填空题

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.