Does Inadequate Sleep Affect Equine Memory? Sponsored by: As a prey species, horses have evolved to sleep in unique ways that will allow them to be nearly constantly vigilant about predators. One way they can stay on alert is dozing while standing up. Though they don't enter the necessary rapid eye movement (REM) phase in this position, they are able to fulfill some sleep requirements. To get deep REM sleep, the horse must lie down. Though horses can cope with short periods of REM sleep, it's imperative that they are able to enter this regenerative sleep phase. Dr. Linda Greening and colleagues from Aberystwyth University created a study to determine the cognitive effects on horses if environmental factors precluded them from entering the REM sleep cycle.