Charles Ellicott Commentary Psalms 121:4

"Behold, he that keepeth Israel Will neither slumber nor sleep." — Psalms 121:4 (ASV)

Slumber nor sleep. —This repetition, with the addition of a synonym, offers a very good instance of the step-like style supposed by many critics to give their name to these psalms. But it must be carefully noticed that there is no climax in the force of the two words, the first, if anything, being the stronger. It is used of the sleep of death (Psalms 76:5).