If Lubavitch is careful not to eat or drink anything outside of the sukka, why does Lubavitch not sleep in the Sukka?
I do not have a source for this, but I have heard that Belz's also doesnt sleep in the sukkah.
The reason why Lubavitch does not sleep in the sukkah is explained by the Rebbe in Likkutei Sichos Vol. 29 p. 211
It is very simple to summarize. There is a halacha that if you are uncomfortable in the sukkah you can leave. When it rains most people don't eat in the sukkah but Lubavitch especially the Rebbe was seen many a pouring rain drenched in the sukkah. How then could we not sleep in it? Well it was bitterly cold in Russia. That is not the answer either. The answer is as follows.
The Sukkah al pi kabbalah represents the Makif D'Binah, loosely translated as a transcendent encompassing level of Understanding of Hashem. If you have any perception of this, it is a quite overwhelming level of G-dliness in the Sukkah. It is possible to eat and learn Torah in the Sukkah, but sleep? Impossible. The Chabad Rebbeim literally could not sleep there. [Some say that the Rebbe did not sleep the entire festival of Sukkos, and was therefore able to fulfill both the Halacha and Kabbalah of Sleeping in the Sukkah, or at least did not go to sleep, but rather would just learn until he fell asleep]. What about us? We don't perceive the Makif D'Binah and we wouldn't make it 7 days without sleeping. The answer is that we have such a level of Hiskashrus to our Rebbe that we are pained that he is pained as well as pained that we can't perceive this level of G-dliness. .
On a personal note, I left Lubavitch some years ago and slept in the Sukkah and always was disturbed by something, ie: a dog barking all night, air mattress going flat, and finally the Sukkos before I made my way back to Lubavitch roots I was viciously bitten by insects to the point that I had at least 50 bites on my body.
That was my Makif D'Binah.
The sources for all of this is found in the Sicha quoted above. If you agree or not, this issue was analyzed by the Rebbe with a microscope and if you actually learn the Sicha it is hard to say he is wrong in what he says regardless if you choose to sleep in the sukkah or not.
The reason Chabad Chasidim do not sleep in the Succah is Kabalistic. The enclosed links explains it.
The Rebbe actually addressed this question in a letter dated 7 Cheshvan, 5715 :
(Excerpt of letter taken from L'chaim # 688 )
BTW, @mochin rehavim wrote:
The Rebbe mentioned this 10 Sivan 5750 – During distribution of dollars for Tzedoka- (the whole conversation can be found in Toras Menachem 5750 Vol 4, P. 325) - where R' Zvi Kahana asked the Rebbe this very question.
PS: To anyone that knows about / read / watched the above 'dollars' encounter and was wondering what was the Rebbe's 'nekudah', well, I saw a blog post about this and one commenter by the name of guravitzer summed it up very well:
I am a Lubavitcher Chassisd myself that learns in a Lithuanian yeshiva. Before Sukkos one year, during a Halacha class, my rebbe was speaking about this topic of sleeping in the sukkah, and I mentioned that in Chabad they don't sleep in the Sukkah because its too holy. He got really angry at that and said "Well we follow like the Shulchan Aruch" and then went on to call that practice a denial and the Rebbe ztk"l a denier of halacha, heaven forbid, for instituting such a practice. I was really disturbed about that for a while so I decided to do some research and I found this: The Rebbe's position was not because of holiness, but rather because of discomfort: one with discomfort is exempt from sukkah. But then poskim ask on this: but discomfort is only physical elements like hot and cold? And the Rebbe answered that question by saying that if one has discomfort it doesn't matter why, one is exempt because of "teishvu k'ein t'duru" (the obligation is to live in a sukkah as one lives in a home). For example, if one would have a holy article in his bedroom (e.g. an antique holy book) he couldn't bring himself to sleep there. So too by Sukkah. However, if one does not sense the holiness and is therefore not discomforted, he has no exemption, which is why I do sleep in the sukkah.