Sick people and their attendants are exempt from the mitzva of sukka. This exemption is not limited to the dangerously ill; even someone who is in no danger – for instance, someone who has a headache and finds sitting in the sukka difficult – is exempt. If a sick person needs help, his aide is exempt as well (Sukka 26a; SA 640:3).
Likewise, a mitzta’er (one who experiences pain or discomfort in the sukka) is exempt, because the mitzva is to reside in the sukka as one resides at home during the year. Just as one would not reside in a place that causes him pain and discomfort, so too on Sukkot, he is not obligated to dwell in the sukka if it causes him discomfort. True, a sukka is a temporary residence and therefore, by its nature, is not as comfortable as a house. This lesser comfort does not exempt someone from sukka, for this is precisely the mitzva. But when an additional factor causes staying in the sukka to entail pain and actual discomfort, one is exempt. The most common case of mitzta’er is rain.
The level of discomfort that warrants exemption from the mitzva of sukka must be significant, of the type that would lead one to move out of his home to a considerably less comfortable place nearby. For example, if one has a very minor leak in his roof, he would prefer to remain at home. Similarly, if there is a minimal amount of rain, one must remain in his sukka. If the rain persists, so the point that it would ruin his food and disturb his sleep, he would move elsewhere, despite the bother of moving, and even if his new quarters were smaller and shabbier. In such a situation, one is considered mitzta’er and is exempt from the sukka. He remains exempt as long as the sekhakh continues dripping enough to ruin his food (SA 639:5; Eshel Avraham [Buczacz] 640:4). If he is mitzta’er with respect to sleeping but not eating, he is exempt from sleeping in the sukka but obligated to eat there (MB 640:16).[12]
If one left the sukka on account of rain, started eating inside, and then the rain stopped, he need not return to the sukka. Rather, he may finish eating in the house. Similarly, if he went to sleep in the house because it was raining, and then the rain stopped, he is not required to return to the sukka. He may sleep at home until morning (SA 639:6-7).[13]
[13]. The basis of this halakha is in Sukka 29a. Also see Beit Yosef and SA 639:6-7, which state that the exemption applies only until he wakes up and dawn breaks. However, if he wakes up after dawn and wants to continue sleeping, he must move to the sukka. Some rule this way in practice. But it seems more reasonable to assume that this is limited to one who regularly wakes up close to dawn. However, one who generally sleeps for another hour or two is not required to move into the sukka before the time he normally wakes up. (See Piskei Teshuvot ad loc. 16; Hilkhot Ḥag Be-ḥag ch. 17 n. 42.) This is certainly true for someone who is worried that if he moves into the sukka he will not be able to fall back asleep, in which case he would have the status of mitzta’er and need not relocate to the sukka. On the other hand, if moving to the sukka is only a minor inconvenience for someone, it would seem that even if he wakes up in the middle of the night, it is proper for him to move into the sukka to sleep. This is in line with MB ad loc. 41.