Chronobiology and the horse: recent revelations and future directions

The circadian system provides animals with a means to adapt their internal physiology to the constantly changing environmental stimuli that exist on a rotating planet. Light information is translated into molecular timing mechanisms within pacemaker cells of the mammalian hypothalamic suprachiasmatic nucleus (SCN) via transcriptional-translational feedback loops. Humoral and neural outputs from this 'master' clock result in circadian rhythms of physiology and behaviour. The larger circadian system involves SCN synchronisation of cellular clocks throughout the organism such that individual organs can adapt their specific function to the time of day. In the short history of this scientific field, the vast majority of mammalian chronobiological research has been conducted using small laboratory animals. This review examines what these studies have revealed, discusses how recent chronobiological findings in the horse compare to what is known and highlights how the principles of circadian biology are applicable to equine husbandry and veterinary care.