Horsetalk.co.nz Light and bedding’s effect on equine sleep explored Image by Olga Itina / Dreamstime.com A recent study examined the effect of environmental factors on equine sleep stages, and whether this would influence cognitive performance during a memory task. As a prey species, horses have evolved to cope with short periods of rapid eye movement (REM) sleep and are known for their ability to sleep, or at least doze, (Non-REM sleep, NREM) while standing up. They still have to lie down for REM sleep which is only possible during recumbency because of the lack of muscle tone that occurs during this sleep phase.