Complex sentence: After about an hour and a half of NREM sleep, the brain waves begin to show a more active pattern again, called REM sleep (rapid eye movement sleep), when most remembered dreaming occurs.
Split into simpler sentences: After about an hour and a half of NREM sleep, the brain waves begin to show a more active pattern again. This sleep state, called REM sleep (rapid eye movement sleep), is when most remembered dreaming occurs.