Based on Watson’s description, Holmes is very different when playing music than when doing detective work, but both are two sides to himself he needs.
Watson describes Holmes as an enthusiastic musician, “being himself not only a very capable performer but a composer of no ordinary merit.” When he loses himself in playing the violin, he is happy and calm.
The swing of his nature took him from extreme languor to devouring energy; and, as I knew well, he was never so truly formidable as when, for days on end, he had been lounging in his armchair amid his improvisations and his black-letter editions.
Holmes enjoys lounging, but gets bored. At this point, he needs to go out and solve a crime. He needs to use his mind. He lets himself loose on the world, on the criminal, and on the public. These are the two sides of himself he needs to balance in order to be successful. Yet he is rarely happy.