These two words that have somewhat similar meanings; but it’s the distinction between the two, the detail of the difference that piques our interest.
“None shall be weary nor stumble among them; none shall slumber nor sleep…” (Isaiah 5:27).
“Behold, he that keepeth Israel shall neither slumber nor sleep” (Psalm 121:4).
Sleep is ‘deeper’ than slumber; it’s a change of state, a condition where all bodily systems rest. We read where Adam and others were in a deep sleep; never will we read a deep slumber. The kings “slept” with their fathers. Lazarus was sleeping (dead); he was not slumbering. Slumber is more of a drowsy state, typically employed as a negative characteristic, closer to lazy and inattentive than to the bodily restoration of sleep.
Notice the use in Proverbs 24:33: a little sleep, a little slumber…these are characteristics of the slothful (v. 30). Slumber, a precursor for sleep, is associated with slothfulness.
Again, Proverbs 6:10: a little sleep, a little slumber…connected with the sluggard (v. 9). Even the first 3 letters (SLU) of sluggard and slumber suggest some kind of etymological connection.
Furthermore, note how slumber is used and how the virgins are rebuked for not watching (they were drowsy, not diligent): “While the bridegroom tarried, they all slumbered and slept” (Matthew 25:5). They should have been watching (v. 13).
God does not slumber (recline from his attention, cease watching, doze off) nor sleep (be in a state of regenerative unconsciousness). While we need regenerating sleep, let us also recognize that slumber is like tares in the field of necessary sleep: it seems the same, and modern bibles/ dictionaries may indicate they are synonymous, but there is clearly a difference. Therefore, let us put away the things that cause drowsiness. Let us not slumber, but rather be watching, ready. Slumber not.