In his Autobiography he assigns various reasons for his illness.

As the result of an accident on his journey from Frankfort to Leipzig he had strained the ligaments of his chest, and the mischief was aggravated by a subsequent fall from his horse; he had suffered from the fumes of the acids he had inhaled in the process of etching; he had ruined his digestion by drinking coffee and heavy beer; and, in accordance with the precepts of Rousseau, he had adopted a _regime_ which proved too severe for his enfeebled constitution.

So he wrote in his old age, but his contemporary letters leave us in little doubt regarding the cause of his breakdown.

He had, in fact, during the latter part of his sojourn in Leipzig lived the life of the average German student of his day.

He had fought a duel, and had been wounded in the arm; he had drunk more than was good for him, and we have seen that he had followed other courses not conducive to his bodily health.
His mental condition was equally unsatisfactory.