Why do horses sleep standing up?

A larger brown horse with a white stripe on its nose is resting it's head on a smaller white horse. They are both snoozing whilst standing in a fenced in grassy field.

If you've ever passed by a field of horses at night, you've probably noticed that they rarely lie down to sleep. So why do these mammals sleep standing up? 

The answer is one of survival: Horses slumber while standing to balance their need for sleep against the ever-looming threat of predators.