A 32-year-old man reports 1 week of feeling unusually irritable. During this time, he has increased energy and activity, sleeps less, and finds it difficult to sit still. He also is more talkative than usual and is easily distractible, to the point of finding it difficult to complete his work assignments. A physical examination and laboratory workup are negative for any medical cause of his symptoms and he takes no medications. What diagnosis best fits this clinical picture?