In Sefer Igros Sofrim there is a letter from the Chasam Sofer zt”l (Siman 2). He writes that the reason for not learning before Chatzos on that night is as follows.
The Goyim go to church and celebrate Mass at midnight. Since they are awake for their Avoda Zara and the Yidden are asleep, it might be a Kitrug in Heaven that the Goyim stay awake for their god but the Yidden do not stay awake for Hashem. Therefore, he writes, it is an ancient custom that the Yidden go to sleep early and do not learn their nightly Seder and get up to learn at Chatzos so that there will not be a Kitrug.
However, not to learn at all that night is no Minhag at all.
Typical lopsided presentation
He gives short shrift to the gedolim that learn on Nittel, and promotes the other side.
So at what time must I slam the Gemorah shut?
No need to slam it, you could close it gently at the Shkiah, and give it a kush. Why the aggression?
He didn’t research the issue enough.
In Sefer Igros Sofrim there is a letter from the Chasam Sofer zt”l (Siman 2). He writes that the reason for not learning before Chatzos on that night is as follows.
The Goyim go to church and celebrate Mass at midnight. Since they are awake for their Avoda Zara and the Yidden are asleep, it might be a Kitrug in Heaven that the Goyim stay awake for their god but the Yidden do not stay awake for Hashem. Therefore, he writes, it is an ancient custom that the Yidden go to sleep early and do not learn their nightly Seder and get up to learn at Chatzos so that there will not be a Kitrug.
However, not to learn at all that night is no Minhag at all.