Dolphins can stay alert and active for 15 days or more by sleeping with one half of their brain at a time, scientists 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, US, 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 . . .