Bars protecting your window from intruders – סורגים – have large gaps between them to let in the light and air.
This is the idea behind the originally-Mishnaic Hebrew word לסירוגין , on and off or intermittently: there are gaps in time between events, such as moments of sleep.
היא זקוקה למנוחה – היא ישנה רק לסירוגין.
She’s in need of rest – she’s been sleeping only on and off.
הם זוג – לסירוגין.
They’re a couple – on and off.