How can a horse sleep standing up?

Related questions

1. Are there any female race horses?
2. Can horses talk to each other?
3. Do the whips hurt horses?
4. Is horse riding exercise?
5. Do race horses know when they win?
6. Are horses intelligent animals?
7. How much do horse trainers make per month?
8. What horse breed is best for eventing?
9. Is English or Western riding easier?
10. What is horse jumping called?

Tags

1. #amount
2. #horses
3. #because
4. #sleep
5. #small
6. #often
7. #comfortable
8. #standing
9. #require
10. #typically
11. #however
12. #while