Sleep is divided into periods of so-called REM sleep, characterized by rapid eye movements and dreaming, and longer periods of non-REM sleep.【C1】______ kind of sleep is at all well-understood, but REM sleep is【C2】______ to serve some restorative function of the brain. The purpose of non-REM sleep is even more【C3】______ . The new experiments, such as those【C4】______ for the first time at a recent meeting of the Society for Sleep Research in Minneapolis, suggest fascinating explanations【C5】______ of non-REM sleep. For example, it has long been known that total sleep【C6】______ is 100 percent fatal to rats, yet,【C7】______ examination of the dead bodies, the animals look completely normal. A researcher has now【C8】______ the mystery of why the animals die. The rats【C9】______ bacterial infections of the blood,【C10】______ their immune systems—the self-protecting mechanism against disease—had crashed. 【C4】