Introduction

Sleep is essential to the life of all animals with a nervous system [1]. Total sleep deprivation leads to a behavioral and physiological syndrome resulting in death within 2–3 weeks in rat models [2]. The relationship between sleep, depression, emotions, cognition, health, and physiological status has been an active area of research in humans and laboratory animals for decades [3,4]. There is a growing interest from animal welfare scientists to further explore these relationships in domestic animals, including farmed ungulates [5,6]. As prey species, studying sleep in farmed ungulates may provide insight into evolutionary adaptations that different species have made to maintain adequate sleep, contributing to important comparative research [7].

Sleep may also help us understand how animals cope with their environment, playing a role in the quality of their lives. The welfare of farmed animals is an important area of concern amongst the public [8]. Thus, research focused on identifying metrics that may contribute to improving the welfare of animals is increasingly being prioritized. A common conceptualization of animal welfare is through three concepts described by Fraser (1997), including the animals’ subjective emotions and moods (“affective states”), their health and normal function of biological systems (“biological functioning”), and their ability to express their normal behaviors in their given environment (“natural living”) [9]. Despite growing interest in sleep from animal welfare researchers, there has been no comprehensive synthesis of how sleep has been studied in farmed ungulates and how it relates to animal welfare outcomes.

Evidence from non-human animals indicates a bidirectional relationship between sleep and different aspects of animal welfare. For example, on the one hand, sleep loss can increase disease susceptibility and negative affective states in animals [3,4,10]. On the other hand, disease and negative affective states, such as pain and anxiety, can also alter sleep patterns [3,11,12]. Thus, the relationship between sleep and animal welfare is likely complex and intertwined.

As a growing and complex area of study, a scoping review of sleep research in farmed ungulates is needed to determine the extent to which sleep has been studied and identify areas for future work. The specific objectives of the proposed scoping review will be to: 1) map existing studies on sleep in farmed ungulates (including the period of publication, animal characteristics, and methods used to assess sleep in studies of farmed ungulates), 2) explore the animal welfare implications of this sleep research, and 3) identify knowledge gaps in need of future research. This scoping review protocol aims to define the scope, outline the inclusion/exclusion criteria, establish methodological rigor, and reduce the risk of bias of the proposed scoping review.

Materials and methods

This scoping review protocol has been developed based on the PRISMA-P guidelines (S1 Appendix). Our objectives align with a scoping review approach as the topic is broad and exploratory. We aim to understand the scope and nature of this research topic, map the existing literature, and identify knowledge gaps, rather than answering a specific research question. The scoping review will follow the PRISMA-ScR guidelines for reporting, including a flow diagram for summarizing the screening and selection process [13].

Selection criteria

Table 1 outlines the criteria that will be used to determine whether articles would be included or excluded from the scoping review. We opted to only include articles in English due to a lack of translation resources.

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Table 1. Inclusion and exclusion criteria for accepting or rejecting studies for a scoping review on sleep research in farmed ungulates.

https://doi.org/10.1371/journal.pone.0343334.t001

Data acquisition and selection

We conducted a pilot study for the scoping review in October 2024. The search strategy was developed with the assistance of a librarian from the University of Prince Edward Island, who has expertise in developing scoping reviews and comprehensive research. In the pilot search, studies were sought in full text across three databases: CAB Abstracts via EBSCOhost, PubMed, and Scopus. Search terms are included in Table 2.

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Table 2. General search terms of a pilot search for a scoping review on sleep in farmed ungulates to be further adapted to each specific database.

https://doi.org/10.1371/journal.pone.0343334.t002

After the initial search, 4,092 articles were imported into Covidence software (Covidence, Melbourne, Australia), where 1,069 duplicate articles were removed (Fig 1). For the pilot search, one primary reviewer (GD) screened the titles, abstract, and full-text of the articles using the inclusion and exclusion criteria. Two co-authors (KP and ET) were consulted to ensure consistency in the application of inclusion and exclusion criteria. In the pilot search, the first screening process removed 2,654 articles based on title and abstract, and the second removed 293 papers after full-text review, resulting in 75 articles. For the scoping review, we will have a secondary reviewer screen a random sample of 20% of the articles to minimize selection bias. Prior to screening, a calibration exercise will be conducted by selecting a sample of 5% of the papers for independent screening by each reviewer. If a 90% or better agreement is not achieved, the reviewers will discuss their points of disagreement, review and revise the inclusion criteria [14].

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Fig 1. Search and screening process of important studies for a scoping review on sleep in farmed ungulates.

https://doi.org/10.1371/journal.pone.0343334.g001

From the pilot search, using 25% of the 75 articles (n = 19), we performed a hand search of the literature to find any missing articles. To do this, we used Google Scholar to search the references as well as any articles that cited of these 19 articles. Articles used for the hand search were pseudo-randomly selected, ensuring all species were represented in the selected studies. Additionally, we searched for articles in review papers of sleep in farmed ungulates. During the hand search, six articles meeting the review inclusion criteria were added to the preliminary list, resulting in a total of 81 studies included in the pilot (S1 Table). The same search strategy will be used for the scoping review. A new search will be conducted to identify articles published since the pilot search.

Discussion

This protocol describes a proposed scoping review with the aims of mapping existing studies on sleep in farmed ungulates, exploring the animal welfare implications of this sleep research, and identifying knowledge gaps in need of future research. To map this literature, we will describe the period of publication, animal characteristics, and methods used to assess sleep in studies of farmed ungulates. The period of publication will allow us to determine trends over time in sleep research and determine whether this is a growing area of scholarly inquiry. An assessment of animal characteristics will help identify any species biases in the work, as well as determine what animal age and sex has been included in current sleep research. As life stage and sex influence sleep in other species [15,16], these factors may be useful for reducing bias in future work assessing sleep in farmed ungulates.

The proposed scoping review will also include a description of methods used to estimate sleep in farmed ungulates. Several methods have been used to assess sleep in animals, including polysomnography [17], sleep-like behaviour [18], accelerometers [19,20], and a combination of physiological indicators such as heart rate and muscle activity [21]. However, not all methods used to assess sleep have been validated. Polysomnography (PSG) is the gold standard method for estimating sleep [17], yet may not be used to researchers because it is labor-intensive and typically requires animals to be housed in restrictive environments to minimize data loss, potentially affecting their natural sleep patterns. In contrast, sleep-like behavior may be easier to measure, but is a poor predictor of sleep in some animals, such as adult cattle [18]. Exploring the methods used to estimate sleep will allow us to assess the consistency of methodology across studies and species, as well as provide a better understanding of which methods have been validated to accurately assess sleep in different species.

To our knowledge, this will be the first scoping review to assess how researchers have studied sleep in animals within a framework of animal welfare. The framework of animal welfare described by Fraser et al. (1997) is relevant for our proposed scoping review, as sleep: 1) supports biological functioning through its role in immunity and disease resistance [4], 2) influences, and can be influenced by, affective states [3], and 3) relates to naturalness, since environmental conditions and the ability to perform normal behaviors shape sleep quantity and quality [22]. Classifying studies using this framework offers insights into future studies at the intersection of animal welfare and sleep and could help guide decisions about housing and management for farmed ungulates to improve their welfare.

We included a pilot study (n = 81) in our protocol as it has been shown to be useful in systematic and scoping review protocols. The primary benefits of a pilot search include helping to test and refine the search strategy, verifying the eligibility criteria, ensuring the feasibility of a review by assessing the volume of literature, and ensuring that the methods are systematically developed and rigorously tested. In this pilot search, we opted to test the coding and classification of studies for observational and experimental studies, and to determine the need for including subcategories. Additionally, the pilot study has helped determine the approach to analyze and summarize the data and will enable us to improve the coding and analysis for the scoping review.

Supporting information

Acknowledgments

The authors would like to acknowledge librarian Kim Mears (University of Prince Edward Island) for helping us develop the search appraisal and code. We would also like to acknowledge some members of the supervisory committee of GD, Drs. Marina von Keyserlingk (University of British Columbia) and Rebecca Meagher (Dalhousie University) for reviewing and providing feedback on this manuscript. Further, we would like to acknowledge Christina Doelling (University of British Columbia) for guidance on review protocol development.