The scheduling of shiftwork depends on several factors, including productivity, costs, and workers’ wellbeing. Indeed, shiftwork has emerged as one of the key translational opportunities for circadian research, together with a chronobiological approach to health issues. Among the different variables that can be optimized is the number of days under nocturnal or diurnal tasks and the rest periods between them. This is a complex issue that should take into account several variables, balancing productivity, presenteeism, accident and incident rates, health and the impact on the social life of workers.1 Since there is a clear link between shift duration, sleep, and health outcomes,20 and the effects of continuous sleep deprivation and irregularity is cumulative,21 a correct decision in terms of the schedule might have long-lasting consequences. Indeed, many other variables contribute to both productivity and wellbeing during shiftwork, including environmental factors (i.e., illumination, feeding schedules, etc.), individual characteristics such as chronotype and even commuting time and distance, and the nature of the work being performed, as well as stress and inherent working hazards. Since shiftwork also affects mood vulnerability,18,22 it is not surprising that such routine impacts both co-workers and their family relationships.
Here we examined the effect of three different working schedules on the sleep-wake performance of workers in the logistics branch of one of the biggest oil companies in South America. In all cases, the duration of sleep during the resting period was significantly less than the recommended 7 hours of nocturnal sleep.23 The “2x2x4” schedule showed increased nocturnal sleep time after diurnal work, decreased sleep regularity and a dampened circadian temperature rhythm. Additionally, this shift exhibited higher levels of insomnia, increased fatigue impact, lower alertness levels, and heightened symptoms of depression (Fig. 3).
Participants in our study only worked 12-h shifts. There is much discussion regarding their advantage over 8-h shifts; including an increased sleep opportunity during rest days. However, some reports state that this schedule might decrease alertness24 and deteriorate domestic life.25 While there is a clear consensus in the fact that any shiftwork schedule will result in circadian and sleep disruption, especially at the extremes of very long or very short types of rotation, there is no clear effect of switching between 8- and 12-h shifts.26 Since oil industry must rely on 24/7 activities, the most usual schedule in companies around the world involves 12-h shifts for 2 or 4 consecutive days (in contrast to the previously implemented 8-h shifts system, with less frequent rotation). However, long work hours are associated with short sleep duration, sleep disruption, and decreased alertness.24,27,28 The impact of shiftwork schedules in the oil industry on the domestic and social life of workers has also been highlighted elsewhere.29
The disruption of sleep patterns and circadian temperature rhythms observed in workers under the "2x2x4" scheme may probably be attributed to the shorter duration of shifts. Sleep regularity, a measure of the consistency of day-to-day variability in sleep–wake timing – has recently received much attention, in terms of its importance for health, safety and productivity). It is known that fluctuations in both environmental and behavioral factors associated with variability in sleep timing can induce circadian disruption30 and, in turn, sleep disturbances.4 In this regard, the increased sleep duration observed in this group could be explained as a compensatory mechanism for the disruption of circadian rhythms. While extending sleep on non-work days is a common human pattern to counter sleep deficiency on work days,30 this practice may not be sufficient in the 2x4 schedule, resulting in additional sleep during work days. Indeed, in a previous study involving long-haul bus drivers working with a two-up operations system, we reported that workers under high fatigue risk working schedules showed increased sleep duration during the trips on the bus.19
The results obtained from the circadian analysis of temperature reinforce what has been discussed so far. The "2x2x4" schedule exhibited lower circadian robustness compared to the "fixed 12-hour” schedule, while the "4x4x4" schedule showed intermediate values, without significantly differentiating from the other groups. Taken together, this reveals a greater circadian disruption in the "2x2x4" schedule compared to the "4x4x4" one. These results are also consistent with the disruption of the expression of the circadian rhythm of wrist temperature by shift-work reported by our group in drivers exposed to high-risk working schedules19 and by others. 31–33
In our work, we observed that workers from “2x2x4” the schedule reported a higher number of days with significant tiredness in the last month, a greater negative impact of fatigue on their daily activities, and more depression symptoms. Also, these workers reported lower levels of alertness in the middle of night shifts than workers from the “4x4x4” schedule. This aligns with the circadian disruption observed in this group, known to potentially result in sleep disturbances, sleep medication consumption, persistent fatigue and mood disorders, and/or burn out, which could compromise safety and health.4 In fact, a report from the 1990s34 investigated oil refinery workers during their night shift, finding significant changes in endocrine rhythms, with a lower amplitude for cortisol and higher peaks for melatonin. It is also well-known that the number of years spent on shiftwork in this industry further affects sleep, health, and productivity.35,36 More recently, studies have demonstrated that impaired sleep quality and shorter sleep duration in oil activity shift workers are associated with increased fatigue-related incidents.6
We also considered chronotype, which might affect the outcome of shift workers, with certain advantages for late-types.37 In our case, we did not find significant differences in midsleep on free days between Schedules “2x2x4” and “4x4x4”. The midpoint of sleep on free days for “2x2x4” schedule was at 05:22 am, while for “4x4x4” schedule it was at 04:50 am, both corresponding to slightly late chronotypes;38 however, it should be noted that the Argentinian is a particularly late culture. Since the alignment of individual chronotype with working hours might reduce circadian disruption,39 more data is needed to assess the impact of this variable on performance and wellness in this particular industry.
According to the guiding principles of the American Academy of Sleep Medicine and the Sleep Research Society (2021),40 the duration of working shifts is linked to adverse performance, safety, and health outcomes, and several countermeasures are proposed to overcome such adverse consequences. The first of these measures is to “schedule interventions to maximize daily sleep opportunity, consistently align work with the biological drive for wakefulness and increase time for recovery after extended duties or multiple shifts.” Here we have compared three different shiftwork schedules for the oil industry, finding that a “2x2x4” shift work schedule showed decreased sleep regularity and a dampened circadian temperature rhythm, associated with higher levels of insomnia, increased fatigue impact, lower alertness levels, and heightened symptoms of depression. There are several confounding factors that should be considered, and were not addressed here, to fully understand the impact of shift work design on sleep, alertness and fatigue. Among these are the nature of tasks performed, the environmental conditions both at work and at home, and the types of jobs. However, we believe that our results contribute to a further understanding of the optimal working conditions that our 24/7 society demands to achieve improved outcomes in health, wellness, and productivity, representing a clear case for translational circadian research.