London, Oct 18 (IANS) Scientists have revealed that dolphins can stay awake for more than 15 days by closing down half their brain at a time.
It allows the mammals to come to the surface when they need to breathe and in staying vigilant against sharks, The Sun reported.
Researchers kept two bottlenose dolphins -- "a male called Nay and a female called Say" -- hunting phantom prey over lengths of time that would have left other animals sleep deprived and exhausted.
The creatures, which find food by sonar (a sound-based system for the detection of objects under water), were bombarded with man-made echoes for five days at a time and found the targets with a success rate of up to 99 percent.
Say, who outperformed her male partner in San Diego, California, was then tested for 15 days and her performance hardly faltered.
Scans confirm that dolphins are capable of "unihemispheric sleep" -- "snoozing with one side of the brain" while keeping an eye open.
"These majestic beasts are true unwavering sentinels of the sea," said researcher Brian Branstetter.