> ...when he's not working is when he's depressed.
The cure for that is known since dawn of time - walking.Holmes, being an exceptionally observant man, definitely would observe that walks raise the mood, allow for (most often silly) ideas to come and, last but not least, increase observation capabilities, attention to details and speed of thought.
Arthur Conan-Doyle did an extensive walks back then, but his hero was written to not to. This is not right.
Holmes is basically a border collie?
As I recall Holmes did in fact do a lot of walking. He vacillated between periods of inactivity(cocaine, violin, shooting V in wall with a revolver) and intense activity (taking up disguises and doing various physical activities including walking all across London and elsewhere.
Just because your logical mind says one thing is good to do and you know you should do it you are not going to always obey your rider, the inertia of the elephant takes over.
So you need a trigger to snap out of it, for Holmes it was a new case.