Psychiatrymiddle twist
A 24-year-old woman is referred because of mood swings since her late teens. Twice in two years she has had six-day spells of sleeping three hours a night, talking over people, cleaning the flat until dawn and spending money she did not have; she kept working and her flatmate called her unstoppable. She has never been admitted and has never had delusions or hallucinations. Between those spells she has had two three-month periods of being unable to get out of bed, sleeping excessively and feeling worthless. She does not drink and her urine drug screen is negative. Which one of the following best accounts for this pattern?