Hi,

A very similar question was asked not so long ago - I gave this reply.

Essentially, you are quite right in the content of your question. There may well be a physiological answer, as much as an issue of thought processes. I think it partly depends on the type of depression you have, the severity of it, the length of time it is experienced and whether any comorbid conditions exist.

The so-called 'diurnal variation' has certainly led to speculation that some depressions, and possibly bipolar affective disorders, may stem from disruption of circadian rhythms (body temperature, hormonal changes, altered patterns in the secretion of cortisol, pituitary-dependent hormones).  A whole raft of studies point to sleep disturbance patterns in depressed people.

Hope this goes some way to answering your question.