Sleep is divided into periods of so-called REM sleep, characterized by rapid eye movements and dreaming, and longer periods of non-REM sleep. ________ kind of sleep is at all well-understood, but REM sleep is ________ to serve some restorative function of the brain. The purpose of non-REM sleep is even more ________ . The new experiments, such as those ________ for the first time at a recent meeting of the Society for Sleep Research in Minneapolis, suggest fascinating explanations________of non-REM sleep.
For example, it has long been known that total sleep________is 100 percent fatal to rats, yet, ________examination of the dead bodies, the animals look completely normal. A researcher has now ________ the mystery of why the animals die. The rats ________ bacterial infections of the blood, ________ their immune systems the self-protecting mechanism against diseases―had crashed.