Yes, dolphins do sleep when they are tired or need to rest, but not in the same way that humans or other land mammals sleep.
Unfortunately dolphins do not have the same luxury as other animals when it comes to getting a good nights rest.
In order to the answer question of whether or not dolphins sleep and how they rest we first need to examine the factors that affect a dolphin’s ability to sleep.
Dolphins are marine mammals and share many of the same characteristics with land animals (such as humans) including the need to breathe air.
Because humans and other mammalian species live on land it is much easier for us to breathe and we do not have to be consistently conscious of our environment or our need for oxygen as we do not have to worry about the possibility of drowning when we go to sleep since there is no water around us that could prohibit our breathing.
In addition to this most humans are also relatively safe from being attacked by other animals when they rest, so its much easier to get a good nights rest when we need to.
Dolphins on the other hand spend their entire lives in the ocean, and while both the dolphin species and fish live in the water fish are born with gills that allow them to pull oxygen directly from the water.
Dolphins do not have this luxury because they are not born with gills; instead they have lungs which means they must always be aware of their surroundings and need to obtain oxygen which can only be found above the surface of the water.
If their oxygen level becomes too low the dolphin must come to the surface of the water in order to get more oxygen otherwise it risks the possibility of drowning.
And even though dolphins can hold their breath for extended periods of time when compared to most land animals they are still very limited in their ability to stay submerged without oxygen.
In order to a better understand what happens when a dolphin sleeps studies using electroencephalography have been performed on dolphins and have shown that when a dolphin rests half of its brain shuts down similar to a humans brain when we sleep.
The other half of the brain however remains active allowing the dolphin to remain semi conscious of their environment in order to prevent the dolphin from drowning and allow it to monitor its oxygen supply.
While it’s possible for dolphins to rest in most environments (including below the surface of the water) most dolphins will be found resting near the surface in order to give them the best opportunity to acquire oxygen if they need to.
Aside from having to consistently be aware of their oxygen level dolphins must also remain aware of potential predators in the area, which also limits their sleep and requires them to remain at least partially aware of their surroundings.
Both of these factors (oxygen and threats) play a significant role in why dolphins rest so differently than humans and land animals.
Note: Because dolphins are constantly aware of their need for oxygen marine biologists refer to dolphins as conscious breathers.