown on the first day in Tokyo by one subject, 

while, at the other extreme, another subject did 

not demonstrate a normal phase shift on any of 

the days in Tokyo. 

3. Behavioral integrity -..vas degraded during 

the transitional period in Tokyo and, to a lesser 

extent, during the period o£ transition back to 

the environment or origin. 

PREFLIGHT 

(TOKYO) 

MEANS FOR AGE GROUPS 

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l<'wum; 5. Comparism1 of mean values of reetal temperature, ;;ubjeetive fatigue, reaction time (solid circles), and 

decision time (solid triangles) based on the three older subjects (dashed lines) assessed prior to return flight 

and the following return to Oklahoma City with the mean values ha:<e<l on the three younger subjects (solid lines) 

assessed during the same periods. 

8 
REFERENCES 

1. Aschoff, J. Tierische Periodik unter dem Einfius.s von 

Zeitgebern. Z. Tierpsychol., 15, 1-30 (1958). 

2. Aschoff, J., and Wever, R. Spontanperiodik des 

Menschen bei Ausschluss aller Zeitgeber. Naturwissenschaften, 49, 337-342 ( 1962) . 

3. Bugard, P., and Henry, M. Quelques Aspects de la 

Fatigue dans Aviation de Transport. La Presse 

Medicale, 69, 1903-1906 ( 1961) . 

4. Bunning, E. The Physiological Clock. Academic 

Pref's, Inc., New York, 1964. 

5. Burton, A. C. The Clinical Importance of the Physiology of Temperature Regulation. Canad. Med. 

Assoc. Journal, 75, 715-7120 (1956). 

6. Flink, E. B., and Doe, R. P. Effect of Sudden Time 

Displacement by Air Travel on Synchronization· of 

Adrenal Function. Proc. Soc. Exp. Biol., New York, 

100, 498--501 1959. 

7. Gerritzen, F. The Diurnal Rhythm in Water, Chloride, Sodium, and Potassium Excretion During a 

Rapid Displacement From East to West and Vice 

Versa. J. Aerospace Med., 33, 697-701 (1962). 

8. Hauty, G. T. Relationships Between Operator Proficiency and Effected Changes in Biological Circadian 

Periodicity .. J. Aerospace Med., 34, 100--105 (1963). 

9 

9. Kleitman, K., and Kleitman, N. Effect of NonTwenty-Four-Hour Routines of Living on Oral Temperature and Heart Rate. J. Appl. Physiol., 6, 283--291 

(1953). 

10. Kleitman, N. Sleep and Wakefulness. Univ. of Chicago Press, Chicago and London, 1963. 

11. Lewis, P. R., and Lobban, M. C. Dissociation of 

Diurnal Rhythms in Human Subjects Living on 

Abnormal Time Routines. J. Exp. Physiol., 42, 371~ 

386 (1957). 

12. Mead, J., and Bommarito, C. L. The Reliability of 

Rec.tal Temperature as an Index of Internal Body 

Temperature. J. Appl. PhysioL, 2, 97 (1949). 

13. Sasaki, T. Effect of Rapid Transposition Around the 

Earth on Diurnal Variation in Body Temperature. 

Proc. Soc. Exp. Bioi. Med., 115, 1129--1131 (1964). 

14. Sasaki, T. and Carlson, L. D. Effect of a Step Change 

in Temperature on Skin Temperature and Blood Flow. 

Proc. Soc. Exp. Biol. Med., 117, 334--338 (1964). 

15. Vaughan, J. A., and Adams, T. Diurnal Patterns· of 

Human Internal Temperature. Oral Present., S.W. 

Div. Proc. Soc. Exp. Bioi. Med. (1964). 

APPENDIX 

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time (solid triangles), aml critical flickers-fusion obtained form Subject 1 assessed prior to flight and following 

arrival in Tokyo. 

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time (solid triangles), and critical flicker-fusion obtained ft·om Subject 2 assessed prior to flight and following 

arrival in Tokyo. 

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time (solid triangles), and critical flicket·-fusion obtained from Subject 1 assessed prior to return flight and following return to Oklahoma City. 

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time (solid triangles), and critical flicker-fusion obtained from Subject 6 assessed prior to return flight and following return to Oklahoma City. 

Appendix 22 

66-3173