I'm half asleep, half awake
In some animals, only half of the brain falls asleep. This allows them to survive in difficult conditions. But people also sometimes show a similar behavior - when they sleep in a strange bed for the first time!
As long as we are awake, we constantly move various muscles and actively perceive our environment. When we sleep, on the other hand, we remain paralyzed most of the time and lose consciousness. The difference can be seen in the electroencephalogram (EEG): A deep sleep phase, for example, can be recognized by the slow, large waves; in the waking state, the EEG waves are significantly faster and run with less deflection.
Because sleeping animals are easy prey for predators, some species such as dolphins and seals, but also some bird species, have developed a special characteristic: they only have one hemisphere of their brains dormant while the other remains alert. Some of them even keep an eye open. Some animals only fall into a unihemispheric slow-wave sleep (USWS) in certain situations, otherwise both hemispheres of their brains are asleep.
One of the first to discover semilateral sleep in animals was the neuroscientist John C. Lilly (1915–2001), who became known for his eccentric self-experiments. In the 1960s, he observed that dolphins only closed one eye at a time when they were resting. Lilly assumed the animals would still survey their surroundings and listen to sounds even when they were asleep. It was only later experiments that brought to light what is actually going on in her brain…