Why did Jesus compare death to sleep & what kind of sleep was he talking about?

The condition of the dead is made clear at Ecclesiastes 9:5, 10, where we read: “The dead know nothing . . . There is no pursuit, no plan, no knowledge or intelligence, within the grave.” (Moffatt) Death, therefore, is a state of nonexistence. The psalmist wrote that when a person dies, “he goes back to his ground; in that day his thoughts do perish.” (Psalm 146:4) The dead are unconscious, inactive.