In this study, we investigated whether time pressure and sleep problems due to thoughts about work were associated with future SA among women and men employed in different sectors, also when adjusting for confounders including familial factors. Along with expectations, sleep problems due to thoughts about work once a week or more often were associated with future SA in the fully adjusted model. However, contrary to expectations, genetic or environmental factors shared by family members did not seem to influence the associations. Other study findings clearly demonstrate that both SA (Gjerde et al. 2013; Svedberg et al. 2012) and sleep (Gasperi et al. 2017; Goel 2017; Åkerstedt et al. 2017) are heritable to some extent. But, the survey question used in the present study differs somewhat from questions used in studies reporting genetic influences. Specifically, our focus on work-related sleep problems, rather than sleep length or sleep quality, may explain the lack of familial confounding. However, the association between sleep problems and SA follows previous research using other measures of sleep disturbances (Lallukka et al. 2013, 2014; Madsen et al. 2016; Westerlund et al. 2008). Modern working life may have blurred the distinction between work and leisure time, which, in turn may influence recovery, including sleep. Furthermore, high work demands may contribute to a sustained activation in the evening and night time, which may have negative health impacts (McEwen and Stellar 1993) and subsequently be associated with SA, particularly without adequate recovery and sleep. In the current study, we adjusted for work-related factors including demands, which somewhat reduced the estimates, but these remained significant. A recent study also found high demands to exacerbate the association of sleep problems with incident SA (Madsen et al. 2016). Our findings align with previous research showing that difficulties to stop thinking about work in the evening, modify the effect of work demands, but still remain the strongest predictor of disturbed sleep (Åkerstedt et al. 2002). Moreover, our findings show that worries about work affecting sleep was more important than time pressure for sickness absence. This has prompted researchers to argue that the ability to recover from stress is an important predictor of health-related outcomes: specifically, inadequate recovery may, due to elevated work stress with worries and thoughts of work contributing a sustained activation and poor sleep, be a pathway for developing ill health. Over time, this may, as our findings suggest, lead to SA for both women and men. Thus, for sickness absence, addressing the importance of recovery and to balance adequately work and leisure time seem more important than reducing time pressure for employees and organizations. Similarly, our findings seem to be independent of occupational sector: even though there were some differences in the estimates, with the highest estimate for state employees, only one interaction effect was significant. However, further research including larger samples, to yield adequate statistical power, is needed to confirm these findings and to explore potential differences when it comes to occupations and positions.
While time pressure causing individuals to miss lunch, work late, or bring home work in some studies have shown a protective effect of SA, we found no statistically significant association between time pressure and SA in the present study, apart from in the crude model. Such differences in study findings may be related to the different measures and questions that have been used, which make comparisons between studies somewhat difficult. Moreover, results of protective effects are more pronounced among the highly educated, among men, and among those with higher positions (Krantz and Lundberg 2006). Similar to sleep problems due to thoughts about work, further research taking into account occupation and position is needed.
This study has several strengths including a large and genetically informed population-based sample, high-quality SA data with complete coverage from a national register and a prospective cohort design. Also, extensive survey data including previously used measures of sleep problems due to thoughts about work, time pressure and relevant confounders were available. A unique strength includes the possibility to control for familial confounding using the discordant twin pairs, i.e. to determine whether an association is likely to reflect a causal relationship (Kujala et al. 2002). However, study limitations should be acknowledged. First, questionnaire data always include some missing data. Yet, in the final sample, those with missing data on sleep problems due to thoughts about work constituted only 0.6% and on time pressure the corresponding percentage was 7.4%. Second, without survey follow-ups, all exposures and covariates were only assessed at a single time-point. Consequently, it is unclear whether reports of time pressure, sleep problems or confounding factors change over time, and if such changes influence the risk of SA. Further, only twins, aged 19–47, born in Sweden were included which reduces generalizability to other groups.
In conclusion, the results show that sleep problems due to thoughts about work is a risk factor for future SA. This aligns with previous research showing that sleep length and disturbances, regardless of the underlying reason, is associated with SA. But, experience of time pressure seems to have no effect on future SA. With familial factors not seeming important for the associations studied, a direct relationship between sleep problems and SA is suggested. Importantly, the inability to stop worrying about work during leisure time may link sleep to SA.