Unihemispherical sleep is an amazing adaptation that has been well studied in several animals, particularly birds. Researchers have used electroencephalograms (EEG) to understand the ability of birds to sleep with only one hemisphere of their brain while the other hemisphere remains active. When one side of the brain is sleeping, the eye on the opposite side of the head is open and alert.
When ducks sleep in a row, those on the end keep one eye open. If a predator is lurking a duck can instantly fly off and alert the rest of the flock. Occasionally they will rotate their bodies to change the awake side so that the other brain hemisphere can get some sleep. Research has shown that these “watch ducks” spend 2 ½ times more in unihemispherical sleep than do the rest of the flock.
It has been postulated that at one time all animals were able to achieve half brain sleep, but this ability has been lost as animals adopt safer places or methods to rest. Studies indicate that humans are also able to achieve unihemispherical sleep during times of severe stress. There may still be a part of our brain that can “switch on” in dangerous situations. So perhaps, there is some truth to the importance of having all your ducks in a row!