Dolphins can sleep with half their brain at a time, allowing them to be aware of predators.
Dolphins' unique ability is the reason why the mammals can appear to stay awake for weeks at a time.
The study used two dolphins and looked at how they used escholocation, a kind of biological sonar, at different time spans.
The researchers made them chase phantom sonar objects.
They found that the dolphins were able to use their sonar for up to 15 days straight with no interruption or impairment.
Smithsonian said that they are able to do this because they only sleep with one hemisphere of the brain at a time.
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The ability to keep half the brain turned on is likely an evolutionary adaptation to protect against sharks.
"These majestic beasts are true unwavering sentinels of the sea," said Brian Branstetter, of the National Marine Mammal Foundation, according to Live Science.
"After being awake for many hours or days, humans and other animals are forced to stop all activity and sleep."
"Dolphins do not have this restriction, and if they did, they would probably drown or become easy prey," he added.
The study was published in PLoS One journal.