A dose-response study of total sleep time and the ability to maintain wakefulness.

The apparent connection between sleep debt, performance decrements and workplace accidents has generated a need for feasible vigilance tests that focus on the quantification of daytime sleepiness in occupational settings. The objective of this study was to evaluate the sensitivity of the Maintenance of Wakefulness Test (MWT) to acute sleep deprivation of various doses. Eight healthy female volunteers, mean age 28.9 years (range 23-36), participated in this laboratory study. After an adaptation night, the subjects were assigned to four counterbalanced, randomly ordered night sleep conditions. These four conditions allowed for a time in bed (TIB) of 0, 2, 4 or 8 h, producing a total sleep time of 0, 113, 218 and 427 min, respectively. The ability to sustain wakefulness was measured after the TIB period at 11.00 and 17.00 hours by the MWT. Analysis of variance with repeated measures was used to study the dependence of MWT sleep latencies on the immediately prior TIB period. Both the latency of stage 1 sleep onset and the appearance of slow eye movements reduced significantly with increased sleep loss. The quantitative relationship between the previous total sleep time and the subsequent MWT sleep latencies followed an exponentially decaying function showing a high sensitivity to acute, severe night sleep loss but low sensitivity to less severe sleep restrictions. It is concluded that the MWT seems to be a sensitive method for the estimation of acute sleep deprivation. The test results appear, however, non-linearly related to the earlier sleep debt.

The effect of four-day round trip flights over 10 time zones on the sleep-wakefulness patterns of airline flight attendants.

To study the effect of a four-day-round trip flight on the sleep-wakefulness of airline flight attendants, subjective sleep-wakefulness and autonomic sleep phases were measured. Forty flight attendants (mean age 33 years, range 21-50) kept daily logs on sleepiness, the time when going to bed, and sleep quality. In addition, the autonomic sleep phases of 21 subjects were studied by the static charge sensitive bed (SCSB) method. After the westward flight, the subjects went to bed approximately 1-3 h local time earlier during the first few days and were very sleepy compared to the week before the flight. There was a significant increase in the number of awakenings and in the feeling of 'not being at all rested' in the mornings. After the return flight eastwards, the subjects were very sleepy on the first evening but slept rather well for about 11 h. During the three following days, sleep restlessness, difficulties in falling asleep, and the feelings of sleepiness in the mornings increased compared to the week before the flights. Four days after the return flight, sleep length and the quality of sleep were, on average, the same as before the flights. According to the SCSB method, there were only small changes in the autonomic sleep phases due to the flights. After the westward flight, quiet sleep increased and intermediate sleep decreased compared to the sleep before the flight. The results indicate that most flight attendants have significant disturbances in sleep quality after transmedian flights. Sleep disturbances increase after both westward and eastward transmedian flights, but differ from each other in specific features.

The effect of four-day round trip flights over 10 time zones on the circadian variation of salivary melatonin and cortisol in airline flight attendants.

The effects of four-day round flights (Helsinki-Los Angeles-Seattle-Helsinki) were studied on the circadian rhythms of salivary melatonin (MT) and cortisol (COR) in 35 flight attendants. The mean age of the subjects was 33 +/- 7 years (median 34, range 21-50). Five 24 h profiles of unstimulated saliva were collected at 2 h intervals (except at 04:00) before, during, and after the four day flight. Salivary MT and COR were determined by radioimmunoassay. Both MT and COR exhibited a clear circadian rhythm with acrophases before the flight at 03:03 (MT) and 09:08 (COR). Two days after the westward flight from Helsinki to Los Angeles, the MT rhythm (circadian acrophase) had delayed 4 h 51 min and the COR rhythm 3 h 55 min compared to the control day before the flight. Two days later, during the last day in the USA, the MT rhythm had delayed 5 h 59 min and the COR rhythm 5 h 29 min as compared to the situation before the flight. After four days of the eastward flight from Seattle to Helsinki, the circadian acrophase of MT was still 1 h 35 min delayed compared to the control day before the westward flight. The results indicate that the restitution time of five days at the home base is on the average proper for recovery, if a four day round flight over 10 time zones takes four days or less. The resynchronization rate of salivary hormones after westward, outgoing flights is faster than the resynchronization rate after the eastward return flights.

Effects of 10 h time zone changes on female flight attendants' circadian rhythms of body temperature, alertness, and visual search.

The aim of the study was to analyse the effects of rapid time zone changes on the circadian rhythms of flight attendants. The mean age of the 40 female subjects was 30.0 (SD = 6.9) years. Measurements of oral temperature, alertness, and visual search were performed at two hour intervals two days before the flight from Helsinki to Los Angeles, during the second and the fourth day in the USA and during the second and fourth day after the return flight to Finland. The body temperature desynchronized and the phases of the alertness and visual search rhythms shifted rapidly in the USA. After the return flight, the acrophases of the circadian rhythms delayed during the second and fourth day in Finland. During the fourth day the acrophase of alertness was 35 min and the acrophases of body temperature and visual search were 2 h 2 min and 3 h 8 min delayed, respectively. The mathematical model based on the C-, S- and W-process theory of alertness explained 25-96% of the variation of observed mean alertness of the subjects in different conditions. It is concluded that the duration of the de- and resynchronization process of the flight attendants' circadian rhythms is on the average longer than 9 days during and after round flights over ten time zones. The mean alertness of the subject can be predicted with considerable accuracy using the mathematical model.

The prediction of the adaptation of circadian rhythms to rapid time zone changes.

The aim of the study was to find out which factors could explain individual differences in the resynchronization speed of circadian rhythms of salivary melatonin and subjective alertness after transmeridian flights over 10 time zones. The mean age of the 40 female subjects was 33.0 +/- 6.9 years. The data were gathered by measurements of the circadian rhythms of melatonin excretion and altertness at 2 h intervals in Helsinki (Finland) two days before westward flight to Los Angeles (USA), where the measurements were repeated on the 2nd day after the flights and on the 2nd day in Finland after return flight. This shift in the acrophases of the two circadian rhythms were used as dependent variables in regression analyses. The predictors used were length of day, marital status, amount of physical exercise, age, neuroticism, extroversion, and eveningness. Age, day length, marital status, and physical exercise explained the acrophase adaptation of the melatonin rhythm after westward flight, and day length, neuroticism, and extroversion after eastward flight. Marital status, neuroticism, and physical exercise explained the variation of the acrophase adjustment of the alertness rhythm after westward flight, and age and eveningness after eastward flight. It is concluded that the amount of daylight and personality are the best predictors of the circadian rhythm adaptation after transmeridian flights.

Performance efficiency and its changes among aging municipal employees.

The aims of the study were to compare the performance efficiency of aging municipal workers in different work categories and to determine the changes that occurred in visual search, short-term memory, and fine motor performance over a four-year follow-up period beginning at about 51 years of age. The mental capacity of the workers in mental work was better than that of the workers in physical and mixed physical and mental work, and the results between the physical and mixed work groups did not differ significantly. In four years, the workers' complex short-term memory weakened 6-7% in all three work content groups. Fine motor speed decreased 2-7% in the physical and mental work groups. It was concluded that the mental capacity of older workers depends on the mental demands of their work tasks. The weakening of performance is 2-7% between the ages of 51 to 55 years.

Associations between functional capacity and work ability among elderly municipal employees.

The relationship between objectively measured physical and mental functional capacity and work ability was studied among 137 workers with a mean age of 55 years. Of the physical capacity tests muscular strength correlated the best with the constructed work ability index. Cardiorespiratory capacity and work ability did not correlate statistically significantly. About 50% of the subjects was classified uniformly, and less than 10% not uniformly, with respect to muscular strength. The correlation between cardiorespiratory capacity and work ability was significant for those without, but not for those with, a musculoskeletal disease. The mental capacity tests had systematically lower correlations with work ability than the tests for physical capacity. The highest statistically significant correlation was found between visuomotor speed and work ability. Objective measurements of muscular strength seem useful for defining work ability, but other tests need to be improved to be more work-related. Furthermore, musculoskeletal diseases should be checked for when cardiorespiratory capacity is assessed.

Changes in maximal cardiorespiratory capacity among aging municipal employees.

Before and after a four-year follow-up, maximal oxygen consumption (VO2max) was determined for 67 municipal employees (initial age about 51 years). The VO2max (ml.kg-1) decreased 6% among the women and increased 8% among the men, the greatest and smallest change occurring for employees doing mental and physical work, respectively. The men's increase was associated with an increase in leisure-time physical activity. Work content was not associated with the level of VO2max for either gender. After the four years the women's absolute VO2max was about 66% of the men's. There were marked changes in VO2max at the individual level, the most marked decreases being associated with the use of medication in some cases. After the four years the women's VO2max of 63% was insufficient according to current recommendations. Therefore physically demanding jobs are not generally recommended for older women.

Flight attendants' desynchronosis after rapid time zone changes.

The aim of the present study was to measure perceived effects of rapid time zone changes on flight attendants' sleep length, quality, adaptation, and recovery time, and to clarify the individual factors related to perceived desynchronosis after time zone changes. The mean age of 285 female subjects was 35.0 years and that of 57 men was 34.1 years. The data were gathered by means of a questionnaire filled out by all Finnish flight attendants who worked on transmeridian routes. The quality of sleep, perceived adjustment, and recovery times were dependent on the flight direction and on the number of time zones crossed. The effects of age, neuroticism, and sex partly explained the variation of perceived desynchronosis, which increased linearly with increasing age and neuroticism.