Dolphins can stay awake and whip-smart for up to 15 days. Why only 15 days? Because the researchers stopped recording after 15 days. No one really knows how long dolphins can go without sleep.

The researchers studied two dolphins at the National Marine Mammal Foundation. They tested the dolphins' echolocation skills (using sound to see the environment around them) while monitoring their sleep patterns. They made the dolphins swim around their pen looking for sonar targets to keep them awake and active, independent.ie says, the dolphins were rewarded with fish when they performed the task correctly.

After five days of being awake, the researchers saw no signs of fatigue in the dolphins. They repeated the tests multiple times, and with the female dolphin, who was more accurate with the task, they extended the test to 15 days. The study was published today, Oct. 17, in the journal PLoS ONE. They saw no decrease in the dolphins' abilities during these trials.

Dolphins are able to put half of their brain to sleep at a time, which lets them stay at least half-alert all the time. It seems, though, that the dolphins are cognitively aware even when half-asleep. Researchers think they developed this ability so they can stay up at the surface of the water to breathe, without drowning. The new study finding — that the dolphins are also cognitively aware during this time — indicates that the need to stay vigilant in the dangerous oceans also played a part in the evolution of this sleeplessness.

"These majestic beasts are true unwavering sentinels of the sea," study researcher Brian Branstetter said in a statement from the journal. "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."