What usually causes insomnia?

Too many things!: Lack of a regular sleep cycle, caffeine, alcohol, medications, over the counter meds, stress, sleep partner who snores, temperature in room, medical issues and goes on.

Psychiatric conditio: Apart form the conditions explained before, insomnia is one of the leading symptom of many psychiatric conditions including, depression, anxiety, and bipolar disorder. A psychiatric evaluation or an initial visit to a primary care should be entertined to rule out such conditions.