Measuring sleep in farm animals: Achievements and challenges

Sleep is a biological necessity for all living mammals. It is known for its restorative properties and plays an important role in various physiological mechanisms. Furthermore, studies in humans could show that sleep also affects psychological processes, which means that sleep is essential only for physical health but also mental well-being. This interconnection makes sleep a promising candidate for assessing animal welfare on a multidimensional scale. However, studies on sleep in farm animals are scarce, even though this field of research can be traced back to the middle of the 20th century. The objective of this narrative review is to give a comprehensive overview over the development and current state of sleep research in cattle, horses and pigs. The focus lies on analyzing studies using polysomnography, the gold standard in human sleep research by which sleep can be identified based on electrophysiological characteristics, and studies using body posture and sleep-associated behaviors as indicators of sleep. By highlighting the strengths and weaknesses of these methods we aim to point out opportunities for improvement. The use of polysomnography to study sleep in farm animals is feasible, though it is time and cost intensive and still highly experimental. Protocols for proper electrode placement and consistent scoring criteria still need to be developed for non-human species. Behavioral observations on the other hand are an established method, however their application for predicting stages of vigilance currently lacks validation. We therefore suggest combining electrophysiological and behavioral recordings in future studies. Further, we hypothesize that including additional parameters such as heart rate, heart rate variability, respiration rate, or body temperature may help to improve validity. Addressing these issues will lay the foundation for studying sleep under on-farm conditions. This, in turn, opens new perspectives for animal welfare research. • Research on sleep in farm animals is scarce and shows a lot of untapped potential. • Polysomnography is the gold standard in sleep research. • Using behavioral indicators to study sleep has advantages for on-farm applications. • We highlight the capabilities and limitations of both methods. • We suggest a combination of methods in future studies to improve validity