Electroencephalographic patterns of equine sleep: comparative description and neurophysiological implications of recumbent and standing sleep

Abstract The present study aimed to describe and compare the electroencephalographic patterns of sleep in horses during recumbent and standing positions, identifying the cortical dynamics and spectral characteristics associated with each stage. Eight clinically healthy adult horses were monitored through polysomnographic EEG recordings, using a double-banana montage with subcutaneous needle electrodes. Sleep stages were classified according to spectral and morphological criteria analogous to human scoring standards, with one sleep cycle analyzed per horse. REM sleep was posture-dependent, with complete paradoxical patterns observed only in recumbent horses. In standing animals, a REM-like state was identified, marked by rapid eye movements but cortical hypoactivation and absence of muscle atonia. Quantitative analysis revealed that N1 and N2 accounted for approximately 90% of total sleep duration, while N3 and REM represented smaller fractions. Standing horses exhibited longer light sleep stages and a smaller proportion of N3 sleep. These findings demonstrate that equine sleep architecture is highly posture-dependent, and behavioral postures such as sternal or lateral recumbency cannot be reliably used as proxies for specific EEG sleep stages. The study provides a methodological framework to support future investigations and facilitate comparisons of equine sleep physiology across studies and species.