Musar zu Tehillim 104:20

This is what our sages said: "'You make darkness, and it is night' (Ps. 104:20) - this refers to this world which is analogous to night." (Bava Metzia 83b).

For after this has become clear to them and likewise after it has become clear to them that the means to perfection are good deeds and traits, they will certainly never consent to diminish these means or be lenient in them. Since, it has already become clear to them that if they diminish in these means or are weak in these means, not employing the full force necessary, they will not attain the true perfection. Rather, it will be reduced in proportion to their reduction in exerting themselves to the necessary extent, leaving them lacking in perfection which is a great calamity and great evil to them.

The pinnacle of this level called "fear of sin", one of the greatest levels, is when a man is constantly afraid and worried lest he have in his hand some trace of sin which obstructs him from the perfection that he is under duty to strive for.