REM's biobehavioral characteristics are paradoxical in that its physiologic correlates appear to be injurious to the health of the organism while its brain correlates suggest social-emotional functions. Unlike REM, NREM biobehavioral characteristics are slightly less paradoxical but still enigmatic. NREM’s physiologic functions may be related to immune system function while its electrophysiologic properties are clearly related to the restorative functions of sleep. Both REM and NREM sleep likely participate in memory processing but so does the waking state. The fact that NREM appears to be associated with the gradual deactivation of a select group of brain structures that are then reactivated during REM suggests that the two sleep states either work in harmony with one another to maintain optimal brain function or that NREM undoes something that REM instantiates.