It has long been known that eveningness correlates with various mental health conditions. In addition sleep is used as a marker for mental state.

Usually depression improves in the evening, sometimes very specifically at dawn. So does anxiety and OCD for some. To the point where there are little to no symptoms. In the morning however, the illness is back in full force.

There are numerous studies on the involvement of the circadian clock in depression and bipolar disorder. Many agents treating those conditions alter the circadian clock.