A 29-year-old female presents to a psychiatric outpatient clinic reporting mood fluctuations over the past 3 years. She describes periods lasting several days where she feels energized, sleeps only 4-5 hours, takes on extra projects, and is more talkative than usual. These alternate with periods of low mood, fatigue, poor concentration, and social withdrawal lasting 1-2 weeks. She has never been hospitalized, has maintained employment, and denies any periods where symptoms caused severe functional impairment or required emergency intervention. She reports that during her "up" periods she does not experience grandiosity, psychotic features, or reckless behavior, and these episodes have never lasted a full week. Her PHQ-9 score is 8 and MDQ screening is positive. She has not been symptom-free for more than 6 weeks at a time during this 3-year period. What is the MOST likely diagnosis?