Nonrestorative sleep: a potential indicator or consequence of job stress

Nonrestorative sleep (NRS) is a symptom of insomnia as per the International Classification of Sleep Disorders 2nd Edition (ICSD-2); however, in 2013, in the revised ICSD 3rd edition (ICSD-3), it was excluded from the symptoms of insomnia, and is now considered a common sleep-related complaint resulting from sleep deterioration caused by various sleep disorders and mental and neurological diseases [1]. Furthermore, NRS is influenced by the direct effects of sleep deterioration and potential biases resulting from mood and cognition, making it a promising indicator of comprehensive daytime dysfunction [2].

NRS predicts future depression more strongly than the three major insomnia symptoms, including difficulty in initiating sleep, difficulty in maintaining sleep, and early morning awakening [3]. It also predicts the onset of cardiovascular and metabolic diseases, such as hypertension [4, 5], obesity, and type II diabetes in insomniacs [5]. Furthermore, based on the findings of a longitudinal cohort study of community residents in the United States, NRS is associated with the risk of all-cause mortality independent of lifestyle habits and the above-mentioned chronic diseases [6].

Sekizuka et al. demonstrated a significant association between job stress and NRS, independent of age, sex, body mass index, comorbidities of lifestyle-related and cardiovascular diseases, sleep duration, and lifestyle habits, among Japanese office workers (odds ratio, 3.68; 95% confidence interval, 3.35–4.03; P < 0.001) [7], suggesting the importance of reducing job stress for maintaining and improving sleep health among the working population. In the study, NRS was evaluated based on responses to a single question asking, “Have you been feeling sufficiently rested after sleeping in the most recent 2–3 months? [7]” However, most previous studies have evaluated NRS using a single questionnaire that assesses the presence or absence of restfulness after sleeping over the past month [3–5], the most recent sleep [6], or NRS scales comprising multiple questions specifically designed for this purpose [8]. Generally, as the evaluation period lengthens, recall bias is expected to increase, and the specificity of the questions is lost. Therefore, it is important to note this limitation when considering the results of their study in conjunction with the findings of previous studies. Furthermore, the said study had a cross-sectional and retrospective design, which requires caution when drawing causal inferences [7].

However, at this point, there are benefits to using NRS as an indicator for assessing the risk of job stress that may harm physical and mental health. In workplace surveys, it may be difficult for employees to accurately and honestly express the stress they experience at work; thus, NRS can act as a potential alternative indicator. Furthermore, it is often easier to implement interventions that reduce NRS rather than directly reducing job stress. Therefore, future prospective longitudinal studies with health outcomes as a primary endpoint could clarify the effects of NRS and job stress, leading to the development of more meaningful work-related health risk intervention methods. Furthermore, elucidating the physiological causal mechanisms between NRS and job stress is important for gaining deeper insights into epidemiological causal inferences and exploring the mechanisms underlying the onset of various chronic diseases where stress and sleep disorders are risk factors [9].

Declarations

Conflict of interest

The authors have no conflicts of interest to declare that are relevant to the content of this article.