I recently read an article (sadly I cannot find the link) about the way that delaying of sleep is a fairly common symptom of depression, as it allows the sufferer to feel some measure of control over their life.

As a sufferer myself, for 42 years, I can confirm this - delaying sleep gives me an extended window away from the problems that surface in a challenging period. Of course, this does not stand up to logical scrutiny, but it is, at best, a semi-logical and highly emotional response to adversity.