A 27-year-old female graduate student presents for evaluation of mood swings. She describes recurrent depressive episodes since age 19. Between depressive episodes, she reports distinct periods lasting 5-6 days during which she feels 'on top of the world,' sleeps only 4 hours without fatigue, takes on multiple new projects, talks faster than usual, and is significantly more productive and sociable than her baseline. Her roommate confirms these periods represent a clear departure from her usual personality. During these episodes she has never been fired, arrested, hospitalized, or required emergency intervention, although her roommate notes she tends to overspend moderately and make impulsive social commitments she later regrets. She has no history of psychotic symptoms. Which diagnosis is most appropriate?