In response to a student's concern about dozing off during study sessions, the Rebbe advises that this is a tactic of the yetzer hara (evil inclination) to disrupt one's service to G-d. He recommends the student request a study partner from the yeshiva administration to invigorate his learning, emphasizing that sincere effort will lead to success, as per the teaching: "If you toil, you will find."

https://www.torahrecordings.com/rebbe/igroskodesh/014/005/5114