A fearful stage of debasement. Sin is the condition without which there can be no repose.

Except they have done mischief - The night is their time for spoil and depredation. And they must gain some booty, before they go to rest. This I believe to be the meaning of the passage. I grant, also, that there may be some of so malevolent a disposition that they cannot be easy unless they can injure others, and are put to excessive pain when they perceive any man in prosperity, or receiving a kindness. The address in Virgil, to an illnatured shepherd is well known: -
Et cum vidisti puero donata, dolebas:
Et si non aliqua nocuisses, mortuus esses.
Eclog. 3: 14.
"When thou sawest the gifts given to the lad, thou wast distressed; and hadst thou not found some means of doing him a mischief, thou hadst died."

For they (f) sleep not, except they have done mischief; and their sleep is taken away, unless they cause [some] to fall.
(f) Meaning that to do evil is more proper and natural to the wicked than to sleep, eat or drink.

For they sleep not, except they have done mischief,.... Or they cannot sleep, as Jarchi and Gersom interpret it. Oftentimes they cannot sleep on their beds for devising mischief, their thoughts are so intensely set on contriving wicked schemes; and when they have so done, they cannot sleep until they have executed them; they are continually restless and uneasy day and night, like the troubled sea, constantly casting up mire and dirt. Who would keep such company as these?
and their sleep is taken away, unless they cause some to fall; into the snares and traps they lay for them, or into sin and calamity by it; the former of which they endeavour by all means to draw men into, and the latter is the unavoidable consequence of it. They imitate their father the devil, both delight in sin, and in the ruin of their fellow creatures; it is a sport to thereto do mischief, and they have no pleasure without it; see Proverbs 11:23. What company are such!

The reason is found in the character of sinners, whose zeal to do evil is forcibly depicted (Proverbs 6:4; Psalm 36:5). They live by flagrant vices (Proverbs 1:13). Some prefer to render, "Their bread is wickedness, their drink violence" (compare Job 15:16; Job 34:7).

In the reason here given the perf. may stand in the conditional clauses as well as in Virgil's Et si non aliqua nocuisses, mortuus esses; but the fut., as in Ecclesiastes 5:11, denotes that they (the רעים and the רשׁעים) cannot sleep, and are deprived of their sleep, unless they are continually doing evil and bringing others into misery; the interruption of this course of conduct, which has become to them like a second nature, would be as the interruption of their diet, which makes them ill. For the Kal יכשׁולוּ, which here must have the meaning of the person sinning (cf. Proverbs 4:19), and would be feeble if used of the confirmed transgressors, the Kerı̂ rightly substitutes the Hiphil יכשׁילוּ, which occurs also 2-Chronicles 25:8, there without an object, in the meaning to cause to fall, as the contrast of עזר (to help).

For - They cannot sleep with quietness.