||Biological rhythms evolved to provide temporal coordination across all tissues and organs and allow synchronisation of physiology with predictable environmental cycles. Most important of these are circadian and circannual rhythms, primarily regulated via photoperiod signals from the retina. Understanding the nature of physiological rhythms in horses is crucially important for equine management. Predominantly, we have removed them from exposure to their natural environmental stimuli; the seasonally changing photoperiod, continuous foraging and feeding activity, social herd interactions and the continuous low intensity exercise of a grassland dweller. These have been replaced in many cases with confined indoor housing, regimental feeding and exercise times, social isolation and exposure to lighting that is often erratic and does not come close to mimicking the spectral composition of sunlight. We have further altered seasonal timing cues through the use of artificial lighting programs that impact reproductive behaviour, breeding efficiency and the development of youngstock. Understanding how these new environmental cues (some stronger, some weaker) impact the internal physiology of the horse in the context of the natural endogenous rhythms that evolved over millennia, is key to helping to improve equine health, welfare and performance, now and into the future. This review provides an overview of the field, highlights the recent discoveries related to biological timing in horses and discusses the implications that these findings may have for the production and management of the elite equine athlete.