1. Introduction
For the past several decades, ergonomics researchers have attempted to optimize shiftwork schedules in order to improve health, safety, welfare, and functional outcomes for shift workers. Factors studied in recent years that have been shown to affect sleep, sleepiness, burnout, and other health and functional outcomes include type of shift (e.g., fixed afternoon/evening, rotating [
1,
2], short rest periods between shifts [
3,
4], unpredictable work hours [
4], split shifts [
4,
5], and shift length [
6,
7]. In the present investigation, we were offered an opportunity to prospectively examine functional outcomes in the context of a planned pilot transition from 8-hour (8-h) to 12-hour (12-h) shift rosters in shift-working ground crew airline managers.
Studies focusing on the length of the shift within the 24-hour day have typically contrasted 8-h and 12-h shifts [
6,
7,
8,
9,
10,
11]. However, few field studies have prospectively assessed the health and functionality of shift workers transitioning from 8- to 12-h shift rosters; these have revealed different and often inconsistent findings [
10,
12,
13,
14,
15,
16]. In a series of studies using the National Institute for Occupational Safety and Health (NIOSH) fatigue and performance test battery in control room operators [
13,
14] and in employees of a natural gas utility [
15], findings consistently showed decline in performance tasks that were attributed to the extra work hours and shorter sleep duration following the transition from 8-h to 12-h shifts. Assessment of performance over consecutive workdays revealed that some performance tasks (e.g., grammatical reasoning response time and accuracy) improved across days in the 12-h but not in the 8-h shift rosters, suggesting that the shorter workweek may offset some of the negative effects of the longer work shift [
13]. Investigators concluded that the extra work hours per shift and the associated sleep debt and increase in fatigue explain the decline in performance during the 12-h shifts, and that despite this decline, the opportunity to condense the work week into fewer shifts is popular among workers who are willing to endure the increased fatigue and compromised performance [
15].
Other studies demonstrated some functional improvements when moving from 8- to 12-h shift rosters in measurements including blood pressure, sleep duration, sleep quality, and subjective level of alertness [
12], psychological health and fatigue, [
16], mood, sleep and general health measures, as well as higher satisfaction regarding domestic and social domains [
10]. However, higher error rates were recorded on performance tests towards the end of the 12-h but not the 8-h shifts [
10]. Beyond the apparent diversity in workers’ professions and in the measured outcomes, we cautiously conclude that workers report improved outcomes regarding their health and wellbeing despite indications of compromised performance. It is important to note that typically, in these studies, only a subset of workers participate in all measurements for both the 8-h and 12-h shifts (e.g., [
10,
14,
15]). Therefore, data analysis was based on independent between-group designs.
Few studies have assessed the effects of shift length on burnout, fatigue, job stress, and job satisfaction, and these were based on cross-sectional studies of hospital nurses [
6,
17,
18]. In a European study, a shift length of 12 hours or more was associated with a 26% increase in reported emotional exhaustion, a 21% increase in experiences of depersonalization, and a 39% decrease in sense of personal accomplishment. Nurses reported less satisfaction with work and schedule flexibility and were more likely to report an intention to leave their job as compared to a shift length of 8-h or less [
6]. Similar findings were reported in a study of U.S. nurses, showing an increase in burnout and job dissatisfaction with increasing shift lengths [
17].
Conversely, one study found lower emotional exhaustion and increased job and schedule satisfaction in nurses working 12-h compared to 8-h shifts [
19]. The urban setting or other demographic characteristics of the sample in this study may explain these discrepant findings. In addition, typical characteristics of hospital nurses, including female gender and associated family responsibilities, low job control, and high psychological demands are likely to contribute to job strain, burnout, and job satisfaction [
20]. For this reason, the subjects may not be comparable to managerial workers in other less stressful settings.
It is evident from the literature that most of the studies that examined the effects of shift length (8-h versus 12-h) compared independent groups of workers or prospectively followed small sample sizes. Very few field studies have been performed on the same individuals transferring from 8-h to 12-h shifts, separately assessing outcomes during both day and night shifts and in the home environment. None of the prospective studies assessed burnout, and the few that assessed sleep quality and duration were based on subjective reporting. Furthermore, most studies were performed over two decades ago. In 2014–2015, we had the opportunity to perform such a naturalistic pilot field study in airline ground crew managers. We tested the feasibility of a transition from a three-shift roster (day shift: 07:00–16:00, evening shift: 16:00–23:00, and night shift: 23:00–07:00 hours), hereafter, the 8-h shift roster, to a two-shift roster (day shift: 08:00–20:00 and night shift: 20:00–08:00), hereafter, the 12-h shift roster.
The objective of the current study was to prospectively compare burnout, objective and subjective measures of sleep quality and duration, and sleepiness during day and night shifts in airline ground crew managers in the transition from 8-h to 12-h shift rosters (see Tables 4 and 5). As previous studies of managerial and low-strain occupations demonstrated a preference to a shorter and condensed workweek despite the hardship of enduring long work shifts, we hypothesized that:
(1) Shift length is associated with burnout, self-reported sleep quality, and objective measures of sleep duration and efficiency. Compared to the 8-h shift roster, we expected to observe decreased burnout, improved sleep quality, and longer sleep duration and efficiency following the transition to a 12-h shift roster.
(2) Shift length is associated with the level of sleepiness recorded during day and night shifts. We expected to observe that sleepiness would increase following the transition from an 8-h to a 12-h shift roster.
3. Discussion
When comparing functional outcomes due to transitions in the length of shifts, it is important to consider additional changes that are implemented in connection with the transition. In order to maintain a comparable number of weekly work hours, 12-h shift rosters in effect compress the number of work days in the week [
21]. Other changes include shift start time [
8] and breaks and rest periods [
6]. These and other changes may be considered necessary adaptations in the workplace, but they challenge research efforts to tease apart the effects of the separate constituents on relevant functional outcomes. Indeed, reviews comparing functional outcomes based on shift length have stressed the need to comprehensively explore the complex interactions between the various factors [
7,
21].
In the present study, burnout was lower and subjective sleep quality was higher following the transition from the 8-h to the 12- shift roster. Sleepiness increased throughout both day and night shifts and was higher during the 8-h than the 12-h day and night shifts. Sleepiness increased throughout the 8-h night shift, whereas during the 12-h night shift, sleepiness was attenuated, likely due to inadvertent napping of some of the participants.
In line with our first hypothesis, burnout on all three subscales and total score improved, as did subjective sleep quality, following the transition from the 8-h to the 12-h shift roster. The rate of workers who exceeded the cutoff score for poor sleep quality was lower in the 12-h than in the 8-h shift roster. These results indicate decreased burnout and better sleep quality following the transition from the 8-h to the 12-h shift rosters. However, actigraphy-based sleep measures did not differ between rosters, with the exception of naps outside the main sleep periods, which showed improved sleep measures (longer time in bed, longer sleep duration, and higher sleep efficiency) during the 12-h shift roster.
Interestingly, 36% of workers reported inadvertent naps during the 12-h night shift, between 03:00–04:00 at night. To account for this potential confounder, we compared all outcome measures in nappers and non-nappers during both rosters. Burnout was higher among the nappers during the 8-h but not the 12-h shift roster, whereas sleep efficiency (measured at home) was higher among non-nappers compared to nappers during the 12-h shift roster only. These findings suggest that nappers and non-nappers are qualitatively different in their adaptation strategies to shiftwork. Nappers may have more difficulty adjusting to the short 8-h shift roster, as indicated by increased reported burnout; however, during the 12-h shift roster, burnout was no longer higher in nappers than in non-nappers. Furthermore, during the 12-h night shift, nappers took advantage of the opportunity to nap, yet their sleep efficiency at home was compromised. It is likely that individual differences that are associated with adaptation to shiftwork, such as age, chronotype, cognitive arousal, responsibilities, and sleep opportunities at home, may underlie these different adaptation strategies [
22,
23].
Our hypothesis regarding the association between sleepiness and shift length was not supported. During the day shift, sleepiness was lower during the 12-h shift than during the 8-h shift. During the night shift, shift length interacted with hour such that during the 8-h shift, sleepiness initially decreased (at 01:00) and subsequently increased linearly, whereas during the 12-h shift, a quadratic function was observed, thus sleepiness showed an increase followed by a plateau, a decrease, and finally, no change. This decrease in sleepiness in the 12-h shift is likely due to the inadvertent naps reported by some of the workers (36%) (despite being clearly non-adherent to study protocol). However, findings also show increased sleepiness in nappers compared to non-nappers, particularly between 02:00–07:00. In a previous study comparing sleepiness and performance with and without a 30-minute nap in a simulated night shift, alertness and performance improved in the nap compared to the no-nap condition following an initial decline due to sleep inertia following the nap [
24]. As napping was not part of the study design, and as only about one third of our sample reported napping, and it was only during the 12-h shift, we are unable to draw any firm conclusions regarding the temporal dynamics of sleepiness with and without naps during 8-h and 12-h shifts.
We have previously demonstrated the efficacy of naps during the night shift on sleepiness and performance [
25]. In a study of shift working hospital nurses, a short scheduled nap benefited both sleepiness and performance regardless of individual differences such as age and chronotype [
25]. In the present study, workers were not expected to nap; however, it appears that nappers were compelled to do so, likely due to increased sleepiness compared to non-nappers.
Another common strategy to cope with sleepiness during the night shift is caffeine consumption [
26]. In the present study, dramatically higher caffeine consumption during the 12-h as compared to the 8-h night shift may indicate that workers anticipated and experienced greater sleepiness during the 12-h shift due to the extended hours that began three hours earlier than the 8-h shift and attempted to battle this sleepiness by increasing their caffeine consumption. However, when evaluating the relationships between caffeine and sleepiness in nappers and non-nappers, we find that increased caffeine was associated with reduced sleepiness only in the non-nappers during the 8-h shift, whereas sleepiness increased with caffeine consumption in both nappers and non-nappers during the 12-h shift, suggesting that caffeine was not effective in combating sleepiness. Whether these findings suggest individual differences in sleepiness, anticipation, behavioral adaptation to shiftwork, or other individual trait-like features is currently unclear. Individual trait-like vulnerability to sleep loss has been suggested in experimentally induced sleep deprivation protocols [
27] and in nighttime shiftwork studies [
22,
28].
This study has some limitations. Due to management considerations of the airline company, we were not able to conduct a crossover design, and therefore order effects (i.e., a shift from 8- to 12-h versus a shift from 12- to 8-h shift rosters) could not be assessed. Furthermore, self-report measures are subject to bias, and in the present study, reported caffeine consumption was particularly high during the 12-h shift. Nevertheless, controlling for caffeine consumption did not substantially affect the main findings. We also cannot rule out the possibility that some participants failed to report inadvertent napping. Additionally, the day shift began one hour later, at 08:00 rather than 07:00 during the 12-h roster. Although we found no differences in outcome measures when computing by clock time rather than by shift start time, it is likely that any differences in outcome measures may be attributed to the additional hour at home during the day, which may be devoted to family activities or an additional hour of sleep, rather than to the difference in shift length. Another limitation is the small number of workers we followed. Thus, our results should be cautiously interpreted, as they may not be indicative of the true effect due to low statistical power. Finally, participants may have been subject to expectation bias, i.e., their improved self-report assessments following the transition to the 12-h shift roster may reflect expectations of the organization and the work environment. Anecdotal evidence obtained by incidental conversations between participants and members of the research team suggested that most of the participants were in favor of the transition. However, it is not possible to determine whether or not these reports were biased by organizational considerations.
Notably, any transition in shift length ultimately affects several other parameters in the work schedule, e.g., start times of each of the shifts, number of shifts per 24 hours, number of work shifts per week, number of commutes per week, order of scheduled shifts and days off, etc. Therefore, it is not entirely possible to separate the distinct characteristics of the two rosters and their relative effects on study outcomes. These limitations are inherent to field studies, which nevertheless provide valuable insights to management, policy makers, and employees.
In conclusion, the transition from 8-h to 12-h shift rosters was associated with lower burnout, improved subjective evaluation of sleep quality, and improved quality of naps in the home environment. Sleepiness was lower during the 12-h compared to the 8-h shift roster during both day and night shifts. The effects of shift on sleepiness interacted with caffeine and napping, suggesting that behavioral strategies aimed to combat night shift sleepiness differentially affected sleepiness depending on shift length. Findings highlight that in addition to factors inherent to scheduling changes, individual differences and behavioral choices should also be taken into consideration as factors that contribute to the outcomes of scheduling changes in the work environment.
4. Materials and Methods
4.1. Study Design
The study was a within-subject prospective repeated measures design comparing measures of burnout, sleep, and sleepiness during 8-h versus 12-h shift rosters. As the study was performed in a real-life setting, it was not possible to counterbalance. All participants were initially assessed during their usual 8-h shift roster and reassessed three months after the transition to a 12-h shift roster.
4.2. Participants
Thirty-nine permanent employees (19 men and 20 women) were recruited from airline ground crew managers at the Ben-Gurion International Airport. All employees worked on rotating shifts full-time and had at least one year of experience in shiftwork.
Exclusion criteria included active chronic disease affecting daily functioning, regular use of medication that affects level of alertness, employees with a child ≤1 year of age, and pregnancy. The ethics committee at Haifa University approved this study (approval # 026/14), and all participants signed informed consent.
4.3. Measures
1. Shirom Melamed Burnout Measure (SMBM) [
29,
30]. Burnout levels were measured using a 14-item questionnaire that included three subscales: physical fatigue, emotional exhaustion, and cognitive weariness. The participants were asked to rate their feelings during the past month on a scale of 1 (almost never) to 7 (almost always); a higher score indicated higher levels of burnout (Cronbach’s alpha 0.94 and 0.95 for 8- versus 12-h shift rosters, respectively).
2. Pittsburgh Sleep Quality Index (PSQI) [
31,
32]. The self-report questionnaire included 19 items grouped into 7 subscales on a 4-point scale (0–3, including subjective sleep quality, sleep latency, sleep duration, sleep efficiency, sleep disorders, use of sleep medications, and daily function) and summed to obtain a total score (total score >5 indicates poor sleep quality). Cronbach’s alpha was 0.71 and 0.83 for 8- versus 12-h shift rosters, respectively. In the current study, we used the total score as a continuous measure, and the cutoff score was used to dichotomize participants with/without poor sleep quality.
3. Activity monitoring: Objective sleep patterns were measured using an activity monitoring device (Actiwatch 2, Phillips Respironics). This small wrist-worn device measures sleep patterns continuously in the natural environment and provides objective data for sleep patterns. Actiwatch output included 3 averaged variables for major sleep episodes and naps, as measured by the Actiwatch algorithm: time in bed (TIB, determined by event markers provided by participants when retiring to and getting out of bed), total sleep time (TST, hours from sleep onset until wake-up time), and sleep efficiency (SE, the percentage of sleep minutes out of time in bed).
4. Karolinska Sleepiness Scale (KSS) [
33]. Participants ranked their level of sleepiness from 1 to 9: 1 = very alert; 3 = alert; 5 = neither alert nor sleepy; 7 = sleepy, but no effort required to stay awake; 9 = very sleepy, great effort required to stay awake. The KSS is widely used and has high content validity.
5. Caffeine intake: To control for the alerting effects of caffeine, participants reported their hourly caffeine consumption during the day and the night shift (number of cups of caffeinated beverages).
4.4. Procedure
The 8-shift roster included 11 days and nights, and the 12-hour shift roster included 7 days and nights (see
Table 4 and
Table 5). All participants were first tested during their usual 8-h shift roster and again under the 12-h shift roster after three months following the transition from 8- to 12-h shift rosters. Burnout (SMBM) and sleep quality (PSQI) measurements were completed once at the beginning of data collection for each of the two rosters (8/12). Participants wore the Actiwatch for the continuous collection of sleep measures during the full respective rosters. Subjective sleepiness (KSS) was measured hourly throughout two night shifts and one day shift per roster.
4.5. Data Analysis
Statistical analyses were performed using the IBM SPSS Statistics version 21. To assess differences in sleep and burnout measures in 8-h and 12-h shift rosters, differences in the 3 burnout subscales were tested using repeated measures analysis, and differences in burnout and subjective sleep quality total scores were tested using paired t-tests. The rate of participants exceeding the PSQI cutoff score between 8-h and 12-h shift rosters were tested by chi-square. Differences in objective sleep patterns were tested using repeated measures analysis. Differences in sleepiness during day and night shifts in both rounds were tested using repeated measures analysis of the overlapping hours. Since there were no significant differences between the two recorded nights in sleepiness, a mean was calculated for each overlapping hour in both night shifts per roster. Mixed models were performed to assess differences in sleepiness levels by hour (clock time), shift (8/12), nap (nappers/non-nappers), and caffeine (number of cups of caffeinated beverages per hour), for parallel hours of both shift rosters (night shift: 00:00–07:00; day shift: 08:00–16:00).