Feeling guilty that you can fall asleep, which is a mitzvah, can be called tzaar and therefore patter you from sleeping? Doesn’t make sense.
Sorry, I don’t get your issue. Is it that why should one feel guilty about doing a Mitzvah? Because it is a very holy place, and you feel bad sleeping (which comes along with other things) in a holy place. So if my house was a Shul, I wouldn’t sleep there either.
But it’s a Mitzvah?
No. There’s no Mitzvah to sleep in a Sukkah. It’s to live in a Sukkah like we do at home. So because we sleep at home, we have to sleep in a Sukkah. But there’s no inherent Mitzvah to sleep in a Sukkah. And the same goes for drinking, or eating (except for the first night). The Mitzvah is to live there like you do at home. Since you eat and drink at home, you have to eat and drink in the Sukkah.
But all Seforim say to sleep in a Sukkah. The Mishna, Gemara, Rishonim, Shulchan Aruch?
Yes, because there’s a Mitzvah to sleep in a Sukkah like you sleep at home. And for hundreds of years, there were two classes of people:
1. People who really grasped the holiness of a Sukkah – and these were Tzaddikim who’s sleep was a different sleep (for example, Shivchei Ari says how the Arizal would learn deep Sisrei Torah while he slept), so for them there was no problem sleeping in a Sukkah, so they had the Chiyuv DeOraisa to sleep in a Sukkah.
2. The regular folk, who didn’t know the Sisrei Torah behind the Sukkah. So they obviously didn’t feel bad about sleeping in a Sukkah, so they had the Chiyuv DeOriasa to sleep in the Sukkah.
It’s just that due to Yeridas Hadoros on one hand, and the spread of Chassidus on the other (so that even relatively simple people have some understanding of what holiness a Sukkah has), that there’s this guilty feeling.
But yes, if you don’t have any guilty feeling, you absolutely are obligated to sleep in a Sukkah.