Sleep is divided into periods of so-called REM sleep, characterized by rapid eye movements and dreaming, and longer periods of non-REM sleep.  51  kind of sleep is at all well-understood, but REM sleep is  52  to serve some restorative function of the brain. The purpose of non-REM sleep is even more  53 . The new experiments, such as those  54  for the first time at a recent meeting of the Society for Sleep Research in Minneapolis, suggest fascinating explanations  55  of non-REM sleep.

For example, it has long been known that total sleep  56  is 100 percent fatal to rats, yet,  57  examination of the dead bodies, the animals look completely normal. A researcher has now  58  the mystery of why the animals die. The rats  59  bacterial infections of the blood,  60  their immune systems—the self-protecting mechanism against disease—had crashed.