In this study, we investigated sleep quality related to direction of shift rotation using large-scale data from shift work-specific health examinations of electronic workers. Our results showed that backward rotation system was associated with poor sleep quality. Additionally, backward rotation system was significantly associated with poor sleep quality in workers ?30 years of age compared with workers 30 years of age.
Previous studies have found an association between direction of shift rotation and sleep quality, however, the results are inconsistent. Orth-Gomer assessed sleep quality and coronary risk factors in a short-term intervention trial that included 45 volunteer policemen [9] and reported that sleep was longer and better with forward rotation. Van Amelsvoort et al. reported that a forward rotation schedule was prospectively related to less work-family conflict and better sleep quality over 32 months of follow-up [10]. Our finding is consistent with these previous study results. However, several other studies indicated that direction of shift rotation did not play a significant role in sleep problems [12, 13]. Cruz et al. assessed subjective ratings of sleep quality, sleepiness, and objective source of sleep/wake data using activity sensors in 28 participants [12]. They reported no effect of rotation condition for any of the sleep measures. Tucker et al. found no effect of direction of rotation on any of the chronic outcome measures despite the association between direction of rotation and sleep duration [13].
The exact mechanisms of increasing risk of poor sleep quality by backward rotation system have yet to be elucidated. Previous studies have found that the circadian rhythms, generated by the suprachiasmatic nucleus (SCN) of the anterior hypothalamus in mammals [20, 21], can be synchronized to external time signals but also can persist in the absence of such signals [22, 23]. In normal conditions, the SCN generates circadian rhythms by receiving light inputs from the retina during the day and from melatonin during the night [20, 21], and SCN neuronal activity drives the circadian variation of the sleep/wake cycle, hormonal secretion, thermoregulation, and other physiologic events [20, 24]. Even in the absence of external signals, SCN neurons have a near-24-h rhythm of electrical activity [21, 25]. This circadian activity reflects the rhythmic pattern of expression of core genes, called clock genes, that are regulated by autoregulatory feedback loops [21, 25, 26]. If the circadian rhythms were driven by external signals, they would persist for a period of exactly 24 h. However, without external signals, the circadian rhythm period is slightly longer than 24 h [27]. Most humans already have a natural tendency to drift slightly later each day; therefore, the human circadian rhythm is more difficult to phase-advance than to phase-delay [28, 29]. It takes less time to reset the circadian rhythm following westward (requiring a phase delay) than eastward (requiring a phase advance) flight [30, 31]. Similarly, adaptation is more rapid after forward rotation (requiring a phase delay) than after backward rotation (requiring a phase advance) [28, 29].
Furthermore, previous studies have found that young age was associated with shift work tolerance, measured as subjective sleepiness, performance tests, recovery after work, and sleep time [32–34]. Some studies have indicated that the critical age for reduced tolerance to shift work is between 40 and 50 years [35, 36]. Results from another study showed that both sleep duration and sleep quality among shift workers decreased with increasing age up to approximately 45 years [37]. These findings might be explained by age-related disruptions of circadian rhythms characterized by changes in both behavior and physiology [38]. Age is associated with decreased electrical, hormonal, and gene-expression activity of SCN cells, which are thought to globally disrupt the body’s circadian activity [38–40]. In elderly humans, rhythm disturbances include fragmented sleep–wake patterns, weak coupling with environmental rhythms, reduced amplitude of daily body temperature rhythms, alterations in the daily rhythm of hormone secretion, high levels of nighttime activity, and reduced daytime cognitive performance [38, 41–43]. These disadvantages may partly explain why the backward rotation system was significantly associated with poor sleep quality in older age.
In our study, we showed that forward rotation systems should be considered to reduce sleep problems. Reportedly, sleep problems have a direct association with accidents or errors at work [44]. Furthermore, in shift workers, sleep problems are represented mainly by the disruption of the circadian rhythm, which adversely affects health [3, 4]. Forward rotation systems are thought to reduce not only sleep problems, but also adverse health effects by decreasing disruption of the circadian rhythm.
Our study had several limitations. First, speed and interval of shift rotation were not identified because the data used in the present study were collected from shift work-specific health examinations. Several studies have suggested that fast-rotating shift systems (change of working hours every 2–3 days) are preferable to slow-rotating shift systems [5]. In addition, several studies have suggested that a shorter shift rotation interval leads to worse sleep quality [6]. To decrease variation in shift rotation, we selected subjects who worked in the electronics industry. Second, we did not take into account several confounding factors such as work condition including type of work and resting time during work [14, 45], socioeconomic status [46], marital status [47], time for chores [48], which may have influenced the association between direction of shift rotation and sleep quality. Third, there is a possibility that current-smoker were underestimated because we did not apply time frame to classify for ex-smoker. Fourth, this was a cross-sectional study, therefore, the temporal relationship could not be determined and the causal relationship between direction of shift rotation and sleep quality could not be investigated. Finally, the subjects included in this study were younger-aged Korean males and females who regularly attended work-related health check-up programs. As a consequence, our findings may not be representative of the general Korean population or of other populations with different demographics.
Prior studies used a small sample size [9–13], focused on males [9, 10], and did not consider that differences between job types may have biased the results [10, 12]. In our study, the large-scale data of electronic workers allowed a greater statistical power and showed the association between direction of shift rotation and sleep quality in the electronics industry. Furthermore, previous studies that suggested an association between direction of rotation and sleep quality, assessed sleep quality using non-standardized methods. In this study, we used the PSQI, a reliable, valid, and standardized measure of sleep quality and a screening tool for sleep dysfunction in non-clinical and clinical samples [18, 49].