It was half-past five before Holmes returned. He was bright, eager, and in excellent spirits,—a mood which in his case alternated with fits of the blackest depression. “There is no great mystery in this matter,” he said, taking the cup of tea which I had poured out for him. “The facts appear to admit of only one explanation.”
"He was bright, eager and in excellent spirits- a mood in which in his case alternated with fits of the blackest depression."
Undoubtedly, Holmes had bipolar disorder...