I think that they race in the dark under flood light when it's cooler anyway. It may also be a different type of heat, i.e. drier. Regardless, they are being forced to race whereas in the wild if they need to run they would do; otherwise they rest under trees, usually with one hoof rested standing head to tail in order to get rid of flies
The Arabs in particular value their Stallions, Saluki and birds of prey ........ think grey horses with flowing manes and tails