After a meeting with Lord Castlereagh in April 1818, the American politician Richard Rush dined at William Wilberforce’s house. Amongst animated queries about the fledgling United States, Wilberforce also spoke to Richard Rush about Mr. Pitt. Wilberforce remembered Pitt as a brilliant mathematician, and said “there was also this peculiarity in his [Pitt’s] constitution that he required a great deal of sleep, seldom being able to do with less than ten or eleven hours; he would often drop asleep in the House of Commons; once he had known him to do so at seven in the evening, and sleep until day-light” (Rush 1872: 175).
Pitt needing a significant amount of sleep, and laying in bed until quite late in the morning (sometimes later than eleven am, and particularly so as he got older), has been mentioned by many of his contemporaries (e.g. George Pretyman Tomline, George Rose, Lord Nelson, Lord Melville, Wilberforce, etc.) so this in all probability was the case. Wilberforce was a close friend of Pitt, and Pitt often stayed at Wilberforce’s villa in Wimbledon before it was sold in 1786, so I firmly trust this anecdote is accurate. Pitt just required a lot of sleep!
Rush, R. (1872) ‘Residence at the Court of London, 3rd edition.’ London: Hamilton, Adams & Co., p. 175.