"During the ealy years in Vienna, Mozart would customarily arise at six o'clock, be at his desk by seven, and compose until nine or ten, when he would make the rounds of his pupils, giving lessons until one o'clock.  'Then I lunch,' he reported to his sister, 'unless I am invited to some house where they lunch at two or even three o'clock. . .I can never work before five or six o'clock in the evening, and even then I am often prevented by a concert.  If I am not prevented, I compose until nine.'

Returning to his room after several hours of social visits, he would again compose for an hour or two:  'I often go on writing until one - and am up again at six.'  He slept five or six hours, though he would have preferred seven.  With variations, that was Mozart's daily routine as he described it in his letters home, perhaps somewhat idealized to impress his father and sister with his diligence but borne out in the main by the objective records his productivity.  After 1784 he gave lessons only in the afternoon, 'in order to keep the morning free for composing.'  Sometimes he was so busy writing that he did not even dress or have his hair done (a friseur woke him each morning); other days the only time he had for composing was in the evenings, 'and of that I can never be sure, as I am often asked to perform at concerts,' in other words, he was in demand as a free-lance accompanist or guest artist. 

'Every minute is precious,' he wrote on one occasion; 'I have so much to do that I often do not know whether I am on my head or my heels,' he wrote again, echoing one of his mother's stock expressions.  He described his enthusiasm while composing Die Entführung aus dem Serail:  'I rush to my desk with the greatest eagerness and remain seated there with the greatest delight.'  'I have so much to compose and not a minute to be lost,' he wrote; 'you know that usually I go on composing until I am hungry.'  His wife came to believe that he 'killed himself with overexertion,' recalling that he 'frequently sat up composing until two and rose at four, an exertion whch assisted to destroy him.'

Although stories of the colossal pace at which he was able to compose have passed into the realm of the legendary, there is no question that he ordinarily worked at an extremely fast rate.  He wrote the violin part of the Sonata in G, K 379/373a, in an hour on the night before the performance - 'but in order to be able to finish it, I only wrote out the accompaniment for Brunetti and retained my own part in my head.'"

Maynard Solomon
Mozart: A Life