Complex sentence: Sleep is a naturally recurring state characterized by reduced or absent consciousness and proceeds in cycles of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep, the order normally being N1 → N2 → N3 → N2 → REM.
Split into simpler sentences: Sleep is a naturally recurring state characterized by reduced or absent consciousness and proceeds in cycles of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. The normal order of sleep stages is N1 → N2 → N3 → N2 → REM.