The Torah tells us (Leviticus 23:42-43) that it is a mitzvah to dwell in the sukkah throughout the Sukkot festival. The Mishnah tells us (Sukkah 2:1) that we don’t fulfil our obligation if we sleep under a bed in the sukkah, the implication is that there is an obligation to sleep in the sukkah, but not under anything that could come between us and the sukkah. In fact, every halachic text that discusses the laws relating to the sukkah and our obligations during Sukkot (see the Shulchan Aruch chapter 639) will mention that sleeping in the sukkah is a mitzvah de’oraita (from the Torah), though I encourage studying the text itself rather than taking my word for it. While halachic texts make it clear that women and children are exempt from sleeping in the sukkah, and that even men are exempt from sleeping in the sukkah if the weather is bad enough to make it uncomfortable in the sukkah, the Chabad-Lubavitch custom to not sleep in the sukkah isn’t based on these rulings. While it goes without saying that the city of Lubavitch and the entire surrounding region is generally cold during Sukkot and that’s enough to exempt most people from sleeping in the sukkah, the Chabad custom is more esoteric in nature.. The sixth Lubavitcher Rebbe (affectionately called “the Frierdriker Rebbe” by Lubavitchers, Yiddish for “the Previous Rebbe”), Rabbi Yosef Yitzchak Schneersohn (1880-1950, Lubavitch, modern-day Russia-Brooklyn, NY, USA) is well known to not have slept in the sukkah. His reason for this practise is based on a teaching of the second Lubavitcher Rebbe (affectionately known as “the Mitteler Rebbe”, Yiddish for “the Middle Rebbe”), Rabbi Dovber Schneuri (1773-1827, Liozna, modern-day Belarus- Nizhyn, modern-day Ukraine) that said that there is a great spiritual light present in the sukkah. Rabbi Schneersohn was extremely sensitive to spiritual matters and could actually sense the spiritual light in the sukkah and was unable to sleep in the sukkah because of the increased spirituality in the atmosphere. From what Chabad rabbis have told me, it’s not that he just didn’t sleep in the sukkah because he didn’t want to or out of defiance to the mitzvah (I would never expect that of such a pious man), but he was simply incapable of sleeping in the sukkah because the spiritual light kept him awake. He knew that the Torah obligated him to sleep in the sukkah, and since he couldn’t sleep in the sukkah due to the intense spiritual light of the sukkah, he never slept at all during Sukkot rather than intentionally violate the Torah. Obviously, since he couldn’t fall asleep in the sukkah, he would be exempt from sleeping in the sukkah on the basis that he simply couldn’t sleep at all. But since he didn’t want to violate the Torah simply because he couldn’t fall asleep in the sukkah, he just stayed awake for the entire holiday so he didn’t miss out on the chance to do the mitzvah. And that is the reason that Lubavitchers don’t sleep in the sukkah today. It’s not really based on an halachic ruling, especially since Lubavitchers don’t even debate that they are still required by the Torah to sleep in the sukkah, but it’s based on their emulation of the Previous Rebbe. While there’s nothing inherently wrong with emulating a tzadik, there a very few, if any, people alive today who are as sensitive to spirituality as the Previous Rebbe. Since there are probably no Lubavitchers alive today on such a lofty spiritual level that the spiritual light in the sukkah would keep them awake, there is no halachic basis for exemption from sleeping in the sukkah for most Lubavitchers. And of those Lubavitchers who may be spiritually sensitive enough to sense the spiritual light in the sukkah, they certainly do sleep inside their houses during Sukkot in violation of the mitzvah.
Ultimately, that what this whole article comes down to, anyone who sleeps indoors during Sukkot because a tzadik was incapable of sleeping in the sukkah due to the intense spiritual light present in the sukkah does so in violation of the Torah. If the spiritual light of the sukkah keeps a person awake during Sukkot, that’s fine because there’s no violation of the mitzvah. However, if a person sleeps indoors only because of the fact that the Previous Rebbe was incapable of sleeping in the sukkah at all, this is a problem because it’s a wilful violation of the Torah.
As an added danger, there is great chance of willingly not sleeping in the sukkah being a chilul HaShem. It’s a danger because, let’s be honest here: How hard is it to sleep in a sukkah in good weather? When a Christian picks up his Bible and reads that we are required to “live in the sukkah” for a week and they notice that their Lubavitcher neighbour only eats in the sukkah and then goes inside to sleep, he’s likely to mention it to someone.. And that someone will spread it until word is spread among the Christians that Jews are so careful to be religious, but Jews have no real relationship with HaShem (which is literally written in the Christian Bible) and Jews are only outwardly religious and not interested in a relationship with our Creator. In addition to being a chilul HaShem, the lack of a relationship with G-d has historically been used as grounds for anti-Semitism, so it could easily develop into that.
To summarise: It is a mitzvah from the text of the Torah itself that any man who sleeps during Sukkot should do so in the sukkah unless the weather makes doing so uncomfortable. Any wilful deviation from this mitzvah without any halachic exemption is a wilful violation of the Torah and, quite frankly, sinful. Furthermore, it is a completely different sin to encourage other Jews to violate the Torah by sleeping indoors when the weather conditions allow sleeping in the sukkah with no physical discomfort. It’s so serious of a sin that the Torah prescribes the death penalty for knowingly and willingly encouraging violation of any mitzvah of the Torah (Deuteronomy 13). Additionally, there is great potential there for chilul HaShem and anti-Semitism. All G-d fearing Jewish men are obligated to sleep in the sukkah if the weather allows it and if they are physically able to sleep in the spiritual atmosphere of the sukkah, and they would do well to not follow the Chabad custom of sleeping indoors when the weather and their spiritual sensitivity level allows sleeping in the sukkah.