Nedarim 15A

כי לא מזדהיר ⁠— בתנאה, אבל באיסורא ⁠— מזדהר.

Sleeping today is not prohibited for him. Rather, it causes sleeping to be prohibited for him tomorrow, because when one is not careful, it is only with regard to a condition. In the former case, sleeping on the 2nd day merely fulfills the condition on which the prohibition was based, causing it to take effect retroactively. Therefore, there is concern that he will not be careful and retroactively cause a violation. However, one is careful with regard to a prohibition. In the latter case, sleeping on the 2nd day is directly prohibited. Therefore, there is no concern that he will violate the prohibition.

תנן: קונם שאני ישן, שאני מהלך, שאני מדבר וכו׳. היכי דמי? אילימא כדקתני, ״שאני ישן״ מי הוי נדרא? והתנן: חומר בשבועות, שהשבועות חלות על דבר שיש בו ממש ועל דבר שאין בו ממש, מה שאין כן בנדרים. ושינה דבר שאין בו ממש הוא! אלא דאמר: ״קונם עיני בשינה״.

The Talmud raises a difficulty with regard to the opinion of Rav Yehuda: We learned in the Mishnah that one who says: Sleeping is konam for me, or: Walking is konam for me, or: Speaking is konam for me, may not violate his vow. What are the circumstances? If we say that the wording of the vow is precisely as the Mishnah teaches, is the vow: Sleeping is konam for me, a valid vow? But didn’t we learn in a baraita: There is stringency with regard to oaths vis-à-vis vows, in that oaths apply to something that has actual substance and to something that does not have actual substance, which is not the case with regard to vows. And sleep is something that does not have actual substance, so how can a vow apply to sleep? Rather, the Mishnah must be referring to a case where he said: Sleeping is konam for my eyes.