How can horses sleep standing up? Is it just horses that can?

Question: How can horses sleep standing up? Is it just horses that can?
- Keywords:
  horse,
  sleep
Asked by invalidusername to Charlotte, Jo, Kevin, Louise, Valeria on 18 Jun 2012. This question was also asked by srogers99.
- Joanna Cruden answered on 18 Jun 2012:
  
  Horses are a prey animal and so it is highly likely that they have evolved to being able to sleep standing up in order to run at a moment’s notice if there is danger . They have bones which lock in place to allow muscles to relax and not to fall over in such a way that they can stand and sleep at the same time.
  
  If you have ever been on the underground during the morning or evening commute you will spot that some humans seem to also be able to sleep standing up!!

Question: How can horses sleep standing up? Is it just horses that can?