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.