Introduction: Atypical working hours, including shift work as well as work during weekends, evenings,
and nights, are known to be risk factors for workers’ health and safety. Fatigue,
including sleepiness, is considered a key factor mediating these effects. Given this
central role of sleepiness, many studies have investigated how it is affected by different
aspects of working hours or work schedules. However, the empirical evidence is mixed
and difficult to generalize, since many studies have focused on specific shift systems
within particular organizations or occupational groups. Therefore, to better understand
the relationships between work schedule characteristics and sleepiness, this study
aims to meta-analytically synthesize research on these relationships. Specifically,
we examine how sleepiness changes with (1) shift length, i.e., time into shift, (2)
shift type (e.g., day versus night), and (3) number of consecutive shifts.
Methods: This review is part of a broader project examining the relationships between different
aspects of working hours and fatigue as well as need for recovery. In this broader
project, we conducted a systematic literature search of peer-reviewed publications
in various databases in February 2024, which yielded 16,804 hits. After removing duplicates,
9,168 abstracts and 805 full texts were screened. We updated the search in January
2025, and after removing duplicates, a further 744 hits were screened. This part of
the study focuses on original studies examining sleepiness measured with the Karolinska
Sleepiness Scale. Statistical analyses are in progress. We plan to calculate mean
overall effect sizes (Hedges’ g) comparing sleepiness for (1) different lengths of
time into shift, (2) different shift types, and (3) different numbers of consecutive
shifts using random-effects models.
Results: We included 38 studies on time into shift, 31 studies on shift type, and 13 studies
on the number of consecutive shifts in the meta-analyses. Most of these studies used
repeated measures design and had small samples, often less than 100 participants.
Nurses were the most frequently studied occupational group. As expected, preliminary
results indicate that sleepiness is higher during night shifts than during day shifts.
However, results regarding time into shift and number of consecutive shifts are less
clear. The final results will be presented at the symposium.
Conclusion: This review provides a comprehensive and up-to-date overview of research on work
schedule characteristics and sleepiness, enhancing the understanding of these relationships.
Besides, its findings will support the evidence-based design of work schedules that
reduce the risk of worker fatigue in the short term, while contributing to reduced
health and safety problems in the long term.
