Sleep is divided into periods of so-called REM sleep, characterized by rapid eye movements and dreaming, and longer periods of non-REM sleep. Neither kind of sleep is at all well understood, but REM sleep is assumed to serve some restorative function of the brain. The purpose of non-REM sleep is even more mysterious. The new experiments, such as these described for the first time at a recent meeting of the Society for Sleep Research in Minneapolis, suggest fascinating explanations for the purpose of non-REM sleep. For example, it has long been known that total sleep deprivation is 100% fatal to rats, yet, upon examination of the dead bodies, the animals look completely normal. A researcher has now cast light on the mystery of why the animals die. The rats develop bacterial infections of the blood, as if their immune systems -- the self-protecting mechanism against disease -- had crashed.