Sleep happens in repeating periods, in which the person body varies between two different modes: REM sleep and non-REM sleep.
Vurhongo byi rhendzeleka hi kuya hi nkarhi, laha miri wu tikumaka e xikarhi ka minxaka yimbirhi ya vurhongo: Vurhongo lebyi fambelanaka na milorho (REM), na vurhongo byorhula (no-REM).