We have already noted the basic difference between a neder and a shevua that is commented on by the Gemara. While a neder acts on an object (e.g. a person declares that meat is forbidden to him), a shevua acts on the person (e.g. he accepts upon himself a prohibition that will keep him from eating meat).
The case of the Mishna on today’s daf, where someone takes a shevua that he will not sleep, will only work if it is an oath, since “sleep” is not an object, and it can only become forbidden by means of a shevua (which will create a prohibition on the person keeping him from sleeping).
The Gemara points out a problem with the Mishna’s ruling that an oath against sleeping takes effect. Rabbi Yoḥanan teaches with regard to shevuot that a person who takes an oath not to sleep for three days is understood to have taken a false shevua – since it is impossible to go without sleep for 72 hours. Therefore, rather than forcing him to attempt the impossible we punish him immediately (for having made a false shevua) and allow him to sleep whenever he wants. The Gemara explains that our Mishna is talking about a case where the person did not specify how long he planned to remain awake, so he will be able to fulfill his oath.
The rishonim are sensitive to the fact that an open-ended shevua sounds as if it should last even longer than three days. While Rabbeinu Ḥananel says that we must be dealing with a case where the person specified an amount of time less than three days, the Rashba suggests that in a situation where the person cannot fulfill the oath that he took, it will be defined as applying only for as long as it is feasible.
In theory it is possible for a person to go without sleep for a period of three days if he is constantly prodded and woken by others whenever he begins to doze off. Nevertheless, withholding sleep from someone for that length of time will likely cause long-term physical and psychological damage, which the Talmudic Sages could not condone.