Tshilidzi (23) reports fluctuating mood patterns over the past two years. He experiences periods of increased energy and decreased need for sleep lasting several days, followed by periods of low mood, fatigue, and poor concentration. None of these episodes has met the full criteria for hypomania or major depression. The mood changes have occurred more than half the time, without a symptom-free period exceeding two months. There is no history of psychosis, mania, or substance use.

Which diagnosis best fits this clinical picture?