Features

• Some animals, particularly sea mammals, experience Unihemispheric Slow-wave Sleep, which is basically a form of being half-awake. During this stage, only half of the brain falls asleep, allowing the other half to stay alert to the arrival of predators or to surface every few minutes to breathe. USWS is common in dolphins, manatees, seals and some lizards. In some species, one eye remains open to check the environment, while the other one is closed. According to experts, animals who experience USWS are considered mammals who don't truly sleep.
  
  

Benefits

• While most newborns--human and other--sleep an average of about twice as much than adults, dolphins and killer whales do not sleep at all during the first month of their lives. This gradually changes as they get older and into more settled patterns, but in the first 30 days they move around in what experts believe is a strategy to avoid predators.
  
  

Considerations

• Only mammals and birds experience REM sleep, the deepest form of sleep. Reptiles and amphibians do not, which experts believe is directly tied to their level of intelligence and cognitive abilities. However, two mammals seem to defy that theory: dolphins, who never achieve it, and platypuses, which sleep more than any other mammal but still rate low in the intelligence scale.
  
  

Types

• There are marked differences in the sleeping patterns of mammals according to their size and their diets. For example, herbivores sleep a lot less than carnivores, and small mammals sleep more than large ones, with just a few exceptions (such as lions). This may be due to their need to stay alert to evade predators or the need to keep warm as outside temperatures drop. Animals also sleep a lot less in winter because body temperature is reduced during sleep and small mammals need to conserve as much body heat as possible to survive the outdoors.
  
  

Expert Insight

• On the other side of the spectrum are the mammals who spend most of their lives sleeping. Aside from hibernation, considered a form of sleep, lions and bats have the record for more hours of sleep in a single day: 19 and 17 hours, respectively. The North American opossum, the owl monkey, the tiger and the squirrel all sleep more than 15 hours per day, despite season or age.