How do fish sleep? If they are swimming all the time.

Question: How do fish sleep? If they are swimming all the time.
- Keywords:
  fish,
  sleep,
  swim
Asked by Guti7713 to Ines on 9 Mar 2017.
- Ines Goncalves answered on 9 Mar 2017:
  
  Fish only really half-sleep. They sleep with one half of their brain then the other. This means that there is at least one half of their brain that is always alert and on the look out for predators. Smart! Some fish, like some shark species, need to swim even when they sleep, because they need water to go through the gills on the side of their heads, to take up oxygen. This is how they breathe, and if water doesn’t go through their gills they suffocate.

Question: How do fish sleep? If they are swimming all the time.