The Rebbe explains the custom of refraining from Torah study on Nittel Nacht until midnight, citing sources like Likutey Taamei HaMinhagim and Hayom Yom (17 Tevet), where the Frierdiker Rebbe attributes it to not adding vitality.
https://www.torahrecordings.com/rebbe/igroskodesh/014/005/5123