Dolphins can stay awake for 15 days or more - by sleeping with one half of their brain at a time, scientists have learned.
Researchers in America tested two bottlenose dolphins' responses to echoes as they swam around a pen.
The mammals kept going for five days non-stop in three separate tests, and one even managed 15 days in another trial.
Experts think the ability is an important survival skill for dolphins.
It means they can surface to breathe when they need to and keep watch for shark attacks.
Dolphins use sound waves to find their way, hunt prey, detect predators and organise groups.