How do dolphins allow half of their brain to fall asleep?

Question: How do dolphins allow half of their brain to fall asleep?
- Keywords:
  brain,
  dolphin,
  rest,
  sleep
Asked by georgieb33 to Charlotte, Jo, Kevin, Louise, Valeria on 17 Jun 2012. This question was also asked by katyyy.
- Joanna Cruden answered on 17 Jun 2012:
  
  it is down to evolution, genetics and survival instincts, if they could not keep half their brain awake they would die
- Valeria Senigaglia answered on 18 Jun 2012:
  
  They slow down their brain activity. Have you ever heard about some person that via deep concentration are able to slow down their heart bit? dolphins do the same with brain waves so half brain can slow down, consume less energy and essentially sleep while the other emisphere keep breathing and vigilance against predators

Question: How do dolphins allow half of their brain to fall asleep?