A 27-year-old male presents for psychiatric evaluation after his girlfriend urged him to seek help for mood instability. He describes a pattern over the past 3 years of alternating between periods of elevated mood, increased energy, and taking on extra projects lasting 2-3 days, followed by stretches of several days of feeling down and withdrawn. During the upbeat periods, he sleeps less and feels productive but the episodes never last 4 consecutive days and he continues functioning at work. During the down periods, he feels low and tired but continues eating and sleeping adequately. He has never experienced a full major depressive episode. This pattern has been continuous for the full 3 years without a symptom-free period lasting longer than 2 months. The most likely diagnosis is: