“How is bipolar disorder related to psychology?”

Any mental illness related with biological, psychiatric and social back ground.

Historically, the most serious difficulties related to mood changes were described as manic depression. Manic states consisted of extreme activation usually with destructive results. More recently the term bipolar has been used to describe just about any fluctuation of mood. When this is treated by a psychiatrist, it will most often be with medication. It is entirely possible, however, for most mood disorders to be treated successfully by any good therapist if they have been well trained and are experienced. The term bipolar is used in both fields: psychology and psychiatry.

It has depression and manic episodes. 

See this link for some insight into your question:
https://psykologtidsskriftet.no/2007/05/psychology-bipolar-disorders