@Rso and @DaasYochid
The Sukkah doesn’t have to cause Tzaar itself. It just can’t be an independent Tzaar (like thinking about dragging blankets while eating):
שהסוכה מצערתו כגון חמה או צינה או סירחה בדברים שסיכך בהן
Sun or cold doesn’t come from the Sukkah itself, and it Patturs one from sleeping in the Sukkah.
The only thing is, Tzaar can’t be something that won’t stop by moving inside (like Aveilus, one’s Meis won’t come back to life if he moves outside the Sukkah). On the other hand, sleeping in the Sukkah is causing me Tzaar, since when I’ll try to sleep in the Sukkah, the Sukkah itself is constantly reminds me that I’m an Oisvarf. In the house, it’s not in my face.