Bottlenose Dolphins
Marine animals frequently experience monohemispheric sleep. The bottlenose dolphin is able to get the necessary restorative sleep that it needs without drowning.
Beluga Whales
These natives of the southern hemisphere can avoid predators by sleeping with one eye open, and it also guarantees that they never go without food.
Blackbirds
Many animals find birds to be easy food, especially medium-sized bird species like blackbirds. These animals may relax while keeping one eye open and avoid becoming prey.
Peregrine Falcons
Despite being somewhat larger than blackbirds, these birds still need to be on the lookout for larger predators like the great horned owl and golden eagle.
Penguins
Penguins often snooze while standing up and keeping one eye open. These animals benefit from unihemispheric slumber by shielding their young from vicious predators.
Mallard Ducks
This species places a strong emphasis on maintaining alertness when sleeping. Predators prey on more than half of the mallard duck population during their early years.
Fruit Bats
The earliest non-marine animal to exhibit the characteristic appears to be fruit bats. While they sleep, these bats keep an eye out for any lurking predators.
Crocodiles
These giants have very few predators, so there may be other reasons they sometimes sleep with an open eye.