halacha - When (specifically pertaining to illness, discomfort, etc.) can one be considered exempt from "positive" mitzvot (mitzvot asei)?

When would it reach a degree of discomfort, illness, or other types of cases of distress that one would be exempt from fulfilling a positive commandment? For example, does the concept of "Nafal BaMishkav", meaning, a desire/need to lay down in bed be considered an "Oines" which is exempt, or would it need to be something more severe?

Can someone explain or elaborate? With Sukkah, it's very simple - if you are uncomfortable, you aren't obligated, period. I am referring to every other case of a positive Asei. Is that a standard criterion, or an individual criterion for each mitsvot, or clusters of Mitsvot?

Thank you.