May 6 (HealthDay News) -- Plenty of people sometimes feel as if they're half-asleep at their desks, but dolphins seem to have really mastered that state of mind.
Dolphins are able to put half of their brains to sleep while the other half remains conscious, an evolutionary trick that lets them rest while continuing to surface for air and stay alert to predators.
New research shows that dolphins can maintain round-the-clock vigilance for at least five days without experiencing any physical signs of sleep deprivation.
Researchers from the U.S. Navy Marine Mammal Program in San Diego and Tel Aviv trained two dolphins to respond to a 1.5-second beep sounded randomly against background noise of half-second beeps every 30 seconds. The beeps were low in volume and barely noticeable as the dolphins swam through their enclosure, according to the study, published in the May 1 issue of the Journal of Experimental Biology.
Even after five days without a break, the dolphins' response to the auditory cue remained as sharp as it was at the outset of the test.
Researchers also tested the dolphins' visual vigilance during half-sleep by training the dolphins to recognize and respond to a visual stimulus of three horizontal red bars or one vertical green bar.
Dolphins' binocular vision is limited, because their eyes are on opposite sides of their heads.
They first trained one dolphin's right eye to recognize either the three horizontal red bars or the vertical green bar. Since half of the brain is asleep during testing, researchers reasoned that the dolphin would see the shapes only through the eye connected to the conscious half of the brain.
When they began training the left eye, the researchers said, they were surprised when the dolphin already recognized the shapes. The information must have been transferred between the two brain hemispheres, they concluded.
The researchers also enticed the dolphins into a bay at night where they could be shown the horizontal and vertical bar shapes. As with the auditory response, the dolphins were as sharp at the end of five days as they had been at the beginning.
When the researchers checked the dolphins' blood for signs of sleep deprivation, they found none.
SOURCE: Journal of Experimental Biology, news release, May 1, 2009