REM sleep (the periods of rapid eye movement during normal sleep) occurs in all species of mammals.

The amount and depth of REM sleep time varies depending on the species' stage of neural development, but all mammals appear to experience negative side effects when denied adequate REM sleep.

Dreaming occurs during the REM stage, and researchers believe that REM sleep is the brain's self-activation mechanism.

Because babies of all mammalian species exhibit much more brain activation in sleep than do adults of the same species, scientists think REM sleep is also an important part of brain development.

Before birth, human fetuses spend nearly 24 hours a day in a first-level state of REM sleep.

Shortly after birth, they require an average of at least 8 hours of REM sleep per day.

Researchers believe babies go "offline" with such long periods of REM sleep so their brains can continue to develop.