Sleeping in the sukka is obligatory, whether one is going to sleep for the night (sheinat keva) or taking a nap (sheinat ara’i). In this, sleeping differs from eating; akhilat ara’i is permitted outside the sukka because people snack outside the home all year long (explained above, section 4). Sleeping is stricter because even sheinat ara’i is significant, as even a short nap can be refreshing, and people generally do not nap outside the home. Therefore, even sheinat ara’i must be in the sukka (Sukka 26a; SA 639:2).
Some people tend to doze off unintentionally while traveling or during lectures. This type of involuntary dozing off is not considered sheinat ara’i and is not prohibited outside the sukka. The difference between the cases is clear: In the case of sheinat ara’i, one puts his head down on a desk or some other support in order to sleep for a little while, and many people are careful not to sleep like this in public. In contrast, one who is dozing off actually wishes to remain awake but dozes off involuntarily and jolts awake periodically.
There are additional issues when it comes to sleeping in the sukka. For a variety of reasons, some people find it hard to sleep in a sukka. The question is: At what point are they considered mitzta’arim who are exempt from sleeping in the sukka? To clarify this basic law, we must first explain the status of a mitzta’er.
If one is traveling to perform a mitzva or to avoid a financial loss, he may lie down on a bench to sleep a little if this is his usual practice, because his travel is for a permissible purpose, the status of the sukka is the same as the status of the home all year long, and some people regularly nap while traveling. However, if one is on a trip or traveling for another non-mitzva reason, he may not sleep a sheinat ara’i outside of a sukka (as explained below, section 14), though involuntary dozing is not forbidden. See Rema 640:3 and Harḥavot.