Night work can be classified as work where the majority of the working time falls between 10:00 p.m. and 6:00 a.m. (Akerstedt, 1998). Working at night is a serious risk factor for the physical and mental health of the worker (Costa, 2003) as well as for work safety and productivity (Folkard & Tucker, 2003). Staffing and supervision are normally reduced at night (Folkard & Tucker, 2003), forcing many to work alone during night shifts. The aim of the present article is to investigate what constitutes discomfort when working alone at night.
Extensive research has already been performed on night work in general, and especially on the health effects of night work. For example, shift work, especially long-term exposure, is associated with an increased risk of cardiovascular disease (Ha & Park, 2005; Karlsson, Knutsson, Lindahl, & Alfredsson, 2003; Nishitani & Sakakibara, 2007) and a higher risk of breast cancer in women (Akerstedt, Knutsson, Narusyte, Svedberg et al., 2015). Night work in particular seems to cause a circadian rhythm sleep disorder called shift work disorder, which includes excessive sleepiness and insomnia related to the work (Waage, Pallesen, Moen, Magerøy et al., 2014). Much research has also focused on countermeasures to improve adaptation to night work (Pallesen, Bjorvatn, Mageroy, Saksvik et al., 2010). This research has mainly focused on shift work scheduling (Costa, Anelli, Castellini, Fustinoni, & Neri, 2014), bright light treatment (Tanaka, Takahashi, Tanaka, Takanao et al., 2011), melatonin treatment, naps at the right time during the work shift, and the use of stimulants (Pallesen et al., 2010). However, possible psychological countermeasures do not receive much attention in this review and other resent research. Researchers need to look at the problem of night work for the individual from different angles to promote employee well-being, reduce safety risk, and improve productivity. Although staffing is reduced at night (Folkard & Tucker, 2003), a closer look at the many individuals working alone at night has not yet been performed in today’s research. Also, because psychological variables are less examined in night work, it seems necessary to investigate the psychological feeling of discomfort related to working alone at night. Moreover, it is important to investigate which work environment variables, state variables, and trait variables contribute to feelings of discomfort when working alone at night. The role social support, feelings of work mastery, digestive troubles, sleep, fatigue, and personality traits play in discomfort stemming from working alone at night may be especially essential.
Working alone at night probably implies less social interaction compared to working in a team. Research suggests that social interaction at work may mitigate the risk of fatigue and may increase alertness during night shifts or sleep-deprivation scenarios (Caruso & Hitchcock, 2010; Pallesen et al., 2010; Pilcher, Vander Wood, & O’Connell, 2011). Social support may reduce the experience of negative stress or strain (Karasek & Theorell, 1990), especially among shift workers (Hamaideh, Mrayyan, Mudallal, Faouri, & Khasawneh, 2008). Having less contact with colleagues may also increase one’s risk of mental health problems (Gamperiene, Nygård, Sandanger, Wærsted, & Bruusgaard, 2006). These research results headline the importance of investigating solitary night work. Still, because working alone signifies no social interaction, social support in itself may not be directly relevant for individuals working solo. However, some individuals may work a schedule that includes both solitary work and teamwork. It is possible that a perception of social support gained from working in teams, or a general feeling of support from supervisors and colleagues, may also sometimes affect individuals when they work alone.
Social support is, together with work demands and control, associated with shift workers’ perceived mastery of their work (Ljoså, Tyssen, & Lau, 2013). Work mastery represents an employee’s self-evaluation of how well his or her effort at work creates desirable work outcomes. Although it has long been known that a general sense of life mastery is associated with better health (Folkman, Lazarus, Gruen, & DeLongis, 1986), few studies have investigated mastery of work directly (Ljoså et al., 2013). Thus, a lack of social support and sense of work mastery may, together with other variables, lead to a feeling of discomfort when working alone at night. An examination of what discomfort means in this context and what is related to discomfort when working at night in particular has not yet been investigated in previous research.
Some individuals tend to tolerate shift work better than others. The term shift work tolerance (SWT) implies the ability to adapt to shift work without adverse consequences (Andlauer, Reinberg, Fourre, Battle, & Duverneuil, 1979). Better SWT means that one experiences less fatigue, fewer sleep difficulties, fewer digestive troubles (Andlauer et al., 1979), and more aggression, sensitivity, and use of sleep medications (Reinberg & Ashkenazi, 2008). General health, well-being, and social variables have also been headlined as important for tolerating shift work in recent research (Saksvik-Lehouillier, Pallesen, Bjorvatn, Mageroy, & Folkard, 2015). Why some individuals experience difficulty when working alone at night while others do not may be attributable to individual differences in SWT or personality, but this link has not yet been examined.
The extent to which individual personality characteristics are related to SWT is debated (Härmä, 1993; Nachreiner, 1998; Saksvik, Bjorvatn, Hetland, Sandal, & Pallesen, 2011). Employees who have lower scores on neuroticism (McLaughlin, Bowman, Bradley, & Mistlberger, 2008; Parkes, 2002) and higher scores on extraversion tend to have better SWT (Korompeli, Sourtzi, Tzavara, & Velonakis, 2009; McLaughlin et al., 2008). Neuroticism is the experience of higher-than-average levels of anxiety, hostility, depression, self-consciousness, impulsivity, and vulnerability (Costa, McCrae., & Kay, 1995). In contrast, extraversion involves a tendency to have a positive orientation toward others, excitement-seeking behaviors, positive emotions, and a high need for stimulation and activity (Costa et al., 1995). The personality dimension of morningness/eveningness is also investigated in relation to SWT. Morningness/eveningness is a dimension often described as a reflection of an individual’s diurnal preference (Roberts & Kyllonen, 1999), determined by when in the day he or she is most alert (Natale & Cicogna, 2002). One study found that low scores on morningness may be preferable when working night shifts (Folkard & Hunt, 2000). However, previous research has not examined whether personality traits contribute to the perception of discomfort when working alone at night.
In the present study, it was hypothesized that the following factors were related to experiencing discomfort when working alone at night: high scores on insomnia and fatigue; more digestive troubles and greater use of sleep medications; high scores on neuroticism; low scores on extraversion and morningness; and low social support and work mastery.
Methods
Sample
A total of 1,106 shift workers employed by a municipality in Norway, including all workers who worked night shifts, were invited by email to participate in an online survey. Of these invitations, 65 were sent to incorrect email addresses. A total of 353 individuals participated (response rate: 33.9%). In total, 283 individuals gave information about working alone at night and were included in the main analyses. Among these, 171 respondents worked alone sometimes or always while 112 never worked alone. These 112 individuals who indicated that they never worked alone were not a part of the final sample and were excluded from the main analyses. The final sample of 171 was primarily female (n=121, 70.8%) and aged 37 years on average (range: 17 to 66 years old). They had been employed for an average of 15.6 years (range: <1 year to 42 years), with an average of 6.36 years of shift work experience (range: <1 year to 32 years). The participants were all employees’ in health and caretaking positions. All together 122 (71.3%) were health personnel, 27 (15.8%) were social workers or held other positions working with children and youth care, while 21 (12.3%) indicated that their job was «other» than the two categories mentioned. It is likely that the respondents indicating that their type of occupation was «other» were employed in either the building and planning department or the nature and development department of the municipality. The broad categories for type of occupation were chosen for anonymity purposes.
Procedure
An online questionnaire was distributed by email during January 2013 to all potential participants. They were given three weeks to complete it. Two email reminders were sent to individuals who did not participate after the first invitation. The email stated that participation was voluntary and anonymous. The study protocol was approved by the Norwegian Data Protection Official for Research (NSD).
Instruments
The survey included questions about employees’ demographics (age and sex), work schedule arrangement, work and night work experience, discomfort working alone at night, SWT (including insomnia, fatigue, digestive troubles, and use of sleep medications), personality, social support and work mastery.
The respondents were asked if they ever worked alone at night and, if yes, how often. Working alone at night was further specified by the phrase «this means that you are the only employee present at work» written in parentheses following the question. To assess if the respondents had experienced discomfort when working alone at night, the respondents were asked if they had felt discomfort when working alone at night, with four response options being given: «always,» «sometimes,» «never,» or «I have never worked alone at night.» A categorical dependent variable, discomfort working alone at night was coded as 0 (for no discomfort working at night) vs. 1 (for any discomfort working alone at night) for analyses. The collapsing of the categories «sometimes» and «always» was performed so that the difference between discomfort and no discomfort would be clearer and because the group who always experienced discomfort working alone was relatively small (n=16).
The Bergen Insomnia Scale (BIS) was used to assess insomnia. BIS consists of six questions measuring the DSM-IV criteria for insomnia including questions asking «how many times per week during the last month have you experienced» any of six different insomnia symptoms (Pallesen, Bjorvatn, Nordhus, Sivertsen et al., 2008). The validity of the BIS has been previously demonstrated (Pallesen et al., 2008). The scale’s Cronbach’s alpha value in the present study was 0.83. The Fatigue Questionnaire (FQ) (Chalder, Berelowitz, Pawlikowska, Watts et al., 1993) measures physical and mental fatigue with 13 questions representing potential fatigue-related problems experienced during the month prior to the survey. The FQ has been validated in a general adult population (Chalder et al., 1993) and in Norway particularly (Loge, Ekeberg, & Kaasa, 1998). Cronbach’s alpha for physical fatigue was 0.91 and for mental fatigue 0.85. The respondents were asked about three different types of sleep medication use, including prescription medications, non-prescription sleep medications, and/or melatonin. Sleep medication question responses were collapsed into one variable, with scores ranging from 0 (i.e. «no» to all three questions) to 3 («yes» to all three questions), as described in a previous shift work study (Saksvik-Lehouillier, Bjorvatn, Hetland, Sandal, et al., 2013). Four of the possible eight digestive trouble questions in the Standard Shift Work Index (SSI) (Barton, Spelten, Totterdell, Smith et al., 1995) were adopted, creating two questions for the present study. These questions included: (1) «How often is your appetite disturbed?» and (2) «How often do you suffer from stomach ache, nausea, or digestive troubles?» The respondents provided their responses on a four-point scale ranging from «almost never» to «almost always.» Cronbach’s Alpha for these two questions together was 0.82.
The ‘Mini-IPIP’ is a 20-item abbreviated version of the full 50-item ‘International Personality Item Pool (IPIP)’ five personality trait instrument (Goldberg, 1999). It includes four statements for each personality trait, and respondents are asked to give a rating on a five-point scale (ranging from «very inaccurate» to «very accurate»). The Mini-IPIP has been found to be useful, effective, and valid (Donnellan, Oswald, Baird, & Lucas, 2006). The Diurnal Scale (DS) has seven questions about what time people prefer to go to sleep and wake up and what times they actually do (Torsvall & Akerstedt, 1980). The questions in this scale include four possible response options, each with different ratings. The validity and consistency of the DS has been shown previously among shift workers (Torsvall & Akerstedt, 1980). Cronbach’s alpha for the two personality traits were 0.69 for neuroticism and 0.79 for extraversion.
Social support was measured with a Norwegian version of the ‘Swedish Demand-Control-Support Questionnaire’ (DCSQ), which has satisfactory psychometric properties in a general Norwegian population (Sanne, Torp, Mykletun, & Dahl, 2005). The social support dimension of the instrument has six items, with response ratings ranging from 1 (fully agree) to 4 (fully disagree). The items are, for example, «There is a good collegiality at work» or «I get along well with my supervisors.» Cronbach’s alpha for this scale was 0.82. Lastly, work mastery was measured using four questions from the ‘Questionnaire for Psychological and Social’ factors at work (QPS-Nordic) (Dallner, Elo, Gamberale, Hottinen et al., 2000). These questions assessed employees’ perceptions of their own quality of work, work load, problem-solving skills, and relationships with colleagues. Respondents were asked how often they experienced specific scenarios on a scale from one (meaning «never or seldom») to five («often or always»). The reliability and validity of the QPS-Nordic scale has been established (Dallner et al., 2000; Wännström, Petterson, Åsberg, Nygren, & Gustavsson, 2009). The Cronbach’s alpha for work mastery in the present study was 0.76.
Statistical analyses
Mean differences on their scores for all factors between the groups of respondents who worked at alone at night and those who did not work alone at night were examined using independent samples t-tests and a Chi-square test for the categorical variable «sex.» Factors that may influence discomfort when working alone at night were identified by comparing mean scores on all factors between those who experienced discomfort when working at night and those who did not experience discomfort when working at night. Independent sample t-tests and Chi-square tests aided in this identification. Lastly, logistic regression analysis was used to examine the relationship between each potentially influential factor to discomfort when working alone at night among night workers. Those independent factors that were statistically significantly different in mean scores were then analyzed all together in a multiple logistic regression model. The variables were entered both separately (crude analysis) and simultaneously (adjusted). All analyses were performed using the statistical software IBM SPSS Statistics.
Results
Among 283 respondents who gave valid answers to the questions about working alone at night, 95 reported experiencing discomfort when working at night, 76 reported no problems, and 112 stated that they never worked alone at night. The 112 who never worked alone at night were excluded from the rest of the analyses. The means and standard deviations of all variables can be found in Table 1.
The distributions of the scores on the continuous variables were also inspected through skewness and kurtosis. Some of the variables were not normally distributed. Use of sleep medication in particular was skewed toward low scores and therefore excluded from further analyses. Insomnia and the fatigue measures were somewhat skewed toward low scores (Kim, 2013), indicating that a high proportion of the sample experienced a low degree of insomnia and fatigue problems. However, as determination of normality is discussed, and especially the use of data transformation is debated (Tabachnick & Fidell, 2001), no transformations of these variables were performed prior to the analyses. There were no statistically significant differences in the mean scores of any of the factors between the participants who worked alone at night vs. those who did not work alone at night (data not shown). Compared to night workers who did not experience discomfort when working alone at night, the night workers who experienced discomfort when working alone at night had significantly higher insomnia, physical fatigue, digestion problem, and neuroticism mean scores and significantly worse mean work mastery scores according to independent sample t-tests (Table 1). Women tended to experience discomfort when working alone more often than men. Age, hours of work per week, mental fatigue, use of sleep medications, extraversion, and morningness were not significantly associated with discomfort when working alone at night.
Logistic regression analyses were performed to measure the associations between the significant variables and the experience of discomfort when working alone at night (Table 2). The assumptions of logistic regression were inspected prior to conducting the analyses, and they were met. Between 11% and 15% of the variability in discomfort when working alone at night was explained by the variables entered in the model. The model correctly classified 70% of the individuals experiencing discomfort when working alone at night. In the simple logistic regression model, all factors tested, including sex, insomnia, physical fatigue, digestive troubles, neuroticism, and work mastery, were statistically significantly associated with discomfort when working alone at night. Only work mastery remained significant in the adjusted model, which adjusted for potential combinatory effects of the factors.
Discussion
The aim of the present study was to identify the nature of discomfort when working alone at night. In the present study, the participants experiencing discomfort when working at night had significantly higher scores on insomnia, physical fatigue, digestion problems, and neuroticism and lower scores on work mastery compared to the participants experiencing no discomfort when working alone at night. However, work mastery was the most important variable.
In all, the present study indicates that work mastery is more important for feeling discomfort related to working alone at night than sleep, fatigue, digestive troubles, and personality. High work mastery indicates that an individual perceives his or her own effort at work as able to create desirable work outcomes (Ljoså et al., 2013). In other words, work mastery is the perception of success of one’s own efforts at work. A previous study found that supervision, safety, and productivity tend to be worse at night (Folkard & Tucker, 2003). Lack of productivity and less access to helpful supervision as well as safety issues may affect the night worker’s perception of success of his or her efforts at work. This tendency may explain the relationship between discomfort with night work and less work mastery. Still, these explanations need further study. The results are nevertheless what one would expect according to previous studies that reported associations between work mastery and social support control of work pacing and decisional demands (Ljoså et al., 2013).
One reason for the finding in the present study that work mastery had an association with feeling discomfort related to night work could have been the way these variables were measured. Discomfort with working at night was measured with the item: «If you work alone on the night shift, do you feel discomfort related to working alone?» Work mastery was measured with questions from the QPS-Nordic questionnaire. These questions ask if the respondent feels competent with his or her quality of work, amount of work, problem solving, and ability to maintain a good relationship with coworkers. Hence, the work mastery questions may be an elaboration of the overall discomfort with working alone at night question. However, results from additional analyses that correlate discomfort when working alone at night with work mastery showed no multi-collinearity between the two variables. Also, the distribution of the scores on work mastery was relatively normally distributed. The answers were slightly skewed toward higher scores, but not so much that the data would not be suitable for statistical analyses.
The results of the present study suggest that worse SWT and less neuroticism are associated with discomfort when working alone at night. Insomnia, physical fatigue, and digestive troubles are core aspects of SWT (Andlauer et al., 1979; Reinberg & Ashkenazi, 2008). In previous research, mental health, fatigue, and alertness were especially found to be affected when working together vs. alone (Caruso & Hitchcock, 2010; Gamperiene et al., 2006). Still, the results of the present study do not show that higher scores on insomnia, physical fatigue, and digestive troubles cause a higher sensitivity to working alone at night. These shift work intolerance symptoms may just be associated with a higher sensitivity toward challenges in life in general or a higher sensitivity toward working at night. Sleep problems (Akerstedt, 2003), disturbed eating habits (Lowden, Moreno, Holmbäck, Lennernäs, & Tucker, 2010), fatigue, irritation, and sensitivity (Pati, Chandrawanshi, & Reinberg, 2001) are known effects of shift work. Nevertheless, the present study illustrates that these problems may be even more evident for individuals experiencing discomfort when working alone at night than for night workers in general.
In the present study, experiencing discomfort when working alone at night was not related to mental fatigue, taking sleep medications, extraversion, or morningness. In previous studies, less extraversion was related to different signs of poor SWT (Korompeli et al., 2009; McLaughlin et al., 2008), and individuals with high morningness scores may have more problems working at night (Folkard & Hunt, 2000). However, these results are concerned with relations to problems associated with shift or night work in general, not to working alone at night in particular. Neuroticism is likely more influential than extraversion and morningness in one’s experience of discomfort when working alone at night.
No statistically significant differences between those working alone at night and those who never worked alone at night were found in the present study. The finding that the two groups of respondents also had different mean neuroticism scores supports the idea that those experiencing discomfort when working alone at night may be more sensitive toward challenges in general. High sensitivity is one of the facets of neuroticism (Costa et al., 1995). Hence, one could assume that more sensitive individuals will experience working at night as a problem and that these individuals are also more inclined to report insomnia, fatigue, and digestive problems. Consequently, although previous research has identified alertness (Pilcher et al., 2011), mental health problems (Gamperiene et al., 2006), and general fatigue (Caruso & Hitchcock, 2010) to be important factors that are influenced by teamwork or solitary work, the present study provides a different perspective by identifying insomnia and other physical health-related problems like physical fatigue and digestive troubles to be especially common among individuals experiencing discomfort when working alone at night.
Strengths and weaknesses
The response rate and sample size were somewhat low in the present study. The response rate was 33.9%, which is close to the standard acceptable response rate of >36% needed for organizational psychology research (Baruch, 1999; Baruch & Holtom, 2008). The study sample consisted of employees in mainly service-oriented, social positions including health personnel and caretaking. The respondents were primarily women. Generalizability of the results to other groups of shift workers, especially groups from very different industries, is limited.
This study is a step toward identifying factors that determine discomfort with working alone at night.
Conclusion
A lack of work mastery in particular was important in experiencing discomfort with working alone at night among the present study’s municipality night workers. Discomfort when working alone at night was associated with higher scores on insomnia, physical fatigue, digestive troubles, and neuroticism. A higher prevalence of these symptoms/behaviors may reflect a general sensitivity to the negative effects of shift work in general. However, it is likely that working alone contributes to the problem and this hypothesis needs further study, especially in relation to work mastery. Prospective longitudinal studies that explore other potentially influential factors would help to better understand why some feel more discomfort than others when working alone at night. The present study is a step toward identifying factors that determine discomfort with working alone at night.
References
Akerstedt, T. (1998). Shift work and disturbed sleep/wakefulness. Sleep Medicine Reviews, 2, 11. doi: 10.1016/S1087-0792(98)90004-1.
Akerstedt, T. (2003). Shift work and disturbed sleep/wakefulness. Occupational Medicine–Oxford, 53(2), 89–94. doi: 10.1093/occmed/kqg046.
Akerstedt, T., Knutsson, A., Narusyte, J., Svedberg, P., Kecklund, G., & Alexanderson, K. (2015). Night work and breast cancer in women: a Swedish cohort study. BMJ Open, 5(4), e008127. doi: 10.1136/bmjopen-2015-008127.
Andlauer, P., Reinberg, A., Fourre, L., Battle, W., & Duverneuil, G. (1979). Amplitude of the oral-temperature circadian-rhythm and the tolerance to shift work. Journal De Physiolologie, 75(5), 507–512. Not available online.
Barton, J., Spelten, E., Totterdell, P., Smith, L., Folkard, S., & Costa, G. (1995). The Standard Shiftwork Index: a battery of questionnaires for assessing shiftwork-related problems. Work & Stress, 9(1), 4–30. doi: 10.1080/02678379508251582.
Baruch, Y. (1999). Response rate in academic studies – A comparative analysis. Human Relations, 52, 421–438. doi: 10.1177/001872679905200401.
Baruch, Y., & Holtom, B. C. (2008). Survey response rate levels and trends in organizational research. Human Relations, 61, 1139–1160. doi: 10.1177/0018726708094863.
Caruso, C. C., & Hitchcock, E. M. (2010). Strategies for nurses to prevent sleep-related injuries and errors. Rehabilitation Nursing, 35(5), 192–197. doi: 10.1002/j.2048-7940.2010.tb00047.x.
Chalder, T., Berelowitz, G., Pawlikowska, T., Watts, L., Wessely, S., Wright, D., & Wallace, E. P. (1993). Development of a fatigue scale. Journal of Psychosomatic Research, 37(2), 147–153. doi: 10.1016/0022-3999(93)90081-P.
Costa, G. (2003). Factors influencing health of workers and tolerance to shift work. Theoretical Issies in Ergonomics Science, 4, 25. doi: 10.1080/14639220210158880.
Costa, G., Anelli, M. M., Castellini, G., Fustinoni, S., & Neri, L. (2014). Stress and sleep in nurses employed in «3 × 8» and «2 × 12» fast rotating shift schedules. Chronobiology International, 31(10), 1169–1178. doi: 10.3109/07420528.2014.957309.
Costa, J. P. J., McCrae., R. R., & Kay, G. G. (1995). Persons, places, and personality: Career assessment using the revised NEO personality inventory. Journal of Career Assessment, 3, 123–139. doi: 10.1177/106907279500300202.
Dallner, M., Elo, A. L., Gamberale, F., Hottinen, V., Knardahl, S., Lindström, K., & al., e. (2000). Validation of the general Nordic questionnaire (QPSNordic) for psychological and social factors at work (No. Nord 2000:12). Copenhagen: Nordic Council of Ministers.
Donnellan, M. B., Oswald, F. L., Baird, B. M., & Lucas, R. E. (2006). The Mini-IPIP scales: Tiny-yet-effective measures of the big five factors of personality. Psychological Assessment, 18(2), 192–203. doi: 10.1037/1040-3590.18.2.192.
Folkard, S., & Hunt, L. J. (2000). Morningness-eveningness and long-term shift work tolerance. In S. Hornberger, P. Knauth, G. Costa, & S. Folkard (Eds.), Shift work in the 21st century, Arbeitswissinshaft in der betrieblichen Praxis (Vol. 17, pp. 311–316). Frankfurt: Peter Lang.
Folkard, S., & Tucker, P. (2003). Shift work, safety, and productivity. Occupational Medicine, 53, 95–101. doi: 10.1093/occmed/kqg047.
Folkman, S., Lazarus, R. S., Gruen, R. J., & DeLongis, A. (1986). Appraisal, coping, health status, and psychological symptoms. Journal of Personality and Social Psychology, 50(3), 571–579. doi: 10.1037/0022-3514.50.3.571.
Gamperiene, M., Nygård, J., Sandanger, I., Wærsted, M., & Bruusgaard, D. (2006). The impact of psychosocial and organizational working conditions on the mental health of female cleaning personnel in Norway. Journal of Occupational Medicine and Toxicology, 1(1), 1–10. doi: 10.1186/1745-6673-1-24.
Goldberg, L. R. (1999). A broad-bandwidth, public-domain, personality inventory measuring the lower-level facets of several five-factor models. In I. Mervielde, F. Deary, F. De Fruyt, & F. Ostendorf (Eds.), Personality Psychology in Europe (Vol. 7, pp. 7–28). Tilburg, Netherlands: Tilburg University Press.
Ha, M., & Park, J. (2005). Shiftwork and metabolic risk factors of cardiovascular disease. Journal of Occupational Health, 47(2), 89–95. doi: 10.1539/joh.47.89.
Hamaideh, S. H., Mrayyan, M. T., Mudallal, R., Faouri, I. G., & Khasawneh, N. A. (2008). Jordanian nurses’ job stressors and social support. International Nursing Review, 55(1), 40–47. doi: 10.1111/j.1466-7657.2007.00605.x.
Härmä, M. (1993). Individual-differences in tolerance to shift work – A review. Ergonomics, 36(1–3), 101–109. doi: 10.1080/00140139308967860.
Karasek, R., & Theorell, T. (1990). Healthy work, stress, productivity, and the reconstruction of working life. New York, NY: Basic Books.
Karlsson, B. H., Knutsson, A. K., Lindahl, B. O., & Alfredsson, L. S. (2003). Metabolic disturbances in male workers with rotating three-shift work. Results of the WOLF study. International Archives of Occupational and Environmental Health, 76(6), 424–430. doi: 10.1007/s00420-003-0440-y.
Kim, H. Y. (2013). Statistical notes for clinical researchers: assessing normal distribution (2) using skewness and kurtosis. Restor Dent Endod, 38(1), 52–54. doi: 10.5395/rde.2013.38.1.52.
Korompeli, A., Sourtzi, P., Tzavara, C., & Velonakis, E. (2009). Rotating shift-related changes in hormone levels in intensive care unit nurses. Journal of Advanced Nursing, 65(6), 1274–1282. doi: 10.1111/j.1365-2648.2009.04987.x.
Ljoså, C. H., Tyssen, R., & Lau, B. (2013). Percieved mastery of work among shift workers in the Norwegian offshore petroleum industry. Industrial Health, 51, 145–153. doi: 10.2486/indhealth.2012-0086.
Loge, J. H., Ekeberg, O., & Kaasa, S. (1998). Fatigue in the general Norwegian population: Normative data and associations. Journal of Psychosomatic Research, 45(1), 53–65. doi: 10.1016/S0022-3999(97)00291-2.
Lowden, A., Moreno, C., Holmbäck, U., Lennernäs, M., & Tucker, P. (2010). Eating and shift work – effects on habits, metabolism, and performance. Scandinavian Journal of Work, Environment and Health, 36, 150–162. doi: 10.5271/sjweh.2898.
McLaughlin, C., Bowman, M. L., Bradley, C. L., & Mistlberger, R. E. (2008). A prospective study of seasonal variation in shift-work tolerance. Chronobiology International, 25(2–3), 455–470. doi: 10.1080/07420520802118269.
Nachreiner, F. (1998). Individual and social determinants of shiftwork tolerance. Scandinavian Journal of Work and Environmental Health, 24(Suppl. 3), 35–42. Retrieved from www.sjweh.fi on June 17, 2015.
Natale, V., & Cicogna, P. (2002). Morningness-eveningness dimension: is it really a continuum? Personality and Individual Differences, 32(5), 809–816. doi: 10.1016/S0191-8869(01)00085-X.
Nishitani, N., & Sakakibara, H. (2007). Subjective poor sleep and white blood cell count in male Japanese workers. Industrial Health, 45(2), 296–300. doi: 10.2486/indhealth.45.296.
Pallesen, S., Bjorvatn, B., Mageroy, N., Saksvik, I. B., Waage, S., & Moen, B. E. (2010). Measures to counteract the negative effects of night work. Scandinavian Journal of Work Environment & Health, 36(2), 109–120. doi: 10.5271/sjweh.2886.
Pallesen, S., Bjorvatn, B., Nordhus, I. H., Sivertsen, B., Hjornevik, M., & Morin, C. M. (2008). A new scale for measuring insomnia: The Bergen insomnia scale. Perceptual and Motor Skills, 107(3), 691–706. doi: 10.2466/pms.107.3.691-706.
Parkes, K. R. (2002). Age, smoking, and negative affectivity as predictors of sleep among shiftworkers in two environments. Journal of occupational health psychology, 7, 17. doi: 10.1037/1076-8998.7.2.156.
Pati, A. K., Chandrawanshi, A., & Reinberg, A. (2001). Shift work: consequences and management. Current Science, 81(1), 32–52. Retrieved from http://www.iisc.ernet.in/currsci/jul102001/32.pdf on July 17, 2015.
Pilcher, J. J., Vander Wood, M. A., & O’Connell, K. L. (2011). The effects of extended work under sleep deprivation conditions on team-based performance. Ergonomics, 54(7), 587–596. doi: 10.1080/00140139.2011.592599.
Reinberg, A., & Ashkenazi, I. (2008). Internal desynchronization of circadian rhythms and tolerance to shift work. Chronobiology International, 25(4), 625–643. doi: 10.1080/07420520802256101.
Roberts, R. D., & Kyllonen, P. C. (1999). Morningness–Eveningness and intelligence: early to bed, early to rise will likely make you anything but wise! Personality and Individual Differences, 27, 1123–1133. doi: 10.1016/S0191-8869(99)00054-9.
Saksvik-Lehouillier, I., Bjorvatn, B., Hetland, H., Sandal, G. M., Moen, B. E., Magerøy, N., Åkerstedt, T. & Pallesen, S. (2013). Individual, situational and lifestyle factors related to shift work tolerance among nurses who are new to and experienced in night work. Journal of Advanced Nursing, 69(5), 1136–1146. doi: 10.1111/j.1365-2648.2012.06105.x.
Saksvik-Lehouillier, I., Pallesen, S., Bjorvatn, B., Mageroy, N., & Folkard, S. (2015). Towards a More Comprehensive Definition of Shift Work Tolerance. Industrial Health, 53(1), 69–77. doi: 10.2486/indhealth.2014-0112.
Saksvik, I. B., Bjorvatn, B., Hetland, H., Sandal, G. M., & Pallesen, S. (2011). Individual differences in shift work tolerance. A systematical review. Sleep Medicine Reviews, 15, 221–235. doi: 10.1016/j.smrv.2010.07.002.
Sanne, B., Torp, S., Mykletun, A., & Dahl, A. A. (2005). The Swedish Demand-Control-Support Questionnaire (DCSQ): Factor structure, item analyses, and internal consistency in a large population. Scandinavian Journal of Public Health, 33(3), 166–174. doi: 10.1080/14034940410019217.
Tabachnick, B. G., & Fidell, L. S. (2001). Using mulitvariate statistics. Boston, MA, USA: Allyn & Bacon.
Tanaka, K., Takahashi, M., Tanaka, M., Takanao, T., Nishinoue, N., Kaku, A., . . . Miyaoka, H. (2011). Brief Morning Exposure to Bright Light Improves Subjective Symptoms and Performance in Nurses with Rapidly Rotating Shifts. Journal of Occupational Health, 53(4), 258–266. doi: 10.1539/joh.L10118.
Torsvall, L., & Akerstedt, T. (1980). A diurnal type scale – construction, consistency, and validation in shift work. Scandinavian Journal of Work Environment & Health, 6(4), 283–290. doi: 10.5271/sjweh.2608.
Waage, S., Pallesen, S., Moen, B. E., Magerøy, N., Flo, E., Di Milia, L., & Bjorvatn, B. (2014). Predictors of shift work disorder among nurses: a longitudinal study. Sleep Medicine, 15(12), 1449–1455. doi: 10.1016/j.sleep.2014.07.014.
Wännström, I., Petterson, U., Åsberg, M., Nygren, Å., & Gustavsson, P. (2009). Psychometric properties of scales in the General Nordic Questionnaire for Psychological and Social Factors at Work (QPSNordic): Confirmatory factor analysis and prediction of certified long-term sickness absence. Scandinavian Journal of Psychology, 50, 231–244. doi: 10.1111/j.1467-9450.2008.00697.x.
