Overnight heart rate variability and next day cortisol response during simulated on-call conditions
Introduction
On-call or stand-by work is an occupational arrangement where an employee must be available to be contacted to start, or resume work, at short notice (Australian Bureau of Statistics, 2016; Bamberg et al., 2012). Over the past decade, irregular work schedules have become increasingly prevalent worldwide (Golden, 2015; Parent-Thirion et al., 2012). Significant percentages of the Australian (25%;) (Australian Bureau of Statistics, 2012), European (10–20%) (Burri et al., 2018; Parent-Thirion et al., 2012), and United States (2–17%) (Katz and Krueger, 2016; Labor, 2005) workforces are involved in on-call work arrangements. On-call work typically benefits organisations by allowing flexibility (i.e., the ability to respond to changes in demand and other contingencies) without the financial cost of full shift coverage on-site (McCrate, 2018; Nicol and Botterill, 2004). However, on-call work is not always beneficial to the worker, as personnel report impaired sleep (Ferguson et al., 2016; Paterson et al., 2016; Vincent et al., 2018a), increased work-family conflict (Jay et al., 2018; Lindfors et al., 2006), compromised social life (Imbernon et al., 1993), and increased stress levels (Dettmers et al., 2016). Heightened physiological stress is known to adversely impact worker health (Chandola et al., 2006), as well as increase the risk of chronic disease (e.g., cardiovascular disease, Type 2 diabetes) (Chrousos, 2009; Rosmond and Björntorp, 2000). Hence, understanding the physiological stress response to on-call work is critical to support worker health and well-being, job satisfaction, and turnover rates (Heponiemi et al., 2008).
The stress associated with on-call work has received recent attention (Hall et al., 2017a, b; Ziebertz et al., 2015), and is thought to result from factors including the inherent unpredictability of calls, rumination about the actual work to be performed when called, and the inability of workers to ‘switch off’ (Bamberg et al., 2012; Paterson et al., 2016). Previous research suggests there is a bi-directional relationship between sleep and physiological stress (Steiger, 2002). This is important to consider given that on-call work is associated with impairments to sleep (Ferguson et al., 2016), even when no calls are received (Torsvall and Akerstedt, 1988; van de Ven et al., 2015). Research highlighting on-call work as an occupational stressor is largely based on self-report measures of stress (Ziebertz et al., 2015). For example, doctors have reported on-call work as a major source of stress and job dissatisfaction (Dowell et al., 2000; Lindfors et al., 2006). Research also shows on-call firefighters report being ‘worried their pager was broken’ after multiple nights of few, or no calls, indicating that stress responses on-call may be present regardless of whether the individual is required to report to work (Paterson et al., 2016). However, self-report measures of stress are not always consistent with physiological measures (Harbeck et al., 2015; Simpson et al., 2016). For example, in response to a 24-h on-call shift, physicians self-reported increased stress without any corresponding increases in physiological measures of stress such as heart rate variability (HRV) or cortisol (Harbeck et al., 2015).
Studies investigating the impact of on-call work on physiological measures of stress are limited and the findings are equivocal. HRV, or the beat-to-beat change in heart rate, is a non-invasive indicator of autonomic nervous system activity (Berntson et al., 1997). Several investigations have noted changes in waking HRV measures in response to real-world on-call work, with some (Amirian et al., 2014; Malmberg et al., 2011; Rauchenzauner et al., 2009; Tobaldini et al., 2013a) but not all (Kikuchi et al., 2018; Thurman et al., 2017) indicating increased physiological stress during on-call work itself. Furthermore, another study reported that when anticipating a stressful task (albeit not on-call), changes in HRV during sleep were observed (Hall et al., 2004). However, no research to date has examined the impact of anticipatory stress preceding a period of on-call work on measures of HRV during sleep. During normal sleep, HRV is influenced by the stage of sleep, for example, rapid eye movement sleep (REM) is characterised by sympathetic predominance and vagal withdrawal, while the opposite is observed during non-rapid eye movement (NREM) sleep (Tobaldini et al., 2013b). Therefore, research is needed to examine the impact that on-call conditions may have on HRV during sleep, while also considering sleep stage.
Cortisol is the most widely accepted biomarker of hypothalamo-pituitary adrenal axis activation (Kirschbaum and Hellhammer, 1989). Some on-call studies have reported no difference in workers’ evening salivary cortisol concentrations (Bamberg et al., 2012), diurnal cortisol levels (Hall et al., 2019), 24-h urinary cortisol (Ernst et al., 2014), and 24-h salivary cortisol (Malmberg et al., 2007). In contrast, other research has demonstrated on-call work is associated with alterations in the functioning of the physiological stress systems, showing a steeper increase in the area under the curve with respect to increase (AUCI) of salivary cortisol in response to waking (Dettmers et al., 2016), increased 24-h urinary excretion of adrenaline (Samel et al., 2004), and noradrenaline (Ernst et al., 2014; Samel et al., 2004). Blunted cortisol awakening response (CAR) peak, and post-awakening cortisol AUCG response (Hall et al., 2019) have also been noted under on-call conditions. Contradictory findings from previous studies are likely the result of differences in research protocols, for example, the on-call conditions themselves (e.g., likelihood of being called, task performed upon waking), use of different self-report and/or physiological stress measures, and sample timing. Well-controlled laboratory studies and standardised measurement techniques (Stalder et al., 2016) may assist to isolate the precise components of on-call work contributing to the physiological stress response.
From existing on-call literature (Ferguson et al., 2016; Hall et al., 2017b), we identified and isolated three factors that could potentially contribute to increased physiological stress when on-call: a) the likelihood of receiving a call; b) the stressfulness of the task performed when called; and c) the possibility of missing a call. From existing on-call literature, we identified and isolated three factors that could potentially contribute to increased physiological stress when on-call: a) the likelihood of receiving a call; b) the stressfulness of the task performed when called; and c) the possibility of missing a call (Ferguson et al., 2016; Hall et al., 2017b). Previous research has reported that the likelihood of receiving a call can differentially impact sleep behaviour when on-call (Jay et al., 2016; Wuyts et al., 2012). For example, when participants were told they ‘could’ be called during a night on-call there were significant differences in sleep outcomes (e.g., longer sleep onset latency, more wake after sleep onset), compared to a night not on-call (Wuyts et al., 2012). Conversely, another study found no differences on on-call and not-on call sleep outcomes, when participants were told they would ‘definitely’ be called on the on-call nights (Jay et al., 2016). Prior studies also indicate that the importance of the task required upon waking following a call may influence anxiety (Âkerstedt, 2006), apprehension (Kecklund and Åkerstedt, 2004) and task performance (Sprajcer et al., 2018a). Finally, in a qualitative study on-call workers who did not receive calls following consecutive on-call nights reported they were ‘worried about missing calls’ and this adversely impacted their sleep (Paterson et al., 2016). However, the relative contribution of each of these on-call factors (likelihood of receiving a call, stressfulness of the task performed, and possibility of missing a call) Vincent et al., 2018b; Larsen et al., 2015. The relative contribution of each of these factors on workers’ physiological stress is unknown, as multiple external confounding variables are usually present including, but not limited to, the amount of prior sleep, timing of calls, noise exposure, and physical activity levels.
To minimise the effect of extraneous variables, three controlled laboratory studies were conducted. Each study examined the relative impact of on-call factors on two measures of physiological stress (i.e., HRV and salivary cortisol concentrations). The aim of this study was to investigate the impact of different on-call factors, namely call likelihood, stressfulness of the task performed when called, possibility of missing a call on, 1) overnight HRV during sleep and; 2) next-day salivary cortisol concentrations. It was hypothesised that on-call nights when on-call demand was greatest (e.g., definitely receiving a call, high-stress task performed upon waking and high-chance of missing a call) would be associated with a) would be associated with reduced HRV measures related to stress and autonomic nervous activity during sleep b) increased next-day salivary cortisol concentrations when compared to on-call nights with less demand (maybe receiving a call, low-stress task performed upon waking and low-chance of missing a call) and nights not on-call.
Section snippets
Participants
Healthy male adults (n = 72) were recruited in Adelaide, Australia. Participant characteristics are reported in Table 1. Participation was voluntary and ethical approval was obtained from the Human Research Ethics Committee of Central Queensland University (H15/07-158). Participants provided written consent and were remunerated financially for their time (AU$480).
A general health questionnaire was used to screen participants to determine whether they were eligible to participate in the study.
Results
Sleep
The sleep data has previously been reported, Study 1 (Sprajcer et al., 2018c), Study 2 (Sprajcer et al., 2018a), Study 3 (Sprajcer et al., 2018b). In all three studies, 25 variables related to sleep quantity and quality were analysed. Overall, in all three studies there was very little difference between conditions in terms of sleep outcomes (4 of the variables tested showed differences between conditions). On occasions where there were statistically significant differences in sleep
Discussion
This is the first study to investigate how different aspects of being on-call (likelihood of receiving a call, task stress, and chance of missing a call) affect physiological stress outcomes. Three separate but related studies utilised an on-call laboratory simulation to investigate how different characteristics of on-call work impact overnight HRV during sleep and next-day salivary cortisol concentrations. Our hypothesis that increased physiological stress would result when participants were
Declaration of Competing Interest
None.
Acknowledgements
This study was funded by an Australian Research Council Discovery grant (DP 150104497). Funding for Madeline Sprajcer’s PhD scholarship was provided by this grant. Dr Grace Vincent is supported by an Early Career Fellowship at Central Queensland University. We would like to thank the staff on the project and the participants for volunteering their time.
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