135:7 On the rest of the nights of Succos, and on all the days, there is no obligation to eat in a succah, but if one wants to eat a fixed meal or to sleep, he must do so in the succah. A fixed meal is more than a k’beitzah of bread (2-3 ounces), even if he does not intend it to serve as a meal. This is true even if it is something like pastry or cake, rather than bread. The same applies to any food made from the five species of grain (wheat, rye, barley, oats and spelt) that is more than a k’beitzah and intended as a meal: one must eat it in the succah and recite the bracha of “leishev basuccah” on doing so.
When it comes to fruit, even if one eats a lot of it and intends it to be his meal, it may be eaten outside of the succah. Also, wine, beverages, meat and cheese may be consumed outside of a succah so long as one did not intend them as a meal. However, if one wants to drink wine or other beverages as part of a fixed meal, or if he wants to eat meat or cheese as part of a fixed meal, then he must do so in the succah. We do not recite “leishev basuccah” on doing so, so one should first eat some bread to enable him to recite the bracha. This is all the basic law but one who is scrupulous and does not even drink water outside of the succah is considered praiseworthy.
135:8 According to the law, sleep – even a nap – should only be done in the succah. The practice of those who are scrupulous in their mitzvah observance is not even to nap outside the succah. Nowadays, many are lenient when it comes to sleeping in the succah and the Acharonim (later Rabbinic authorities) gave a number of reasons to justify this leniency. Nevertheless, a G-d-fearing person should be scrupulous and, if possible, build a succah that he can live in with his wife as he does in the house the rest of the year. It should at least be sufficient for him to sleep in alone. If the succah is unfit for this, then it is invalid, even after the fact. (“Unfit for sleeping” means, for example, that it won’t keep the wind out. In cold climates, a succah is valid for eating even though one might not be able to sleep in it – Mishnah Brurah 640:18.)