Tosafot Yom Tov on Pirkei Avot 3:4

והמפנה לבו לבטלה. ונ"א ומפנה. והיא גי' הר"ב שכ"כ הנעור וכו' ומחשב וכו'. וכ"כ במדרש שמואל על לשון הר"ב דגרס ומפנה ול"ג והמפנה. וכתב עוד בשם החסיד ז"ל שטעות הוא שנפל בספרים שכתוב והמפנה כי לא ימנע או הנעור מפנה או לא. אם מפנה היינו המפנה. ואי לא למה יתחייב. ולית נגר דיפרקיניה ע"כ. ובשם הרשב"ם כתב הנעור בלילה ואפי' בבית והמהלך בדרך אפילו ביום. ובשניהם ביחידי עסקינן. אבל אין גורסין הנעור בלילה יחידי. ומסתכן לפי שמזיקין פוגעים בו. ע"כ. ולדבריו יש לקיים גרסת והמפנה. וכן לפירוש הר"ר מנחם שבמדרש שמואל ואין להאריך ובדרך חיים עשה לגירסא זו עקרית ומפרש הנעור בלילה ניזוק דלא אברי ליליא אלא לשינתא. או לגרסא. כדאמרינן בעירובין (דף ס"ה.). והמפנה לבו לבטלה ואפי' ביום. וכלומר שהוא נמשך אחר הבטלה ומבקש הבטול אבל הנעור בלילה היינו אע"פ שאינו מבקש הבטול כל שאינו ישן או לומד או עיסק במלאכתו שכל תורה שאין עמה מלאכה סופה בטלה. ולגרסת ומפנה צריך לדחוק דהא דלא קאמר רק והמפנה לבו לבטלה. בלילה. משום דאי הכי הוה משמע שמפנה לבו לבטלה כדי שיישן. ע"כ. ועיין מה שכתבתי במשנה ח': AND ONE WHO TURNS HIS MIND TO IDLE THINGS. Other versions have “and turns his mind,” which is what Rav had, as he writes: One who is up at night… and turns his mind… Midrash Shmuel, too, notes in discussing Rav’s commentary that he had “and turns his mind” as opposed to “and one who turns his mind.” He also writes in the name of Chasid Ya’avetz that the version “and one who turns his mind” is a typographical error that has crept into various editions. For “one who is up” must either be turning his mind to idle things or not. If he is, that is nothing more than the case of “one who turns his mind to idle things.” If he is not, then what is his sin? He concludes that there is no way to read this version of the mishna.
He writes in the name of Rashbam that “one who is up at night” could mean even in his home, and “one who is walking along the road” could be even during the day. In both cases he is alone, and the text does not read “one who is up at night alone.” And the danger is because mazikin might attack him. Based on Rashbam, we can properly read the version “and one who turns,” as we also can based on the words of R. Menachem Meiri quoted in Midrash Shmuel, but I am not going to go into this at length.
Maharal in Derech Chaim writes that this version is the main version, and explains that “one who stays up at night” will be damaged because “night was created only for sleeping or Torah study,” as the Talmud says in Eruvin 65a. The case of “one who turns his mind to idle things” refers even to one who does so during the day, meaning that he is drawn to idleness and seeks to remain idle. But “one who stays up at night,” even if he does not seek to remain idle, he “forfeits his life” unless he is sleeping, studying, or plying his trade—because any Torah that is not accompanied by a trade eventually turns to naught. Whereas if we have the version “and turns his mind,” we must resort to the forced explanation that the reason the mishna doesn’t simply say “and one who turns his mind to idle things at night” is that this would seem to include even someone who turns his mind to idle things in order to fall asleep.
Cf. my comments on mishna 8.