These two words mean the same thing. Here to "slumber" means to stop protecting. The negative form strengthens the statement. Alternate translation: "will not sleep and stop protecting you" or "will always protect you" (See: [[rc://en/ta/man/translate/figs-doublet]] and [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-doublenegatives]])