Who s Not Sleepy at Night? Individual Factors Influencing Resistance to Drowsiness during Atypical Working Hours

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1 Who s Not Sleepy at Night? Individual Factors Influencing Resistance to Drowsiness during Atypical Working Hours Halszka OGINSKA Department of Ergonomics, Jagiellonian University, Kraków, Poland Abstract: Sleepiness is one of the major factors degrading performance and impairing safety at work. It is hypothesised that some personality and chronotype characteristics influence alertness at night; in other words, some individuals are less fatiguable at physiologically abnormal times. The report presents an analysis of data gathered from employees (mainly operators) in an iron and steel plant. Only 10 % of the subjects did not experience sleepiness during the night. These resistant subjects needed on average half an hour less sleep to feel good and fit the next day in comparison to the drowsy ones (p<0.02). They had probably longer sleep latency, slept better in the daytime than their counterparts, and were less neurotic (p<0.05) and less reactive (p<0.01). The distribution of morning and evening types was regular and similar in both groups, while the distribution of small medium large amplitude types differed significantly: only a few subjects among the resistant workers showed a large amplitude of the circadian activity rhythm. Descriptions of individual temperamental traits may be useful for initial orientation as to one s own chances to meet the system s demands and to work out effective coping strategies. 1. Introduction Sleepiness is one of the major factors leading to performance degradation and impaired safety at work. According to various authors, it may cause problems with information processing and short-term memory, may impair vision, reaction time, and judgement, and may result in decreased motivation, lowered mood, and increased aggressive behaviours. It reduces vigilance and produces episodes of microsleep. The set of symptoms resembles the effects of alcohol use. This resemblance recently has been studied [1], the conclusion being that performing while sleep-deprived is like working (driving) under the influence of a moderate, but in many countries illegal (for drivers), dose of alcohol. Drowsiness at a time when wakefulness is required has been a problem since biblical times according to Wedderburn [2], and it seems it will be so forever, as full adjustment of the body and mind to nighttime work is not possible. Note, however, that sleepiness stemming from chronobiological factors does not regard only night work but also other delicate times of the day, like the post-lunch period. The state of sleepiness/alertness may be assessed by various objective and subjective methods. Their interrelations are still not known: some of the studies show physiological measures to be uncorrelated with subjective feelings of sleepiness [3], while others consider the subject s own insight into sleepiness to be a valid tool for assessing the actual state [4]. It is the same with more general individual self-reports and declarations as to susceptibility to sleepiness at work, as well as other subjective estimates of sleep-related items, such as the need for sleep or symptoms of insomnia. This study concentrates on the individual experience of sleepiness during night work in the context of some individual factors and traits. It is hypothesised that some personality and 330

2 chronotype characteristics may favour maintenance of appropriate alertness in the night, that is, some individuals are less fatiguable than others at physiologically abnormal times. Is it possible to define the configuration of individual factors underlying or modifying resistance to drowsiness? 2. Material and Methods Subjective data gathered from employees of an iron and steel works in Krakow were analysed. The main study was oriented on women s shift work, so the proportion of female and male subjects in the group examined was the reverse of the actual one in the plant (in our group 80 % women vs. 20 % men). Nearly 600 shift workers were interviewed by psychologists and nurses as to various aspects of shiftwork-related psychological, social and health problems. The set of questionnaires they filled in was under constant development, so different measures were taken at different times this explains the varying number of observations at different points. The main set included the Eysenck Personality Inventory (extraversion, neuroticism, L-scale), Strelau s temperament questionnaire (level of reactivity), a measure of morningness (the Horne & Oestberg Questionnaire, our own Diurnal Activity Questionnaire, or the morningness-eveningness scale of our own Chronotype Questionnaire), and some measures of other chronotypological traits ( languidness and flexibility from the Circadian Type Inventory or distinctness scale of our own Chronotype Questionnaire). The main analysis covered 572 shiftworkers, men (n=119) and women (n=457) employed in the same continuous shift system 4M : 4A : 4N, forward rotation, with a 48-h break after each shift block. Eight-hr shifts started at 0600, 1400, and Over 60 % of all the examined subjects worked as operators of various industrial machines (mainly cranes); the rest were involved in quality control, laboratory work or services. One of the questions in the interview regarded sleepiness experienced at work. Our subjects reported problems with drowsiness on all shifts. The severity of the problem varied from slight discomfort to sleep attacks, irresistible sleepiness requiring an immediate break. Part of the group, however, reported no problem at all, declaring themselves never sleepy on the night shift. These subjects were described in comparison to the others. 3. Results The proportion of workers experiencing drowsiness during shift work is illustrated in Fig.1. Only 10 % of the subjects described themselves as not having problems maintaining alertness during the night shift lasting from 2200 to This group will be further referred to as resistant, in contrast to the drowsy ones. The proportion of the latter is in accordance with the results of other studies, for example Wedderburn s [2]. An intriguing phenomenon is the really high proportion of workers feeling drowsy on the morning shift (up to 73 %). This may be associated with the early start of the shift (06.00), forcing a very early awakening for some workers ( ), which can result in a sleep deficit and drowsiness as a secondary effect. The average worker slept about 6.4 h before the morning shift, no more than 84 % of their declared sleep requirement. 331

3 100% 10 80% % 40% no sometimes yes 20% % 8 M A N Figure 1. Proportion of workers experiencing sleepiness on various shifts A general comparison of the group of resistant workers to the drowsy ones showed similar ages ( and ), the same proportion of females and males (81 % vs. 19 %), and the same proportion of operators to other jobs (60 % vs. 40 %). The timing and length of sleep is forced by work hours and there is not a lot one can do about it, so there were no differences there between the groups (Table 1). drowsy at work length of sleep ( mins; mean +SD) average need for sleep ( mins; mean +SD) actual sleep vs. need for sleep sleep deficit (% of subjects complaining) Table 1. Drowsiness at work and length of sleep drowsy (n=406) resistant (n=57) significance of the difference N 100 % 0 % M 77.6 % 50.9 % *** A 30.0 % 7.0 % *** M A N main N nap (37 % nappers) (35 % nappers) N total day off * M A ** N day off M % 22.8 % * A 14.8 % 3.5 % *** N 53.7 % 15.8 % *** day off 5.7 % 3.5 % * p<0.05 ** p<0.01 *** p<

4 Table 2. Sleep disorders proportion of complaints (%) difficult to fall asleep frequent awakening waking too early generally poor sleep quality feeling fatigued on awakening (not properly rested) drowsy (n=406) resistant (n=57) significance of difference M * A ** N day off ** M 39, A N day off M A N day off M A N ** day off M A N *** day off * p<0.05 ** p<0.01 *** p<0.001 Table 3. Individual traits means + SD or proportion (%) drowsy resistant significance of difference EPI extraversion n= n= EPI neuroticism * EPI L scale ** reactivity n= n= ** DAQ eveningness n= n= MEQ morningness n= n= CHQ eveningness n= n= CHQ distinctness of rhythm CTI flexibility n= n= CTI languidness *** morning type n= % n= % intermediate type 55.2 % 51.2 % evening type 21.3 % 25.6 % small amplitude n= % n= % medium amplitude 52.4 % 63.9 % large amplitude 23.6 % 2.8 % ** * p<0.05 ** p<0.01 *** p<

5 An important parameter is the declared individual need for sleep. Resistant subjects said they needed on average half an hour less sleep to feel good and fit the next day (this is a significant difference, p<0.02). Comparing the index built on the relation of the actual amount of sleep to the declared sleep requirement, one sees higher values in all cases for the resistant group. However, only in the case of the A-shift is it significant. Considering the percentage of workers complaining of sleep deficit, in all three cases (M, A and N shift) the resistant workers were in a better situation. The subjects were asked about the main types of sleep disorders, that is, three types of insomnia: difficulty in falling asleep, frequent awakening during sleep, and waking up too early (Table 2). Resistant workers did complain more often of difficulties in falling asleep both on working days and on days-off. This may suggest that they simply have longer sleep latency. But once they fall asleep, they sleep as well (or as badly) as the other shift workers, waking too early before the morning shift, probably due to fear of oversleeping. They were also asked to assess their sleep quality by two criteria: general sleep quality and feeling fatigued on awakening / not properly rested. There was a significant difference regarding sleep after the night shift resistant subjects slept better in the day than their counterparts. As to personality and chronotype traits (Table 3), the resistant workers were significantly less neurotic and less reactive. The latter scale is scored from high values to low. These two scales are in accord. No differences in morningness-eveningness scores of any type scale were found between the drowsy and resistant groups (however, the groups were quite small), and we found only one strong difference in the vigorousness-languidness scale of Simon Folkard s CTI [5]. The scale was meant to deal with the amplitude of the circadian rhythm of activation. On the basis of questionnaire scores, we were able to identify the subjects as morning, intermediate, or evening types, using the values of the lower and upper quartiles of each scale. Similarly, we described them as showing small, medium or large amplitudes (distinctness) of the diurnal activation rhythm. The distribution of M-E types was regular and similar in the two analysed groups, while the distribution of S-M-L amplitude types differed significantly; there were only a few subjects with large amplitude among the resistant workers. 4. Discussion and Conclusions It seems we can identify a set of individual variables that suggest greater resistance to sleepiness during night work: naturally short sleep requirement, longer sleep latency, low neuroticism, low reactivity, vigorousness (less distinct circadian rhythm of activation). Eveningness, a trait that used to be considered a good predictor of night work fitness, does not seem to play an important role, as both larks and owls are sleepy at night the first perhaps earlier and the second later. 334

6 The set of variables given above can be justified: short sleepers are more resistant to drowsiness because their sleep deficit (which is unavoidable in a shift system) is less than that of medium and long sleepers; long sleep latency means they do not use the chance to fall asleep even if the external conditions are momentarily favourable (dim light, white noise, comfortable seat, etc.); low neuroticism (low reactivity) means they are more resistant to fatigue, so the homeostatically regulated component of the need for sleep does not affect them as much as it does the others; the second important consequence of lower reactivity is that they are not so sensitive to disturbances of daytime sleep; the less distinct rhythm (small amplitude) means they are not so vulnerable to the time of day (or night), that is, to the circadian component of the need for sleep. Is this an answer to the question asked at the beginning? It looks like an answer, but it is rather only an indication of where further research should be directed. At least two questions should be kept in mind: (1) what are the practical possibilities of predicting general fitness for night work on the basis of descriptions of individual factors, and (2) what are the interrelations between subjective declarations (self-assessments) and objective measures, in other words between subjective alertness and performance? On the basis of our previous experience of predicting tolerance to shiftwork [6], it seems that prognoses are not accurate enough to be practically useful, and not accurate enough to recommend selection procedures as a valuable tool for improving safety. However, descriptions of individual temperamental traits may be useful in gaining an initial orientation as to one s own chances to meet the system s demands and to work out effective coping strategies. The study did not cover motivational factors that may be at least as important to resistance to fatigue and sleepiness at work. References: [1] N. Lamond, D. Dawson, Equating the Effects of Fatigue and Alcohol Intoxication on Performance. XIV Int. Symp. on Night- and Shift work, Wiesensteig, Germany, 1999, Abstracts, p. 23. [2] A. Wedderburn, Sleeping on the Job: The Use of Anecdotes for Recording Rare but Serious Events, Ergonomics 30 (1987), [3] H. Danker-Hopfe, S. Kraemer, H. Dorn, A. Schmidt, I. Ehlert, W.M. Herrmann, Time-of-Day Variations in Different Measures of Sleepiness (MSLT, Pupillography, and SSS) and Their Interrelations, Psychophysiology 38 (2001), [4] S.D. Baulk, L.A. Reyner, J.A. Horne, Driver Sleepiness Evaluation of Reaction Time Measurement as a Secondary Task, Sleep 24 (2001), [5] J. Barton, G. Costa, L. Smith, E. Spelten, P. Totterdell, S. Folkard, The Standard Shiftwork Index: A Battery of Questionnaires for Assessing Shiftwork-Related Problems, Work and Stress 9 (1995), [6] H. Oginska, J. Pokorski, A. Oginski, E. Pietsch, Predicting Individual Shiftwork Tolerance Practical Aspects. In: T. Marek, H. Oginska, J. Pokorski, G. Costa, S. Folkard (eds.), Shiftwork 2000: Implications for Science, Practice, and Business. Institute of Management, Jagiellonian University, Krakow, 2000, pp