My sleep-deprived wife's reaction when I read this to her was "duh!"
Anyway, our Cartesian culture lives as if this is not true. One can especially see this on college campuses. Kids really think they are thinking well even though they don't get enough sleep, exercise, fresh air, and good food. If I were a young adult who was in recovery from this lifestyle I would carefully sift through all the judgments about important matters I was so certain about while I was in college, especially negative or pessimistic ones. Avery Dulles talks about his own experience of this in his college years at Harvard in his autobiography, A Testimony to Grace.
A.G. Sertillanges, O.P. (are you tired of me quoting him yet?) says:
In men of otherwise equal gifts, it is certain that sickness is a serious handicap. It lessens the output; it interferes with the freedom of the soul at the moment of its delicate operations; it sidetracks attention; it may warp the judgment by effects on the imagination and the nervous reaction that suffering brings about. A disease of the stomach changes a man's character, his character changes his thoughts. If Leopardi [a 19th century Italian agnostic poet--very influential on Msgr. Giussani] had not been delicate and deformed, would he have been among the pessimists? (Sertillanges, The Intellectual Life, p. 36)
Interestingly, Henri de Lubac suffered most of his adult life from severe headaches cause by a war injury. What might he have done had he been well?