Albert Barnes Commentary Psalms 139:8

"If I ascend up into heaven, thou art there: If I make my bed in Sheol, behold, thou art there." — Psalms 139:8 (ASV)

If I ascend up into heaven – The word “heaven” here, in the original, is in the plural number – “heavens” – and includes all that there is above the earth – the highest worlds.

If I make my bed – Properly, “If I strew or spread my couch.” This means if I should seek that as the place where to lie down.

In hell – Hebrew, “Sheol.” See the notes at Isaiah 14:9, where the word is fully explained.

The word here refers to the underworld – the abodes of the dead. In the psalmist’s understanding, it corresponds in depth with the word “heaven” in height. The two represent all worlds, above and below; and the idea is that, in neither direction, above or below, could he go where God would not be.

Thou art there – Or, more emphatically and impressively in the original, “Thou!” That is, the psalmist imagines himself in the highest heaven, or in the deepest abodes of the dead – and behold! God is there also! He has not gone from “him”! He is still in the presence of the same God!