Several species of seals and dolphins sleep in a very unique sort of way. Like humans, their brains are divided into two hemispheres. These animals have the ability to allow one hemisphere of their brains to sleep while the other halves are awake. One hemisphere will show sleeping patterns of slow brain waves while the other will show signs of wakefulness. Whales and manatees are also believed to use unihemispheric sleep, as do even mallard ducks, as reported in 1999 news articles about sleep research.