Better than Navy SEALs, these dolphins showed no signs of fatigue after 5 days without sleep (Image: US Navy)

Remaining vigilant for five days and nights without a break would reduce any human to an incoherent, sleep-deprived daze – but dolphins can string together all-nighters without any detectable mental impairment.

Sam Ridgway, a marine mammal biologist at the University of California, San Diego, and his colleagues trained captive dolphins to listen to a series of broadcast sounds. Most of the time, a short beep sounded every 30 seconds, but a few times per hour it was replaced by a slightly longer beep. When that happened, the dolphin had to press a lever for a food reward.

Previously, Ridgway’s team had shown that dolphins could stay alert and perform this task for up to 120 consecutive hours without any decline in accuracy.


However, sleep-deprived humans show their fatigue most quickly in more complex mental tasks rather than such simple physical skills. To see if this is true of dolphins, too, Ridgway’s team gave the dolphins a more complex task – in which they had to make different sounds in response to two different visual stimuli – at intervals during the 120-hour vigilance test. Here, too, the dolphins showed no sign of losing their sharpness as the days wore on.

The dolphins’ remarkable alertness may result from their ability to sleep on one side of the brain at a time. This unihemispheric sleep allows dolphins and other cetaceans to get the benefits of sleep even as they remain aware of their pod mates and predators – and, presumably, experimental sounds, says Ridgway.

Journal reference: Journal of Experimental Biology (DOI: 10.1242/jeb.027896)