Kiddushin 80b: One woman may be alone with two men. This was only said in the city, but on the road there must be three men, because if there were only two, one of them might need to go off and relieve himself, leaving the other man alone with the woman.
Even Hoezer 22:5: The same applies to night time: there must be three men.
קידושין פ ע”ב: לא יתייחד אדם עם שתי נשים, אבל אשה אחת מתייחדת עם שני אנשיםֹ. ושם בדף פא ע”א: אמר רב יהודה אמר רב: ל״ש אלא בעיר, אבל בדרך ־ עד שיהיו שלשה, שמא יצטרך אחד מהם להשתין, ונמצא אחד מתייחד עם הערוה.
שו”ע אה”ע כ”ב ס”ה ברמ”א: וכ״ז בעיר אבל בשדה או בלילה אפילו בעיר בעינן ג׳ (ב״י בשם הראב״ד).
Rabbi Meir Brandsdorfer was once asked to serve as the sole shomer for yichud for a chosson and kallah (following a chupas nidah). He sat learning at a shtender all night long.
Usually, at night we require two shomrim, because one might fall asleep. Here, R’ Meir Brandsdorfer either was not afraid of falling asleep, or else he held that a nap on a shtender would not be considered sleeping for this purpose.
[This depends on how we learn the Raavad, who is the source of the halacha that you need two shomrim at night. The Beis Yosef brings the Raavad as follows:
וכתב הראב”ד בספר בעלי הנפש שי”א דכי אמרינן דאשה מתיחסת עם שני אנשים כשרים ה”מ ביום אבל בלילה לא ישן עמה בבית אחד עד שיהיו שם ג’ אנשים מפני שהוא דומה לדרך דאמרינן לא שנו אלא בעיר אבל בדרך עד שיהיו ג’ שמא יצטרך א’ מהם לנקביו ונמצא זה מתיחד עם הערוה וה”נ חיישינן דילמא אדנאים חד מינייהו אזיל חד ועביד איסורא.
According to this version of the Raavad, he is talking about people who are awake at night, and the fear is that if there is only one shomer, the shomer might fall asleep and the other man might go and commit a sin. Therefore we need two shomrim (three men total), so that the likelihood of them both falling asleep is low.
But in the Sefer Baalei Hanefesh that we have, the girsa is אדנאים חד מינייהו איתער חד ועביד איסורא: “While one is asleep, the other will wake up and commit a sin.” This implies that the case is that they are all sleeping, and the fear is that if there are only two men, one man might wake up and sin. Therefore we require three, so that if one is tempted, he will be afraid to sin lest one of the two shomrim wake up. But if the shomer is trying to stay awake, there is no problem, either because we are not afraid he will fall asleep, or because the temporary dozing of someone trying to stay up all night is not a deep enough sleep to make the other man unafraid of being caught.]
Source: Chukei Chaim, Parshas Bo 5780 (#165)
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