Dolphins can remain alert for two weeks at a time with no sign of fatigue because only half of their brains sleep at a time, U.S. researchers say. Researchers at the National Marine Mammal Foundation in San Diego and colleagues found dolphins can use echolocation with near-perfect accuracy continuously for as long as 15 days, identifying targets and monitoring their environment. The researchers studied two dolphins, one male and one female, and found that they were capable of this task with no signs of fatigue for 5 days, and the female dolphin performed additional waking tasks for a 15-day period. How much longer they could have continued was not studied. Writing in the journal PLoS ONE, they said sleeping with only one half of the brain at a time -- known as unihemispheric sleep -- probably evolved in dolphins to enable them to breathe at the surface of water even when half-asleep. The new study suggests the need to remain vigilant may also have played a role in the evolution of this \"half-awake\" sleeping behavior. \"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 behavior through echolocation,\" foundation researcher Brian Branstetter said.