Adopted from “Four Chassidic Masters” by Rabbi Dr. Abraham Twerski:
The Chofetz Chaim used to go into the beis midrash late at night and order the talmidim to go to sleep. He said that their drive to learn into the early hours of the morning does not come from the yetzer tov, but rather the yetzer hara that wants them to be so exhausted the next day that they will not be able to concentrate.
The Chofetz Chaim pointed out that we pray, “Remove Satan from before us and from behind us.” The “before us” is understandable, because Satan may stand in the way of one’s doing mitzvos, but what does “behind us” mean? The Chofetz Chaim says that sometimes Satan stands behind a person and pushes him to do mitzvos; e.g., learn until 3 a.m. so that one is tired the next day, fast so that one will be weak, give away everything one owns to tzeddakah so that he becomes destitute. So we ask that Hashem protect us from Satan pushing us from behind.