Do dolphins sleep?

| David Lewis | Nature

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Yes and no. Dolphins sleep, but not in the way you and I define sleep. 

Dolphins do not breathe spontaneously. In other words, they have to actively decide when to breathe. Hence, in order to breathe and survive, they have to remain conscious, which essentially contradicts sleeping as we know it. 

To get around this, dolphins practice a unihemispheric sleep. Namely, only one-half of their brains sleep at a time; the other half stays alert to enable the dolphin to continue breathing and look out for dangers.

They literally sleep with one eye open! 

Bonus fact: This lack of ability to spontaneously breathe underwater is probably due to the fact that dolphins are not fish, but mammals – like you and me!  Indeed, another famous mammal that moved his habitat, the whale, has to think about every breath it takes and scientists believe that, like dolphins, they sleep with one eye open and one half of their brain awake.

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And for all of the marine mammals enthusiasts, here are some more facts:

Marine mammals are an example of a group of animals that have made an extreme transition from one living environment to another. This type of transition, from land to sea, is quite rare in evolution. However, transition back from land to water – that is to say for animals that initially live in the sea – is more common and has been performed several times by:

  • Reptiles (lizards, snakes, turtles and crocodiles)
  • Birds (such as penguins) 
  • Marine mammals, especially whales and dachshunds that do not ascend to land at any stage of their lives. 

This is a complex transition because they had readjust themselves both biologically and behaviorally to life underwater. 

Fish swim by moving their tails side to side right to left and vice versa. But Marine mammals are unable to do so since their spine was originally evolved to fit a terrestrial environment. That is why while the structure of the fish tail is mostly vertical while the fin-like structure of marine mammals is horizontal. Therefore the typical form of swimming is in moving the back up and down, rather than side to side. 

And for all of the marine mammals enthusiasts, here are some more facts:

Marine mammals are an example of a group of animals that have made an extreme transition from one living environment to another. This type of transition, from land to sea, is quite rare in evolution. However, transition back from land to water – that is to say for animals that initially live in the sea – is more common and has been performed several times by:

  • Reptiles (lizards, snakes, turtles and crocodiles)
  • Birds (such as penguins) 
  • Marine mammals, especially whales and dachshunds that do not ascend to land at any stage of their lives. 

This is a complex transition because they had readjust themselves both biologically and behaviorally to life underwater. 

Fish swim by moving their tails side to side right to left and vice versa. But Marine mammals are unable to do so since their spine was originally evolved to fit a terrestrial environment. That is why while the structure of the fish tail is mostly vertical while the fin-like structure of marine mammals is horizontal. Therefore the typical form of swimming is in moving the back up and down, rather than side to side.