When thou liest down thou shalt not be afraid. This is beautifully illustrated by what David says in Psalms 4:8, "I will both lay me down in peace and sleep: for thou, Lord, only makest me dwell in safety."
No fear is to be apprehended where Jehovah is Protector (see Psalms 3:5, Psalms 3:6; Psalms 46:1-3; Psalms 91:1-5; Psalms 121:5-8). When, (im) is rendered "if" by the Vulgate, LXX; Targum Jonathan. Thou liest down; tish'kav, "thou shalt lie down," kal future, like shakavta, kal perfect, in the corresponding hemistich, is from shakav, "to lie down," specially to lay one's self down to sleep, as in Genesis 19:4; Psalms 3:6.
Vulgate, si dormieris; cf. Proverbs 6:22, "when thou sleepest" בְּשָׁכְבְּךָ, b'shok'b'ka'. The LXX. rendering, "if thou sittest" ( κάθη), arises from reading תֵּשֵׁב (teshev) for תִּשְׁכַב (tish'kav) Yea, thou shalt lie down; b'shok'b'ta, as before, with] prefixed, equivalent to the future, as in the Authorized Version; LXX; καθεύδῃς.
Shall be sweet; arvah, from arav, "to be sweet," or "pleasant," perhaps "well mixed," as arev, equivalent to "to mix." Thy sleep shall be full of pleasing impressions, not restless, as in Deuteronomy 28:66 and Job 7:4, but sweet, because of the sense of safety, and from confidence in God, as well as from a good conscience (cf.
Job 11:18, "Thou shalt take thy rest in safety," from which the idea is probably taken).