Dolphins can stay alert and active for 15 days or more by sleeping with one half of their brain at a time, scientists in the United States have learned.
The trick of keeping half the brain continuously awake is vital to the sea mammals' survival, experts believe.
It allows them to come to the surface every so often to breath, and remain constantly vigilant for sharks.
Scientists in California tested the ability of two bottlenose dolphins to echolocate accurately over periods of time which would have left other animals sleep-deprived and exhausted. The dolphins, a male called Nay and female called Say, had to swim around a pen looking for phantom sonar targets.
Each of the eight targets consisted of a device that picked up dolphin sound pulses and sent back "phantom" echoes. When a dolphin detected an echo from an activated target, it responded by pressing a paddle. Correct detection triggered a tone, signalling success, and the dolphin was rewarded with a fish. False alarms led to no tone and no reward.
Over three sessions of five continuous days both dolphins did well, with success rates of up to 99%, but Say outperformed her male partner. The scientists then went on to test Say further by repeating the same experiment over a period of 30 days.
In the event, a storm cut the trial short after 15 days had elapsed. However, during this time Say's performance hardly deteriorated at all. The findings were published in the online journal Public Library of Science ONE.
Lead researcher Dr Brian Branstetter, from the National Marine Mammal Foundation in San Diego, said: "These majestic beasts are true unwavering sentinels of the sea. The demands of ocean life on air breathing dolphins have led to incredible capabilities, one of which is the ability to continuously, perhaps indefinitely, maintain vigilant behaviour through echolocation."
The scientists wrote: "From an anthropomorphic viewpoint, the ability of the dolphin to continuously monitor its environment for days without interruption seems extreme. However, the biological, sensory and cognitive ecology of these animals is relatively unique and demanding.
"If dolphins sleep like terrestrial animals, they might drown. If dolphins fail to maintain vigilance, they become susceptible to predation. As a result, the apparent 'extreme' capabilities these animals possess are likely to be quite normal, unspectacular, and necessary for survival from the dolphin's perspective."