18. Sharks
Sharks have diverse sleeping habits depending on the species. Some, such as the nurse shark, can rest motionless on the ocean floor, taking breaks from constant movement. Others, like great whites, must keep swimming to breathe and may sleep with only parts of their brain inactive.
This partial rest allows them to stay aware of their surroundings while still getting the downtime they need to survive in the vast, unpredictable ocean. Live Science