How do Dolphins Sleep???

How Do Dolphins Sleep?

While sailing or cruising with MANA Cruises, we come across Hawaiian Spinner Dolphins almost daily along the south and west shores of Oahu, sometimes even multiple times a day!  Seeing them is always a joy and never gets old.  We will normally come across 20-40 dolphins in a pod in the mornings and when we do, it’s almost always in shallow waters of about 30-40 feet deep – and they’re usually very docile.  During the evenings, we will see them in much deeper waters and much more active. One thing you may notice is that we never come across dolphins that seem like they’re sleeping.  They’re always moving around and need to swim to breath… 

So how do they sleep, and how do they sleep without drowning?! 

Hawaiian spinner dolphins, like all dolphins, are known for their ability to remain active for long periods of time without rest. This is because dolphins, unlike most mammals, do not sleep in the same way that humans do. Instead of entering a state of unconsciousness, dolphins enter a state of “sleep” where one half of their brain remains active and alert while the other half enters a state of rest. This is known as unihemispheric slow-wave sleep.

Unihemi-what?

During unihemispheric slow-wave sleep, dolphins are able to swim around and remain aware of their surroundings while still getting some rest. This is important for dolphins because they are social animals and need to be constantly on the lookout for predators. They also need to be able to breathe at the surface of the water, which requires them to remain conscious.

Another reason they may need to enter unihemispheric slow-wave sleep is due to the fact that their calves don’t sleep for the first couple of weeks that they are alive!! During periods of sleep, the eye on the opposite side of the sleeping hemisphere is almost always closed. This eye closing behavior was almost never seen in newborn dolphin and killer whale calves or their mothers for the first few weeks after the calves were born. The frequent turning and surfacing behavior of the calves also make it less likely that they got any sleep.

Come witness this in action on one of our morning trips out of Ko Olina or Waikiki!

Sources:

• “Dolphin Sleep: How Do Dolphins Sleep Without Drowning?” – National Geographic (https://www.nationalgeographic.com/animals/mammals/d/dolphin/dolphin-sleep/)
• “Dolphin Sleep: How Do They Do It?” – MarineBio Conservation Society (https://marinebio.org/animals/dolphins/dolphin-sleep/)
• “Newborn Dolphins and Orcas Don’t Sleep: Their Mothers Don’t Sleep Much Either” – U of Washington
  (https://faculty.washington.edu/chudler/ordol.html)S