Halacha-Yomi - Torah.org

Chapter 135:8
Dwelling in the Sukkah

8. According to halacha ("Mi'dinah"), one must sleep in a sukkah, even if one is taking a short nap. This is the practice of those who are "precise in the observance of the mitzvos" ("me'dakdekin be'mitzvos"), [that is], they do not even nap outside a sukkah. At present, many are accustomed to being lenient regarding [sleeping in the sukkah] (1), and the later Rabbinic authorities ("Acharonim") have offered several rationales to explain their conduct (2). However, it is proper for every G-d-fearing person to construct a sukkah in which he can dwell together with his wife in the manner that he dwells throughout the entire year [in his house], if possible. At the very least, it should be fit for him to sleep in there alone. If it is not fit for this, it is unacceptable ("pesulah") even after the fact ("bedi'avad") (3).

FOOTNOTES:

(1) That is, they do not sleep in their sukkah.

(2) The most prominent rationale offered to explain the leniency is the cold weather at Succos time in Europe and North America (See Ramah 639:2 and the Mishna Berura). As we've seen, one who will experience discomfort while eating or sleeping in the sukkah, is exempt from doing so.

(3) This is not referring to unavoidable situations, such as those who live in a cold climate, and thus any sukkah would be unfit for sleeping in. Rather, this is referring to a case in which one built one's sukkah in a place that would make it uncomfortable to sleep in, for example, due to insects, high speed winds, foul odors, or fear of bodily harm inflicted by animals or humans (and one could have chosen a place where it would have been comfortable to sleep in the sukkah). There are authorities who rule that even though one should not build a sukkah in place that would make it unfit to sleep in, if one did so, the sukkah would still be "kosher" for eating in. That is, one can still fulfill one's obligation [to eat in a sukkah], by eating in it (See Mishna Berura 640:18 and 20).