A 33-year-old female nurse is referred by her primary care provider with a suspected diagnosis of bipolar II disorder. She describes a 4-year history of mood cycling. During 'up' periods lasting 2-3 days, she feels energized, sleeps only 4 hours without fatigue, talks rapidly, and takes on extra shifts at work. During 'down' periods lasting 5-7 days, she feels sad, fatigued, and withdraws from friends, but she continues eating normally, denies anhedonia, maintains her hygiene, and keeps going to work. She has never had a period lasting longer than 5 weeks without mood fluctuations. Her PCP started her on lamotrigine for presumed bipolar II. On detailed interview, the PMHNP establishes that her elevated periods have never lasted 4 or more consecutive days, she has never engaged in reckless behavior or grandiosity during elevated periods, and her depressive periods have never included suicidal ideation, worthlessness, psychomotor changes, or concentration deficits beyond what she attributes to fatigue. She has never met full criteria for a major depressive episode. What is the most accurate diagnosis?