Although Rav Moshe says that the Rabbis were not concerned lest one be unable to fall asleep one night, it's interesting that this seems to be proved from the lack of mention of a concern, rather than an explicit mention of lack of concern.הא איכא זמן דא"א לאדם לחיות כלל בלא שינה, וכשבא לזמן כזה הא ברור לו שיישן. וגם ברובא דרובא פעמים שכל אדם ישן כהרגלו, ואין זה אף בחשיבות מיעוט שנחוש לזה. וגם חזינן שלא חשו לזה דהא התקינו לברך ברכת המפיל בכל לילה קודם השינה, ולא חשו כלל לשמא לא יישן אף ללילה אחת, וצ"ע.Behold, there is a time for which it is not possible for a person to live at all without sleep and when a time like this arrives, it is certain to this person that this person will fall asleep. Also, a majority of most of the time, everybody will sleep as they usually do, and we do consider the rarity [of not falling asleep] about which we may concern for it. Also, we see that [the Talmudic Rabbis] did not worry about this, since they established for people to bless the blessing of hamapil every night before sleep and they did not worry at all lest one would not sleep even one night. And further looking into the matter is necessary.
* Rabbi Moshe Feinstein, Igerot Moshe, vol. 8, ed. Rabbi Shabsai Avraham HaKohen Rapaport (Jerusalem: Rabbi D. Feinstein, 1996), 222. Rav Moshe wrote this on 25 February 1982.