Giraffes can go without sleep for weeks, while brown bats sleep for nearly the entire day. The golden dormouse carefully balances itself on the branch of a tree to sleep, and any quiver of the twig wakes it up immediately.
From the miniscule tree shrew to the most physically imposing of mammals, animals have varying sleep patterns and habits. Rats have similar sleep needs to humans, requiring rest to become alert and learn new tasks for the upcoming day. Certain canines have even helped scientists in treating serious sleep disorders.
"The only way to understand human sleep is to study animals," says Jerome Siegel, PhD, professor of Psychiatry at the UCLA Center for Sleep Research. "If we could better understand animal sleep, we could better understand the core aspects of sleep."
The common denominator of both (non-human) mammals and humans is the existence of rapid eye movement (REM) sleep, the sleep state that is associated with dreams. Both humans and all other mammals display the same level of brain activity and increased heart rate variability during REM sleep. For example: dogs often bark or twitch their legs during REM sleep; platypuses make movements imitating the process where they kill crustacean prey before eating it; and humans often talk in their sleep.
"[Mammals] all have the same fundamental sleep cycle," says Adrian Morrison, DVM, PhD, professor of Behavior Neuroscience at the University of Pennsylvania Veterinary Center. "During REM sleep, you see the same kind of eye movement, paralysis and twitching across species."
Scientists still don't know—and probably never will—if animals dream during REM sleep, as humans do. "How can you prove that another person has dreams? You ask them," says Siegel.
Scientists do know, however, that the brain wave pattern during REM sleep among animals is similar to humans.
Sleep schedules also greatly vary from animal to animal. Siegel proposes that these differences are based on the brain metabolism rate of the animal. Smaller animals, who often have higher rates of brain metabolism, tend to require more sleep, while larger animals generally get less