מַה שֶׁנּוֹהֲגִים שֶׁלֹּא לִלְמוֹד בְּנִיתְּל, שָׁמַעְתִּי מֵאַאַמוּ"ר הַטַּעַם

I heard from my revered father, the Rebbe [Rashab], that the reason that one customarily refrains from studying [Torah] on nittel night

שֶׁהוּא כְּדֵי שֶׁלֹּא לְהוֹסִיף חַיּוּת

is to refrain from adding vitality [to the kelipos]. 

פַּעַם אָמַר אַאַמוּ"ר

Once my revered father, the Rebbe [Rashab], said:

דִי מַתְמִידִים וואָס דִי אַכט שָׁעָה אִיז זֵיי נוֹגֵעַ בְּנֶפֶשׁ, האָב אִיך נִיט האָלט

“I do not care for those scholars who are so [unduly] diligent that the loss of those eight hours3 touches their very souls.”

וְהוּא רַק עַד חֲצוֹת לַיְלָה

[We refrain from studying] only [from sunset] until midnight.

Text and Translation courtesy of Sichos in English