Charles Ellicott Commentary Psalms 121:6

"The sun shall not smite thee by day, Nor the moon by night." — Psalms 121:6 (ASV)

Smite you. —The mention of shade leads to the amplification of the figure. The evil effects of sunstroke are too well known to need comment. They are often mentioned in the Bible (2 Kings 4:18; 2 Kings 4:20; Jonah 4:8).

Nor the moon by night. —Possibly there is an allusion to the belief, so common in former times, of the harmful influence of the moon’s light—a belief still recalled in the word lunacy. It is a fact that temporary blindness is often caused by moonlight (see authorities referred to by Ewald and Delitzsch).

Others, again, think that the injurious cold of the night is here placed in antithesis to the heat of the noonday sun (Jeremiah 36:30), the impression that intense cold burns being common in the East, as indeed everywhere. Tennyson speaks of the moon being “keen with frost.” But it is also possible that the generally harmful effects of night air are intended.