Crocodiles sleep with one eye trained on nearby stimuli, researchers report in the Journal of Experimental Biology. Their observations suggest that crocodiles are engaging in "unihemispheric sleep," which occurs in marine animals, such as dolphins and sharks, and in birds. In marine mammals, this behaviour helps groups stick together even while sleeping (or in the case of sharks, allows them to keep breathing*). Birds do more half-brain sleeping in the presence of a predator, suggesting that they mainly use this strategy to keep safe. Birds may even sleep during flight: with half a brain awake and one eye open, they can continue to navigate on long migrations while also getting some shut-eye.
The researchers in this study looked at juvenile crocodiles, who preferentially kept one eye open and trained on a human stimulus (if there was one present). The observations did not extend to brain activity, so all that we know for sure is that they keep one eye open (and trained on a target) while "sleeping;" however, it seems likely that this is indicative of unihemispheric sleeping.
If so, what does this discovery mean for the evolution of unihemispheric (versus "all-consuming") slumber? One study author says, "To me, the most exciting thing about these results is they provide some evidence to think that the way we sleep might be novel, in an evolutionary sense." That is, maybe we evolved from critters that were capable of half-brain sleeping. Did half-brain sleeping originate independently several times, or did it evolve once in archosaurs (the common relative of birds and non-avian reptiles) or even farther back in evolutionary time?
A common question dots sleep-related discussion boards across the internet: are humans capable of half-brain sleeping? Sadly (or not, depending on your point of view), the answer seems to be no. The closest we can get is through extreme sleep deprivation, or "local sleep in wake," whereby some neurons turn off by themselves (despite their owner still appearing awake and motile). This method is not recommended.
* But see a wealth of odd research on shark sleeping preferences, which reveals that some sharks can actively pump water across their gills while motionless, while others find areas of existing current and camp out. "In 1969, intrepid free diver Ramon Bravo discovered streamlined sharks apparently sleeping in a cave at Isla Mujeres, off the Yucatan Peninsula. Funded by the National Geographic Society, Eugenie Clark and co-workers traveled to Isla Mujeres in 1972 and 1973 to investigate this unprecedented phenomenon. Their field research revealed the species to be the Caribbean Reef Shark (Carcharhinus perezi) and that the water inside these caves have an unusually high oxygen content and reduced salinity, possibly due to freshwater upwellings from the Mexican mainland. The sharks remained in the caves for hours at a time, actively respiring by pumping water over their gills 20 to 28 times per minute. Clark speculates that the enhanced oxygen content may make breathing easier for the motionless sharks and may produce a narcotic effect the sharks enjoy."
Writing in the Journal of Experimental Biology, the researchers say the crocs are probably sleeping with one brain hemisphere at a time, leaving one half of the brain active and on the lookout. Consistent with this idea, the crocs in the study were more likely to leave one eye open in the presence of a human. They also kept that single eye trained directly on the interloper, said senior author John Lesku.