One can eat a light snack outside the sukkah but not take a catnap. But with tefillin, catnap is allowed; what is the difference? Is it because he can appoint an overseer who will wake him up? Rav Mesharshia objected: that's not an explanation - guarantor needs his own guarantor, and your overseer may fall asleep as well! Rather, one should not nap outside the sukkah because even a nap refreshes him and counts as sleep. With tefillin, the concern is that he should not pass gas, and that won't happen while catnapping.
Rabbi Yochanan ben Zakkai was extra strict with himself and ate and drank even small amounts only in the sukkah. Nevertheless, this kind of strictness is permitted and is not considered presumptuous.
Art: The Sick Child by J. Bond Francisco