Why is Sherlock Holmes a late riser?

In the opening of "The Adventure of the Speckled Band," Watson writes:
It was early in April in the year '83 that I woke one morning to find Sherlock Holmes standing, fully dressed, by the side of my bed. He was a late riser, as a rule, and as the clock on the mantelpiece showed me that it was only a quarter-past seven, I blinked up at him in some surprise, and perhaps just a little resentment, for I was myself regular in my habits.
One of the things that appeals to Sherlock Holmes fans is the enviable lifestyle of the famous detective. When Watson calls him "a late riser" he is just dropping another example of Holmes' independence, comfort, and self-indulgence. By seven-thirty in the morning most of the men in England would have gotten up on a cold, foggy morning and would have been on their way to work in the offices, factories, and warehouses of the great city. But Holmes probably didn't think about getting up much before around 9 A.M. And even then he would put on a robe and stroll out to the living room for a cup of coffee and a smoke while he browsed through the morning papers. The same was true of Dr. Watson when he lived with Holmes at Baker Street. Watson had been wounded in India and was living on a government pension. So he could sleep as late as Holmes.