Theories of REM and NREM functions we have been assuming that the sleep state is doing something for the wake state; i.e., that NREM SWS restores energy for waking consciousness or that REM supports emotional memory consolidation for waking consciousness. But it is also possible that the functions of REM and NREM have more to do with the sleep states themselves rather than with waking consciousness. REM may be undoing something that NREM is doing since REM typically follows NREM in the sleep cycle. Or conversely, NREM SWS may be doing something important for the organism (e.g., immune system repair) but that function is costly, so REM functions to complete, complement, repair, or undo something that NREM had to do to accomplish its primary functions. In this scenario, SWS sleep repairs the immune system each night, but that is so onerous a job that NREM then requires REM to restore NREM’s functional capacity so that it can do its immune system repair again the following night.