Hazrat Moulana Ashraf Ali Thanwi (rahmatullahi ‘alaih) once mentioned:
People have become very brazen and bold in committing sins, and these sins thereafter result in various spiritual illnesses. Through committing sins, the heart continues to lose its noor and lustre and the darkness within it increases.
Sins, in their very nature, contain darkness, and create the effect of darkness within a person’s life as well. In one hadeeth, Rasulullah (sallallahu ‘alaihi salam) mentioned that when a person commits a sin, a black stain of darkness appears on his heart. If he sincerely repents to Allah Ta‘ala for committing the sin, then Allah Ta‘ala removes the stain of darkness from his heart. However, if he persists in committing the sin, then the black stain of darkness begins to spread, until it eventually engulfs his entire heart. (Sunan Tirmizi #3334)
In regard to this black stain of darkness that settles in the heart, Allah Ta‘ala mentions in the Quraan Majeed:
کَلَّا بَلۡ رَانَ عَلٰی قُلُوۡبِهمۡ مَّا کَانُوۡا یَکۡسِبُوۡنَ
No! Rather, the stain of that which they earn (through committing sins) has covered their hearts. (Surah Mutaffifeen v. 14)
Moulana then quoted the following poetry:
ہر کنہ زنگے ست مرات دل دل شود زيں زنگ ہاخوار وخجل
چوں زيادت گشت دل راتيرگي نفس دوں رابيش گردوخیرگي
Every sin forms a black stain of darkness on the surface of the heart, due to which the heart becomes disgraced.
When the darkness in the heart increases due to the black stain increasing, then the uneasiness and confusion within this wretched soul increases (i.e. he remains restless and unhappy).
(Malfoozaat Hakeemul Ummat 1/58)