Individual vulnerability to insomnia, excessive sleepiness and shift work disorder amongst healthcare shift workers. A systematic review

Summary

Shift workers often experience reduced sleep quality, duration and/or excessive sleepiness due to the imposed conflict between work and their circadian system. About 20–30% of shift workers experience prominent insomnia symptoms and excessive daytime sleepiness consistent with the circadian rhythm sleep disorder known as shift work disorder. Individual factors may influence this vulnerability to shift work disorder or sleep-related impairment associated with shift work. This paper was registered with Prospero and was conducted using recommended standards for systematic reviews and meta-analyses. Published literature that measured sleep-related impairment associated with shift work including reduced sleep quality and duration and increased daytime sleepiness amongst healthcare shift workers and explored characteristics associated with individual variability were reviewed. Fifty-eight studies were included. Older age, morning-type, circadian flexibility, being married or having children, increased caffeine intake, higher scores on neuroticism and lower on hardiness were related to a higher risk of sleep-related impairment in response to shift work, whereas physical activity was a protective factor. The review highlights the diverse range of measurement tools used to evaluate the impact of shift work on sleep. Use of standardised and validated tools would enable cross-study comparisons. Longitudinal studies are required to establish causal relationships between individual factors and the development of shift work disorder.

Introduction

With the increasing demand to operate as a 24/7 society, approximately a quarter of the workforce worldwide now undertake work hours outside normal daytime employment times [1], ∗[2]. Work hours that result in non-standard sleep/wake times can cause sleep disruption, shorter sleep duration and/or sleepiness due to the conflict between the imposed sleep-wake cycle and the individual's circadian system ∗[2], [3]. For the majority of shift workers this circadian misalignment is temporary (depending on the degree of the circadian phase shift experienced) with recovery within a few days after returning to a normal sleep wake schedule. For some people it substantially impairs their ability to sleep, resulting in insomnia, severe sleep debt and daytime sleepiness with a more prolonged recovery period [4], [5], ∗[6], [7], known as the circadian rhythm sleep disorder called shift work disorder (SWD) [5].

SWD effects approximately 10–30% of the shift worker population, depending on the definition [8], ∗[9], ∗[10], ∗[11]. The International Classification of Sleep Disorders: Diagnostic and Coding Manual (ICSD) defined SWD as a circadian rhythm sleep disorder in 2005 [12], [13]. The diagnostic criteria for SWD include consistent symptoms of insomnia and/or excessive sleepiness temporally associated with work schedules resulting in a reduction in total sleep time, unsatisfactory sleep or impaired alertness over the course of at least one month [14]. Measures used to diagnose SWD include excessive sleepiness, sleep latency, sleep duration, sleep quality and insomnia symptoms assessed by sleep diary, actigraphy or questionnaires [2]. However, validated measurement tools to screen for SWD were not developed until 2012; the Bergen shift work sleep questionnaire (BSWSQ) [15] and the shift work disorder questionnaire (SWDQ) [16], which were constructed to assess insomnia and sleepiness symptoms related to sleep-wake disturbances from shift work.

Shift workers are at greater risk of physical and mental health conditions such as obesity, type 2 diabetes, cardiovascular disease, high blood pressure, digestive problems, depression, and anxiety [17], [18]. Shift workers are also at higher risk for work place injuries, motor vehicle accidents, increased sick leave and absenteeism than daytime workers [19], [20], ∗[21]. Individuals identified as having SWD or sleep-related impairments associated with shift work, are at an even greater risk of these problems than other shift workers. Occupational and road incidents from excessive sleepiness cost the economy $71–93 billion per annum in the United States [5], ∗[6], ∗[21]. The impact of SWD and sleep-related impairment on an individual, their family and society results in an increased burden on the healthcare system, the workplace and the economy. SWD is considered to be an under-recognized, under-diagnosed, and under-treated sleep disorder that requires further exploration [4], [5], ∗[6], [7].

Healthcare shift workers make up the largest proportion of shift workers (15–20%) [1], ∗[2]. Factors such as an increase in population and an ageing demographic with people living longer will see a further increase in demand on healthcare, however the retention of healthcare workers is weak [22], [23]. Healthcare workers often have highly stressful work environments requiring them to deal with traumatic situations, terminally ill patients and grieving individuals, all of which can exacerbate sleep-related impairments and make recovery back to a normal circadian rhythm cycle from shift work more difficult [24], [25]. Additionally, a large percentage of healthcare workers are also female compared to other shift work industries such as truck drivers, police and miners [26]. Female workers are more likely than males to have further responsibilities outside of work, such as domestic duties with family, children and social commitments, which can further restrict the availability of time for sleep and recovery from shift work [25], [27], [28]. Circadian misalignment in shift workers may result in irregular and longer menstrual cycles compared to non-shift workers [29], [30]. Furthermore, there is accumulating evidence that circadian disruption can increase the risk of breast cancer in women, possibly due to altered light exposure and reduced melatonin secretion [30]. Hence, the healthcare sector is an important industry that requires specific investigation to determine individual characteristics that make some workers more vulnerable than others to disturbed sleep due to shift work.

Work demands, work schedules, salary, lack of control, effort-reward imbalances, job satisfaction and other organisational factors have been explored as factors that may influence an employee's ability to cope with shift work [31], [32]. These factors do not, however, explain all the individual vulnerability to sleep-related impairment associated with shift work when under similar work conditions and schedules ∗[33], [34]. SWD and sleep-related impairment associated with shift work varies between individuals, despite similar schedules and operational circumstances. This suggests that some individuals are more adversely affected by shift work, struggle to adapt to circadian phase shifting and are not be able to quickly rebound back into a normal sleep wake cycle after shift work [35], [36].

Understanding which individual factors make someone more vulnerable or resilient to sleep-related impairments associated with shift work may help to inform interventions and management of SWD/sleep-related impairment associated with shift work. A large range of biological, behavioural, social and personality factors may contribute to an individual's vulnerability to sleep-related impairment associated with shift work [25], [35], [36], [37].

Diurnal preference appears to be a predictive factor for SWD and sleep-related impairment associated with shift work, with morning-types having greater difficulty adapting to evening or night shifts compared to evening-types ∗[6], [38], [39]. Furthermore, subjective circadian rhythm stability (Flexible/Rigid) and amplitude (Languid/Vigorous) impact on how an individual's circadian system adapts to shift work [40], [41]. Flexibility (the ability to sleep and work at odd times) and languidity (amount of difficulty overcoming drowsiness and feelings of lethargy following a reduction in sleep) are also predictive of sleep-related impairment associated with shift work [38], [41], [42], [43]. Reduction in melatonin concentration and earlier timing of the circadian pacemaker occur with older age, causing vulnerability to circadian desynchronization, which may impair the ability to rebound back from shift work [44], [45], [46], [47]. Therefore, older shift workers may have greater difficulty in circadian adjustment to night work and increase in sleep-related impairment [47], [48], [49], [50]. Being female may play a role in increasing vulnerability to poor sleep during shift work [51], [52] however, this is not a consistent finding [53], [54], [55], [56]. Lifestyle factors can further account for individual variability in sleep-related impairment associated with shift work. Shift workers tend to consume higher amounts of alcohol and caffeine, are smokers and have an increased body mass index (BMI), contributing to poorer sleep in the sub-population of shift workers with greater use [57], [58], [59], [60], [61]. Finally, personality traits could explain the differences in thinking, feeling, behaviour and reaction to shift work. Hardiness, defined as general resilience and reaction to stress and illness, appears to be a protective factor against sleep-related impairment ∗[9], [35], [42], while neuroticism has a negative impact on sleep quality [43], [62] in shift workers.

This review extends a previous systematic review published in 2011 by Saksvik et al. [63]. Saksvik et al. analysed papers published up until 2009 that explored individual differences to shift work tolerance (SWT). SWT encompasses a wider variety of consequences to shift work compared to SWD. SWT is the ability to adapt to shift work without adverse consequences such as physical health (digestive problems, diabetes, obesity); mental health disorders (fatigue, depression or anxiety); or sleep-related impairment [64], [65]. This present review however focused only on sleep- and circadian-related impairment and no other comorbid medical disorders related to shift work. Due to the growing area and limited validated tools for SWD, in order to capture all previous research this review included studies that measured sleep-related impairment associated with shift work, not just SWD. This included studies that measured sleep outcomes such as reduced sleep quality, duration and increased daytime sleepiness in relation to healthcare shift workers. The following question was then investigated: In the healthcare sector, what individual characteristics are associated with sleep-related impairment (insomnia, reduced sleep quality, duration or excessive sleepiness) related to shift work?