All the animals that sleep less than four hours on average are large land mammals, such as the elephant, cow, and giraffe. Land-grazing animals also use so much time eating, they don't have much time left for sleep.
Horses stand 98% of the time, making it difficult to find a time to obtain REM sleep.
Other experts, like Morrison, believe sleep schedules are often set based on the danger each animal faces. Some birds sleep with one eye open, with the open eye keeping track of potential predators in its midst. REM sleep is deliberately short in birds, lasting only seconds, so they can reduce the risk of being attacked when they are stationary.
"Birds primarily sleep from an anti-predator point of view. Birds do sleep in vulnerable areas. If birds had a lot of REM sleep, they'd be putting themselves in danger," says Charles Amlaner, Jr., PhD, Director of Animal Research at Indiana State University. Dr. Amlaner also notes that birds exposed on the edge of a group are much more likely to keep one eye open when sleeping than birds protected by the flock.
Mammals also factor danger in their sleep patterns. To avoid predators, the African Papio papio baboon sleeps on its heels at the tops of trees in an awkward position that makes it difficult to get a sound sleep.
Other animals prefer safer spots so they can get more sleep. Marine mammals have unique sleep habits. When most species of marine mammals are asleep, there is always one hemisphere of their brain that is awake. This allows dolphins, for example, to swim and surface to breathe when they are sleeping. They enjoy the benefits of sleep but still perform many of the same processes done when awake. Experts believe that understanding marine mammals' sleep is a key to understanding sleep in humans. "What are the functions that the dolphin needs to do to become well-rested?" asks Siegel. "That's the mystery."
For many years, scientists struggled to identify the brain abnormality in humans that causes narcolepsy. Little did