Sleep architecture and lying behavior in horses across age groups and two housing systems: an overnight non-invasive EEG study.

Sleep is a crucial factor for the well-being of horses, yet assessing equine sleep in common housing conditions remains challenging. This study investigated the effects of changes in horses' immediate environment and the influence of age on their sleep and resting behavior using a novel, non-invasive electroencephalography (EEG) mask. Ten horses from the teaching herd at the University of Veterinary Medicine Vienna were observed over four consecutive nights in two different housing systems (a large communal stable vs. two separate, adjacent individual stalls). Data collection included EEG recordings, video monitoring, electrocardiography for heart rate variability (HRV) analysis, and fecal cortisol metabolite (FCMs) measurements. The results showed significant differences in sleep stages and lying behavior between the two housing systems. Over time, the horses, in the single stable setting, exhibited an increase in REM sleep and lying-down frequency, while in the large barn setting, they showed longer duration of lying episodes, suggesting an adaptation effect. Younger horses exhibited more REM, N1 and N2 sleep, more complete sleep cycles and longer lying episodes compared to older horses. Objective stress monitoring via HRV analysis showed trends toward increased parasympathetic activity, while FCMs showed no significant changes. These findings suggest that housing conditions and age should be considered when assessing equine sleep as well as well-being. and reinforce the need for management practices that promote adequate rest. Further research is required to establish normative values and validate EEG protocols in equine medicine and to investigate the associations between sleep, stress, and well-being.