to sleep: to fall into and rest in full bodily sleep, the deep nightly rest as opposed to light dozing
This is the ordinary Hebrew word for real, deep sleep, the kind of nightly rest a person sinks into, as distinct from merely drowsing or nodding off. In story after story it comes with its natural bookends: a person lies down, sleeps, and then wakes.
From that plain base the word stretches in several directions. It can picture safe, trusting rest, someone lying down in peace because they are protected. It can serve as a test of watchfulness, since a guard, an army, or even God can be described as sleeping or, more pointedly, as never sleeping. And it becomes a gentle way of naming death, the still rest of the grave.
That last picture has a story of its own across the Bible. Job longs for it as quiet relief; Jeremiah slams it shut over fallen Babylon, calling their end a sleep from which they will never wake; and centuries later the promise is pried open, that many who sleep in the dust will wake again. Because sleep by its nature points toward waking, it became the natural language of hope beyond death, later carried into Greek as "those who have fallen asleep."
The picture of a sleeping God is handled with special care. Israel could dare to cry out and accuse God of sleeping in a lament only because it was certain he never truly sleeps; the same image thrown at a rival god was pure mockery.