Why Do Horses Sleep Standing Up — Question of the Day

I know it's random, but I was wondering yesterday why a lot of animal sleep standing up?

Not just horses, elephants, elephants, cows, come to find out most hoofed animals, birds and large ones like giraffes all sleep standing up.

But why? I have a two year old son and I know at some point in time he's going to ask me this and I wanna be prepared!

So if you have a theory, tell me about it below!

Or waste some more time checking out these videos of standing sleepers:
Dog

Baby