Abstract
BACKGROUND: Horses must lie down to go into vital rapid-eye movement (REM) sleep. If they are not lying down for sufficiently long periods they can become so sleep-deprived that they collapse uncontrollably, which results in a risk of injuries. OBJECTIVES: To investigate how recumbency as a prerequisite to REM sleep on the experimental days and the following nights was influenced by changes in social and spatial environment throughout the day. STUDY DESIGN: Cross-over design in which subjects experienced each experimental condition twice. METHODS: Observations were conducted on a horse farm. Ten young horses in training were observed on days when they were alone indoors in a box, on an outdoor paddock alone, on the same paddock in pairs, and in the following night stabled alone. The number of lying bouts and the total lying duration throughout the day and night were assessed automatically using 3D-accelerometers and data were evaluated using mixed-models. RESULTS: Horses had a higher number of lying bouts during the days (p = 0.05, by a factor of 1.21 [95% CI: 1.00-1.45]) and longer lying duration at night (p < 0.001, by a factor of 11.25 [6.47-18.40]). On average, the number of lying bouts and the duration of lying increased from being indoors alone, to being outdoors alone, and outdoors in pairs although this could not be statistically supported (bouts: p = 0.5, by a factor of 1.08 [0.84-1.36] and 1.17 [0.91-1.48]; p = 0.6, duration: by a factor of 1.39 [0.73-2.93] and 1.38 [0.68-2.78]). MAIN LIMITATIONS: A small number of horses were observed and there was large variability between days within horses. CONCLUSIONS: We found some indications that open space and a social companion throughout the day increased time lying down in the day as well as during the following nights thus allowing for more REM sleep.