Everything I've read about Nick and listening to his music indicates to me that he was depressed, but not manically depressed. Many people use the term 'manic depression' to mean a severe or long lasting depression, not realising the mania side of it construes a different disorder. It is a strange task to attempt to diagnose someone who has been dead for nearly a quarter of a century, and someone who I've never interviewed or given psychological tests. Firstly to define depression and mania. Depression is a sad, low and lethargic state in which life seems black and overwhelming.
Mania is the opposite of depression and is a state of frenzied energy and euphoria.
Usually, moods such as happiness or sadness are transient responses to life events. In people diagnosed as having a mood disorder the moods last a long time, colour all their interactions with the world and disrupt their normal functioning. In depression the person suffers recurrent depression but has no history of mania. Bipolar disorder entails alternating periods of depression and mania, usually with intervening periods of normal mood. Most bipolar mood disorders begin with a manic episode.